ABS: Anti-lock braking system. An electro-mechanical braking
system which is designed to minimize or prevent wheel lock-up during braking.
ABSOLUTE PRESSURE: Atmospheric (barometric) pressure plus
the pressure gauge reading.
ACCELERATOR PUMP: A small pump located in the carburetor
that feeds fuel into the air/fuel mixture during acceleration.
ACCUMULATOR: A device that controls shift quality by
cushioning the shock of hydraulic oil pressure being applied to a clutch or
ACTUATING MECHANISM: The mechanical output devices of a
hydraulic system, for example, clutch pistons and band servos.
ACTUATOR: The output component of a hydraulic or electronic
ADVANCE: Setting the ignition timing so that spark occurs earlier before the
piston reaches top dead center (TDC).
ADAPTIVE MEMORY (ADAPTIVE STRATEGY): The learning ability of
the TCM or PCM to redefine its decision-making process to provide optimum shift
AFTER TOP DEAD CENTER (ATDC): The point after the piston
reaches the top of its travel on the compression stroke.
AIR BAG: Device on the inside of the car designed to inflate
on impact of crash, protecting the occupants of the car.
AIR CHARGE TEMPERATURE (ACT) SENSOR: The temperature of the
airflow into the engine is measured by an ACT sensor, usually located in the
lower intake manifold or air cleaner.
ALDL (assembly line diagnostic link): Electrical connector
for scanning ECM/PCM/TCM input and output devices.
AIR CLEANER: An assembly consisting of a housing, filter and
any connecting ductwork. The filter element is made up of a porous paper,
sometimes with a wire mesh screening, and is designed to prevent airborne
particles from entering the engine through the carburetor or throttle body.
AIR INJECTION: One method of reducing harmful exhaust
emissions by injecting air into each of the exhaust ports of an engine. The
fresh air entering the hot exhaust manifold causes any remaining fuel to be
burned before it can exit the tailpipe.
AIR PUMP: An emission control device that supplies fresh air
to the exhaust manifold to aid in more completely burning exhaust gases.
AIR/FUEL RATIO: The ratio of air-to-gasoline by weight in
the fuel mixture drawn into the engine.
ALIGNMENT RACK: A special drive-on vehicle lift
apparatus/measuring device used to adjust a vehicle's toe, caster and camber
ALL WHEEL DRIVE:
Term used to describe a full time four wheel drive system or any other vehicle
drive system that continuously delivers power to all four wheels. This system
is found primarily on station wagon vehicles and SUVs not utilized for
significant off road use.
ALTERNATING CURRENT (AC): Electric current that flows first
in one direction, then in the opposite direction, continually reversing flow.
ALTERNATOR: A device which produces AC (alternating
current) which is converted to DC (direct current) to charge the car battery.
AMMETER: An instrument, calibrated in amperes, used to
measure the flow of an electrical current in a circuit. Ammeters are always
connected in series with the circuit being tested.
AMPERAGE:The total amount of current (amperes) flowing in a
AMPLIFIER: A device used in an electrical circuit to
increase the voltage of an output signal.
AMP/HR. RATING (BATTERY): Measurement of the ability of a battery to
deliver a stated amount of current for a stated period of time. The higher the
amp/hr. rating, the better the battery.
AMPERE: The rate of flow of electrical current present when
one volt of electrical pressure is applied against one ohm of electrical
ANALOG COMPUTER: Any microprocessor that uses similar
(analogous) electrical signals to make its calculations.
ANODIZED: A special coating applied to the surface of
aluminum valves for extended service life.
ANTIFREEZE: A substance (ethylene or propylene glycol)
added to the coolant to prevent freezing in cold weather.
ANTI-FOAM AGENTS: Minimize fluid foaming from the whipping
action encountered in the converter and planetary action.
ANTI-WEAR AGENTS: Zinc agents that control wear on the
gears, bushings, and thrust washers.
ANTI-LOCK BRAKING SYSTEM: A supplementary system to the base
hydraulic system that prevents sustained lock-up of the wheels during braking
as well as automatically controlling wheel slip.
ANTI-ROLL BAR: See stabilizer bar.
ARC: A flow of electricity through the air between two
electrodes or contact points that produces a spark.
ARMATURE: A laminated, soft iron core wrapped by a wire that
converts electrical energy to mechanical energy as in a motor or relay. When
rotated in a magnetic field, it changes mechanical energy into electrical
energy as in a generator.
ATDC: After Top Dead Center.
ATF: Automatic transmission fluid.
ATMOSPHERIC PRESSURE: The pressure on the Earth's surface
caused by the weight of the air in the atmosphere. At sea level, this pressure
is 14.7 psi at 32<deg.>F (101 kPa at 0<deg.>C).
ATOMIZATION: The breaking down of a liquid into a fine mist
that can be suspended in air.
AUXILIARY ADD-ON COOLER: A supplemental transmission fluid
cooling device that is installed in series with the heat exchanger (cooler),
located inside the radiator, to provide additional support to cool the hot
fluid leaving the torque converter.
AUXILIARY PRESSURE: An added fluid pressure that is
introduced into a regulator or balanced valve system to control valve movement.
The auxiliary pressure itself can be either a fixed or a variable value. (See
balanced valve; regulator valve.)
AWD: All wheel drive.
AXIAL FORCE: A side or end thrust force acting in or along
the same plane as the power flow.
AXIAL PLAY: Movement parallel to a shaft or bearing bore.
AXLE CAPACITY: The maximum load-carrying capacity of the
axle itself, as specified by the manufacturer. This is usually a higher number
than the GAWR.
AXLE RATIO: This is a number (3.07:1, 4.56:1, for example)
expressing the ratio between driveshaft revolutions and wheel revolutions. A
low numerical ratio allows the engine to work easier because it doesn't have
to turn as fast. A high numerical ratio means that the engine has to turn more
rpm's to move the wheels through the same number of turns.
BACKFIRE: The sudden combustion of gases in the intake or
exhaust system that results in a loud explosion.
BACKLASH: The clearance or play between two parts, such as
BACKPRESSURE: Restrictions in the exhaust system that slow
the exit of exhaust gases from the combustion chamber.
BAKELITE: A heat resistant, plastic insulator material commonly
used in printed circuit boards and transistorized components.
BALANCED VALVE: A valve that is positioned by opposing
auxiliary hydraulic pressures and/or spring force. Examples include mainline
regulator, throttle, and governor valves. (See regulator valve.)
flexible ring of steel with an inner lining of friction material. When
tightened around the outside of a drum, a planetary member is held stationary
to the transmission/transaxle case.
BALL BEARING: A bearing made up of hardened inner and outer
races between which hardened steel balls roll.
BALL JOINT: A ball and matching socket connecting suspension
components (steering knuckle to lower control arms). It permits rotating
movement in any direction between the components that are joined.
BARO (BAROMETRIC PRESSURE SENSOR): Measures the change in
the intake manifold pressure caused by changes in altitude.
BAROMETRIC MANIFOLD ABSOLUTE PRESSURE (BMAP) SENSOR:
Operates similarly to a conventional MAP sensor; reads intake manifold pressure
and is also responsible for determining altitude and barometric pressure prior
to engine operation.
BAROMETRIC PRESSURE: (See atmospheric pressure.)
BALLAST RESISTOR: A resistor in the primary ignition circuit
that lowers voltage after the engine is started to reduce wear on ignition
BATTERY: A direct current
electrical storage unit, consisting of the basic active materials of lead and
sulfuric acid, which converts chemical energy into electrical energy. Used to
provide current for the operation of the starter as well as other equipment,
such as the radio, lighting, etc.
BEAD: The portion of a tire that holds it on the rim.
BEARING: A friction reducing, supportive device usually located
between a stationary part and a moving part.
BEFORE TOP DEAD CENTER (BTDC): The point just before the
piston reaches the top of its travel on the compression stroke.
BELTED TIRE: Tire construction similar to bias-ply tires,
but using two or more layers of reinforced belts between body plies and the
BEZEL: Piece of metal surrounding radio, headlights, gauges
or similar components; sometimes used to hold the glass face of a gauge in the
BIAS-PLY TIRE: Tire construction, using body ply reinforcing
cords which run at alternating angles to the center line of the tread.
BI-METAL TEMPERATURE SENSOR: Any sensor or switch made of
two dissimilar types of metal that bend when heated or cooled due to the
different expansion rates of the alloys. These types of sensors usually
function as an on/off switch.
BLOCK: See Engine Block.
BLOW-BY: Combustion gases, composed of water vapor and
unburned fuel, that leak past the piston rings into the crankcase during
normal engine operation. These gases are removed by the PCV system to prevent
the buildup of harmful acids in the crankcase.
BOOK TIME: See Labor Time.
BOOK VALUE: The average value of a car, widely used to determine
trade-in and resale value.
BOOST VALVE: Used at the base of the regulator valve to
increase mainline pressure.
BORE: Diameter of a cylinder.
BRAKE CALIPER: The housing that fits over the brake disc.
The caliper holds the brake pads, which are pressed against the discs by the
caliper pistons when the brake pedal is depressed.
BRAKE HORSEPOWER(BHP): The actual horsepower available at
the engine flywheel as measured by a dynamometer.
BRAKE FADE: Loss of braking power, usually caused by excessive
heat after repeated brake applications.
BRAKE HORSEPOWER: Usable horsepower of an engine measured
at the crankshaft.
BRAKE PAD: A brake shoe and lining assembly used with disc
BRAKE PROPORTIONING VALVE: A valve on the master cylinder
which restricts hydraulic brake pressure to the wheels to a specified amount,
preventing wheel lock-up.
BREAKAWAY: Often used by Chrysler to identify first-gear
operation in D and 2 ranges. In these ranges, first-gear operation depends on a
one-way roller clutch that holds on acceleration and releases (breaks away) on
deceleration, resulting in a freewheeling coast-down condition.
BRAKE SHOE: The backing for the brake lining. The term is,
however, usually applied to the assembly of the brake backing and lining.
BREAKER POINTS: A set of points inside the distributor,
operated by a cam, which make and break the ignition circuit.
BRINNELLING: A wear pattern identified by a series of
indentations at regular intervals. This condition is caused by a lack of lube,
overload situations, and/or vibrations.
BTDC: Before Top Dead Center.
BUMP: Sudden and forceful apply of a clutch or band.
BUSHING: A liner, usually removable, for a bearing; an
anti-friction liner used in place of a bearing.
CALIFORNIA ENGINE: An
engine certified by the EPA for use in California
only; conforms to more stringent emission regulations than Federal engine.
CALIPER: A hydraulically activated device in a disc brake
system, which is mounted straddling the brake rotor (disc). The caliper
contains at least one piston and two brake pads. Hydraulic pressure on the
piston(s) forces the pads against the rotor.
CAPACITY: The quantity of electricity that can be delivered
from a unit, as from a battery in ampere-hours, or output, as from a generator.
CAMBER: One of the factors of wheel alignment. Viewed from
the front of the car, it is the inward or outward tilt of the wheel. The top
of the tire will lean outward (positive camber) or inward (negative camber).
CAMSHAFT: A shaft in the engine on which are the lobes
(cams) which operate the valves. The camshaft is driven by the crankshaft, via
a belt, chain or gears, at one half the crankshaft speed.
CANCER: Rust on a car body.
CAPACITOR: A device which stores an electrical charge.
CARBON MONOXIDE (CO): A colorless, odorless gas given off as
a normal byproduct of combustion. It is poisonous and extremely dangerous in
confined areas, building up slowly to toxic levels without warning if adequate
ventilation is not available.
CARBURETOR: A device, usually mounted on the intake manifold
of an engine, which mixes the air and fuel in the proper proportion to allow
CASTER: The forward or rearward tilt of an imaginary line
drawn through the upper ball joint and the center of the wheel. Viewed from the
sides, positive caster (forward tilt) lends directional stability, while
negative caster (rearward tilt) produces instability.
CATALYTIC CONVERTER: A device installed in the exhaust
system, like a muffler, that converts harmful byproducts of combustion into
carbon dioxide and water vapor by means of a heat-producing chemical reaction.
CENTRIFUGAL ADVANCE: A mechanical method of advancing the
spark timing by using flyweights in the distributor that react to centrifugal
force generated by the distributor shaft rotation.
CENTRIFUGAL FORCE: The outward pull of a revolving object,
away from the center of revolution. Centrifugal force increases with the speed
CETANE RATING: A measure of the ignition value of diesel
fuel. The higher the cetane rating, the better the fuel. Diesel fuel cetane
rating is roughly comparable to gasoline octane rating.
CHECK VALVE: Any one-way valve installed to permit the flow
of air, fuel or vacuum in one direction only.
CHOKE: The valve/plate that restricts the amount of air
entering an engine on the induction stroke, thereby enriching the air/fuel
CHUGGLE: Bucking or jerking condition that may be engine
related and may be most noticeable when converter clutch is engaged; similar to
the feel of towing a trailer.
CIRCLIP: A split steel snapring that fits into a groove to
hold various parts in place.
CIRCUIT BREAKER: A switch which protects an electrical
circuit from overload by opening the circuit when the current flow exceeds a
pre-determined level. Some circuit breakers must be reset manually, while most
CIRCUIT: Any unbroken path through which an electrical current
can flow. Also used to describe fuel flow in some instances.
CIRCUIT, BYPASS: Another circuit in parallel with the major
circuit through which power is diverted.
CIRCUIT, CLOSED: An electrical circuit in which there is no
interruption of current flow.
CIRCUIT, GROUND: The non-insulated portion of a complete circuit
used as a common potential point. In automotive circuits, the ground is
composed of metal parts, such as the engine, body sheet metal, and frame and is
usually a negative potential.
CIRCUIT, HOT: That portion of a circuit not at ground
potential. The hot circuit is usually insulated and is connected to the
positive side of the battery.
CIRCUIT, OPEN: A break or lack of contact in an electrical
circuit, either intentional (switch) or unintentional (bad connection or broken
CIRCUIT, PARALLEL: A circuit having two or more paths for
current flow with common positive and negative tie points. The same voltage is
applied to each load device or parallel branch.
CIRCUIT, SERIES: An electrical system in which separate
parts are connected end to end, using one wire, to form a single path for
current to flow.
CIRCUIT, SHORT: A circuit that is accidentally completed in
an electrical path for which it was not intended.
CLAMPING (ISOLATION) DIODES: Diodes positioned in a circuit
to prevent self-induction from damaging electronic components.
CLEARCOAT: A transparent layer which, when sprayed over a
vehicle's paint job, adds gloss and depth as well as an additional protective
coating to the finish.
CLUTCH: Part of the power train used to connect/disconnect
power to the rear wheels.
CLUTCH, FLUID: The same as a fluid coupling. A fluid clutch
or coupling performs the same function as a friction clutch by utilizing fluid
friction and inertia as opposed to solid friction used by a friction clutch.
(See fluid coupling.)
CLUTCH, FRICTION: A coupling device that provides a means of
smooth and positive engagement and disengagement of engine torque to the
vehicle powertrain. Transmission of power through the clutch is accomplished by
bringing one or more rotating drive members into contact with complementing
COAST: Vehicle deceleration caused by engine braking
COEFFICIENT OF FRICTION: The amount of surface tension
between two contacting surfaces; identified by a scientifically calculated
COIL: Part of the ignition system that boosts the relatively
low voltage supplied by the car's electrical system to the high voltage
required to fire the spark plugs.
COMBINATION MANIFOLD: An assembly which includes both the
intake and exhaust manifolds in one casting.
COMBINATION VALVE: A device used in some fuel systems that
routes fuel vapors to a charcoal storage canister instead of venting them into
the atmosphere. The valve relieves fuel tank pressure and allows fresh air into
the tank as the fuel level drops to prevent a vapor lock situation.
COMBUSTION CHAMBER: The part of the engine in the cylinder
head where combustion takes place.
COMPOUND GEAR: A gear consisting of two or more simple gears
with a common shaft.
COMPOUND PLANETARY: A gearset that has more than the three
elements found in a simple gearset and is constructed by combining members of
two planetary gearsets to create additional gear ratio possibilities.
COMPRESSION CHECK: A test involving removing each spark
plug and inserting a gauge. When the engine is cranked, the gauge will record a
pressure reading in the individual cylinder. General operating condition can
be determined from a compression check.
COMPRESSION RATIO: The ratio of the volume between the
piston and cylinder head when the piston is at the bottom of its stroke (bottom
dead center) and when the piston is at the top of its stroke (top dead center).
COMPUTER: An electronic control module that correlates input
data according to prearranged engineered instructions; used for the management
of an actuator system or systems.
CONDENSER: 1. An electrical device which acts to store an
electrical charge, preventing voltage surges. 2. A radiator-like device in the
air conditioning system in which refrigerant gas condenses into a liquid,
giving off heat.
CONDUCTOR: Any material through which an electrical current
can be transmitted easily.
CONNECTING ROD: The connecting link between the crankshaft
CONSTANT VELOCITY JOINT: Type of universal joint in a
halfshaft assembly in which the output shaft turns at a constant angular
velocity without variation, provided that the speed of the input shaft is
CONTINUITY: Continuous or complete circuit. Can be checked
with an ohmmeter.
CONTROL ARM: The upper or lower suspension components which
are mounted on the frame and support the ball joints and steering knuckles.
CONVENTIONAL IGNITION: Ignition system which uses breaker
CONVERTER: (See torque converter.)
CONVERTER LOCKUP: The switching from hydrodynamic to direct
mechanical drive, usually through the application of a friction element called
the converter clutch.
COOLANT: Mixture of water and anti-freeze circulated through
the engine to carry off heat produced by the engine.
CORROSION INHIBITOR: An inhibitor in ATF that prevents
corrosion of bushings, thrust washers, and oil cooler brazed joints.
COUNTERSHAFT: An intermediate shaft which is rotated by a
mainshaft and transmits, in turn, that rotation to a working part.
COUPLING PHASE: Occurs when the torque converter is
operating at its greatest hydraulic efficiency. The speed differential between
the impeller and the turbine is at its minimum. At this point, the stator
freewheels, and there is no torque multiplication.
CRANKCASE: The lower part of an engine in which the
crankshaft and related parts operate.
CRANKSHAFT: Engine component (connected to pistons by connecting
rods) which converts the reciprocating (up and down) motion of pistons to rotary
motion used to turn the driveshaft.
CURB WEIGHT: The weight of a vehicle without passengers or
payload, but including all fluids (oil, gas, coolant, etc.) and other equipment
specified as standard.
CURRENT: The flow (or rate) of electrons moving through a
circuit. Current is measured in amperes (amp).
CURRENT FLOW CONVENTIONAL: Current flows through a circuit
from the positive terminal of the source to the negative terminal (plus to
CURRENT FLOW, ELECTRON: Current or electrons flow from the
negative terminal of the source, through the circuit, to the positive terminal
(minus to plus).
CV-JOINT: Constant velocity joint.
CYCLIC VIBRATIONS: The off-center movement of a rotating
object that is affected by its initial balance, speed of rotation, and working
CYLINDER BLOCK: See engine block.
CYLINDER HEAD: The detachable portion of the engine, usually
fastened to the top of the cylinder block and containing all or most of the
combustion chambers. On overhead valve engines, it contains the valves and
their operating parts. On overhead cam engines, it contains the camshaft as
CYLINDER: In an engine, the round hole in the engine block
in which the piston(s) ride.
DATA LINK CONNECTOR
(DLC): Current acronym/term applied to the federally mandated,
diagnostic junction connector that is used to monitor ECM/PC/TCM inputs,
processing strategies, and outputs including diagnostic trouble codes (DTCs).
DEAD CENTER: The extreme top or bottom of the piston stroke.
DECELERATION BUMP: When referring to a torque converter clutch
in the applied position, a sudden release of the accelerator pedal causes a
forceful reversal of power through the drivetrain (engine braking), just prior
to the apply plate actually being released.
DELAYED (LATE OR EXTENDED): Condition where shift is expected
but does not occur for a period of time, for example, where clutch or band
engagement does not occur as quickly as expected during part throttle or wide
open throttle apply of accelerator or when manually downshifting to a lower
DETENT: A spring-loaded plunger, pin, ball, or pawl used as
a holding device on a ratchet wheel or shaft. In automatic transmissions, a
detent mechanism is used for locking the manual valve in place.
DETENT DOWNSHIFT: (See kickdown.)
DETERGENT: An additive in engine oil to improve its
DETONATION: An unwanted explosion of the air/fuel mixture in
the combustion chamber caused by excess heat and compression, advanced timing,
or an overly lean mixture. Also referred to as "ping".
DEXRON<reg.>: A brand of automatic transmission fluid.
DIAGNOSTIC TROUBLE CODES (DTCs): A digital display from the
control module memory that identifies the input, processor, or output device
circuit that is related to the powertrain emission/driveability malfunction
detected. Diagnostic trouble codes can be read by the MIL to flash any codes or
by using a handheld scanner.
DIAPHRAGM: A thin, flexible wall separating two cavities,
such as in a vacuum advance unit.
DIESELING: The engine continues to run after the car is shut
off; caused by fuel continuing to be burned in the combustion chamber.
DIFFERENTIAL: A geared assembly which allows the
transmission of motion between drive axles, giving one axle the ability to
rotate faster than the other, as in cornering.
DIFFERENTIAL AREAS: When opposing faces of a spool valve are
acted upon by the same pressure but their areas differ in size, the face with
the larger area produces the differential force and valve movement. (See spool
DIFFERENTIAL FORCE: (See differential areas.) digital
readout: A display of numbers or a combination of numbers and letters.
DIGITAL VOLT OHMMETER: An electronic diagnostic tool used to
measure voltage, ohms and amps as well as several other functions, with the
readings displayed on a digital screen in tenths, hundredths and thousandths.
DIODE: An electrical device that will allow current to flow
in one direction only.
DIRECT CURRENT (DC): Electrical current that flows in one
DIRECT DRIVE: The gear ratio is 1:1, with no change
occurring in the torque and speed input/output relationship.
DISC BRAKE: A hydraulic braking assembly consisting of a
brake disc, or rotor, mounted on an axle shaft, and a caliper assembly
containing, usually two brake pads which are activated by hydraulic pressure.
The pads are forced against the sides of the disc, creating friction which
slows the vehicle.
DISPERSANTS: Suspend dirt and prevent sludge buildup. double
bump (double feel): Two sudden and forceful applies of a clutch or band.
DISPLACEMENT: The total volume of air that is displaced by
all pistons as the engine turns through one complete revolution.
DISTRIBUTOR: A mechanically driven device on an engine which
is responsible for electrically firing the spark plug at a pre-determined point
of the piston stroke.
DOHC: Double overhead camshaft.
DOUBLE OVERHEAD CAMSHAFT: The engine utilizes two camshafts
mounted in one cylinder head. One camshaft operates the exhaust valves, while
the other operates the intake valves.
DOWEL PIN: A pin, inserted in mating holes in two different
parts allowing those parts to maintain a fixed relationship.
DRIVELINE: The drive connection between the transmission and
the drive wheels.
DRIVE TRAIN: The components that transmit the flow of power
from the engine to the wheels. The components include the clutch, transmission,
driveshafts (or axle shafts in front wheel drive), U-joints and differential.
DRUM BRAKE: A braking system which consists of two brake
shoes and one or two wheel cylinders, mounted on a fixed backing plate, and a
brake drum, mounted on an axle, which revolves around the assembly.
DRY CHARGED BATTERY: Battery
to which electrolyte is added when the battery is placed in service.
DVOM: Digital volt ohmmeter
DWELL: The rate, measured in degrees of shaft rotation, at
which an electrical circuit cycles on and off.
DYNAMIC: A sealing application in which there is rotating or
reciprocating motion between the parts.
EARLY: Condition where shift occurs before vehicle has
reached proper speed, which tends to labor engine after upshift.
EBCM: See Electronic Control Unit (ECU).
ECM: See Electronic Control Unit (ECU).
ECU: Electronic control unit.
ELECTRODE: Conductor (positive or negative) of electric
ELECTROLYSIS: A surface etching or bonding of current
conducting transmission/transaxle components that may occur when grounding
straps are missing or in poor condition.
ELECTROLYTE: A solution of water and sulfuric acid used to
activate the battery. Electrolyte is extremely corrosive.
ELECTROMAGNET: A coil that produces a magnetic field when
current flows through its windings.
ELECTROMAGNETIC INDUCTION: A method to create (generate)
current flow through the use of magnetism.
ELECTROMAGNETISM: The effects surrounding the relationship
between electricity and magnetism.
ELECTROMOTIVE FORCE (EMF): The force or pressure (voltage)
that causes current movement in an electrical circuit.
ELECTRONIC CONTROL UNIT: A digital computer that controls
engine (and sometimes transmission, brake or other vehicle system) functions
based on data received from various sensors. Examples used by some
manufacturers include Electronic Brake Control Module (EBCM), Engine Control
Module (ECM), Powertrain Control Module (PCM) or Vehicle Control Module (VCM).
ELECTRONIC IGNITION: A system in which the timing and firing
of the spark plugs is controlled by an electronic control unit, usually called
a module. These systems have no points or condenser.
ELECTRONIC PRESSURE CONTROL (EPC) SOLENOID: A specially designed
solenoid containing a spool valve and spring assembly to control fluid mainline
pressure. A variable current flow, controlled by the ECM/PCM, varies the
internal force of the solenoid on the spool valve and resulting mainline
pressure. (See variable force solenoid.)
ELECTRONICS: Miniaturized electrical circuits utilizing
semiconductors, solid-state devices, and printed circuits. Electronic circuits
utilize small amounts of power.
ELECTRONIFICATION: The application of electronic circuitry
to a mechanical device. Regarding automatic transmissions, electrification is
incorporated into converter clutch lockup, shift scheduling, and line pressure
ELECTROSTATIC DISCHARGE (ESD): An unwanted, high-voltage
electrical current released by an individual who has taken on a static charge
of electricity. Electronic components can be easily damaged by ESD.
ELEMENT: A device within a hydrodynamic drive unit designed
with a set of blades to direct fluid flow.
ENAMEL: Type of paint that dries to a smooth, glossy finish.
END BUMP (END FEEL OR SLIP BUMP):Firmer feel at end of shift
when compared with feel at start of shift.
END-PLAY: The clearance/gap between two components that
allows for expansion of the parts as they warm up, to prevent binding and to
allow space for lubrication.
ENERGY: The ability or capacity to do work.
ENGINE: The primary motor or power apparatus of a vehicle,
which converts liquid or gas fuel into mechanical energy.
ENGINE BLOCK: The basic engine casting containing the cylinders,
the crankshaft main bearings, as well as machined surfaces for the mounting of
other components such as the cylinder head, oil pan, transmission, etc..
ENGINE BRAKING: Use of engine to slow vehicle by manually
downshifting during zero-throttle coast down.
ENGINE CONTROL MODULE (ECM): Manages the engine and
incorporates output control over the torque converter clutch solenoid. (Note:
Current designation for the ECM in late model vehicles is PCM.)
ENGINE COOLANT TEMPERATURE (ECT) SENSOR: Prevents converter
clutch engagement with a cold engine; also used for shift timing and shift
EP LUBRICANT: EP (extreme pressure) lubricants are specially
formulated for use with gears involving heavy loads (transmissions,
ETHYL: A substance added to gasoline to improve its
resistance to knock, by slowing down the rate of combustion.
ETHYLENE GLYCOL: The base substance of antifreeze.
EXHAUST MANIFOLD: A set of cast passages or pipes which
conduct exhaust gases from the engine.
FAIL-SAFE (BACKUP) CONTROL: A substitute value used by the
PCM/TCM to replace a faulty signal from an input sensor. The temporary value
allows the vehicle to continue to be operated.
FAST IDLE: The speed of the engine when the choke is on.
Fast idle speeds engine warm-up.
FEDERAL ENGINE: An engine certified by the EPA for use in
any of the 49 states (except California).
FEEDBACK: A circuit malfunction whereby current can find
another path to feed load devices.
FEELER GAUGE: A blade, usually metal, of precisely predetermined
thickness, used to measure the clearance between two parts.
FILAMENT: The part of a bulb that glows; the filament
creates high resistance to current flow and actually glows from the resulting
FINAL DRIVE: An essential part of the axle drive assembly
where final gear reduction takes place in the powertrain. In RWD applications
and north-south FWD applications, it must also change the power flow direction
to the axle shaft by ninety degrees. (Also see axle ratio).
FIRING ORDER: The order in which combustion occurs in the
cylinders of an engine. Also the order in which spark is distributed to the
plugs by the distributor.
FIRM: A noticeable quick apply of a clutch or band that is
considered normal with medium to heavy throttle shift; should not be confused
with harsh or rough.
FLAME FRONT: The term used to describe certain aspects of
the fuel explosion in the cylinders. The flame front should move in a
controlled pattern across the cylinder, rather than simply exploding immediately.
FLARE (SLIPPING): A quick increase in engine rpm accompanied
by momentary loss of torque; generally occurs during shift.
FLAT ENGINE: Engine design in which the pistons are horizontally
opposed. Porsche, Subaru and some old VW are common examples of flat engines.
FLAT RATE: A dealership term referring to the amount of
money paid to a technician for a repair or diagnostic service based on that
particular service versus dealership's labor time (NOT based on the actual time
the technician spent on the job).
FLAT SPOT: A point during acceleration when the engine
seems to lose power for an instant.
FLOODING: The presence of too much fuel in the intake
manifold and combustion chamber which prevents the air/fuel mixture from
firing, thereby causing a no-start situation.
FLUID: A fluid can be either liquid or gas. In hydraulics, a
liquid is used for transmitting force or motion.
FLUID COUPLING: The simplest form of hydrodynamic drive, the
fluid coupling consists of two look-alike members with straight radial varies
referred to as the impeller (pump) and the turbine. input torque is always
equal to the output torque.
FLUID DRIVE: Either a fluid coupling or a fluid torque
converter. (See hydrodynamic drive units.)
FLUID TORQUE CONVERTER: A hydrodynamic drive that has the
ability to act both as a torque multiplier and fluid coupling. (See
hydrodynamic drive units; torque converter.)
FLUID VISCOSITY: The resistance of a liquid to flow. A cold
fluid (oil) has greater viscosity and flows more slowly than a hot fluid (oil).
FLYWHEEL: A heavy disc of metal attached to the rear of the
crankshaft. It smoothes the firing impulses of the engine and keeps the
crankshaft turning during periods when no firing takes place. The starter also
engages the flywheel to start the engine.
FOOT POUND (ft. lbs. or sometimes, ft. lb.): The amount of
energy or work needed to raise an item weighing one pound, a distance of one
FREEZE PLUG: A plug in the engine block which will be pushed
out if the coolant freezes. Sometimes called expansion plugs, they protect the
block from cracking should the coolant freeze.
FRICTION: The resistance that occurs between contacting
surfaces. This relationship is expressed by a ratio called the coefficient of
FRICTION, COEFFICIENT OF: The amount of surface tension
between two contacting surfaces; expressed by a scientifically calculated
FRONT END ALIGNMENT: A service to set caster, camber and
toe-in to the correct specifications. This will ensure that the car steers and
handles properly and that the tires wear properly.
FRICTION MODIFIER: Changes the coefficient of friction of
the fluid between the mating steel and composition clutch/band surfaces during
the engagement process and allows for a certain amount of intentional slipping
for a good "shift-feel." full throttle detent downshift: A quick
apply of accelerator pedal to its full travel, forcing a downshift.
FRONTAL AREA: The total frontal area of a vehicle exposed to
FUEL FILTER: A component of the fuel system containing a
porous paper element used to prevent any impurities from entering the engine
through the fuel system. It usually takes the form of a canister-like housing,
mounted in-line with the fuel hose, located anywhere on a vehicle between the
fuel tank and engine.
FUEL INJECTION: A system replacing the carburetor that
sprays fuel into the cylinder through nozzles. The amount of fuel can be more
precisely controlled with fuel injection.
FULL FLOATING AXLE: An axle in which the axle housing extends
through the wheel giving bearing support on the outside of the housing. The
front axle of a four-wheel drive vehicle is usually a full floating axle, as
are the rear axles of many larger (_ ton and over) pick-ups and vans.
FULL-TIME FOUR-WHEEL DRIVE: A four-wheel drive system that
continuously delivers power to all four wheels. A differential between the
front and rear driveshafts permits variations in axle speeds to control gear
wind-up without damage.
FUSE: A protective device in a circuit which prevents
circuit overload by breaking the circuit when a specific amperage is present.
The device is constructed around a strip or wire of a lower amperage rating
than the circuit it is designed to protect. When an amperage higher than that
stamped on the fuse is present in the circuit, the strip or wire melts, opening
FUSIBLE LINK: A piece of wire in a wiring harness that performs
the same job as a fuse. If overloaded, the fusible link will melt and interrupt
FWD: Front wheel drive.
GAWR: (Gross axle weight rating) the total maximum weight
an axle is designed to carry.
GCW: (Gross combined weight) total combined weight of a tow
vehicle and trailer.
GARAGE SHIFT: initial engagement feel of transmission,
neutral to reverse or neutral to a forward drive.
GARAGE SHIFT FEEL: A quick check of the engagement quality
and responsiveness of reverse and forward gears. This test is done with the
GEAR: A toothed mechanical device that acts as a rotating
lever to transmit power or turning effort from one shaft to another. (See gear
GEAR RATIO: A ratio expressing the number of turns a smaller
gear will make to turn a larger gear through one revolution. The ratio is found
by dividing the number of teeth on the smaller gear into the number of teeth on
the larger gear.
GEAR REDUCTION: Torque is multiplied and speed decreased by
the factor of the gear ratio. For example, a 3:1 gear ratio changes an input
torque of 180 ft. lbs. and an input speed of 2700 rpm to 540 Ft. lbs. and 900
rpm, respectively. (No account is taken of frictional losses, which are always
GEARTRAIN: A succession of intermeshing gears that form an
assembly and provide for one or more torque changes as the power input is
transmitted to the power output.
GEL COAT: A thin coat of plastic resin covering fiberglass
GENERATOR: A device which produces direct current (DC) necessary
to charge the battery.
GOVERNOR: A device that senses vehicle speed and generates a
hydraulic oil pressure. As vehicle speed increases, governor oil pressure
GROUND CIRCUIT: (See circuit, ground.)
GROUND SIDE SWITCHING: The electrical/electronic circuit
control switch is located after the circuit load.
GVWR: (Gross vehicle weight rating) total maximum weight a
vehicle is designed to carry including the weight of the vehicle, passengers,
equipment, gas, oil, etc.
HALOGEN: A special type of lamp known for its quality of
brilliant white light. Originally used for fog lights and driving lights.
HARD CODES: DTCs that are present at the time of testing;
also called continuous or current codes.
HARSH(ROUGH): An apply of a clutch or band that is more
noticeable than a firm one; considered undesirable at any throttle position.
HEADER TANK: An expansion tank for the radiator coolant. It
can be located remotely or built into the radiator.
RANGE: A term used to describe
the ability of a spark plug to carry away heat. Plugs with longer nosed
insulators take longer to carry heat off effectively.
HEAT RISER: A flapper in the exhaust manifold that is
closed when the engine is cold, causing hot exhaust gases to heat the intake
manifold providing better cold engine operation. A thermostatic spring opens
the flapper when the engine warms up.
HEAVY THROTTLE: Approximately three-fourths of accelerator
HEMI: A name given an engine using hemispherical combustion
HERTZ (HZ): The international unit of frequency equal to one
cycle per second (10,000 Hertz equals 10,000 cycles per second).
HIGH-IMPEDANCE DVOM (DIGITAL VOLT-OHMMETER): This styled
device provides a built-in resistance value and is capable of limiting circuit
current flow to safe milliamp levels.
HIGH RESISTANCE: Often refers to a circuit where there is an
excessive amount of opposition to normal current flow.
HORSEPOWER: A measurement of the amount of work; one
horsepower is the amount of work necessary to lift 33,000 lbs. one foot in one
minute. Brake horsepower (bhp) is the horsepower delivered by an engine on a
dynamometer. Net horsepower is the power remaining (measured at the flywheel
of the engine) that can be used to turn the wheels after power is consumed
through friction and running the engine accessories (water pump, alternator,
air pump, fan etc.)
HOT CIRCUIT: (See circuit, hot; hot lead.) hot lead: A wire
or conductor in the power side of the circuit. (See circuit, hot.)
HOT SIDE SWITCHING: The electrical/electronic circuit
control switch is located before the circuit load.
HUB: The center part of a wheel or gear.
HUNTING (BUSYNESS): Repeating quick series of up-shifts and
downshifts that causes noticeable change in engine rpm, for example, as in a
4-3-4 shift pattern.
HYDRAULICS: The use of liquid under pressure to transfer
force of motion.
HYDROCARBON (HC): Any chemical compound made up of hydrogen
and carbon. A major pollutant formed by the engine as a by-product of
HYDRODYNAMIC DRIVE UNITS: Devices that transmit power solely
by the action of a kinetic fluid flow in a closed recirculating path. An
impeller energizes the fluid and discharges the high-speed jet stream into the
turbine for power output.
HYDROMETER: An instrument used to measure the specific
gravity of a solution.
HYDROPLANING: A phenomenon of driving when water builds up
under the tire tread, causing it to lose contact with the road. Slowing down
will usually restore normal tire contact with the road.
HYPOID GEARSET: The drive pinion gear may be placed below or
above the centerline of the driven gear; often used as a final drive gearset.
IDLE MIXTURE: The mixture of air and fuel (usually about
14:1) being fed to the cylinders. The idle mixture screw(s) are sometimes
adjusted as part of a tune-up.
IDLER ARM: Component of the steering linkage which is a
geometric duplicate of the steering gear arm. It supports the right side of the
center steering link.
IMPELLER: Often called a pump, the impeller is the power
input (drive) member of a hydrodynamic drive. As part of the torque converter
cover, it acts as a centrifugal pump and puts the fluid in motion.
INCH POUND (inch lbs.; sometimes in. lb. or in. lbs.): One
twelfth of a foot pound.
INDUCTANCE: The force that produces voltage when a conductor
is passed through a magnetic field.
INDUCTION: A means of transferring electrical energy in the
form of a magnetic field. Principle used in the ignition coil to increase
INITIAL FEEL: A distinct firmer feel at start of shift when
compared with feel at finish of shift.
INJECTOR: A device which receives metered fuel under relatively
low pressure and is activated to inject the fuel into the engine under
relatively high pressure at a predetermined time.
INPUT: In an automatic transmission, the source of power
from the engine is absorbed by the torque converter, which provides the power
input into the transmission. The turbine drives the input(turbine)shaft.
INPUT SHAFT: The shaft to which torque is applied, usually
carrying the driving gear or gears.
INTAKE MANIFOLD: A casting of passages or pipes used to
conduct air or a fuel/air mixture to the cylinders.
INTERNAL GEAR: The ring-like outer gear of a planetary
gearset with the gear teeth cut on the inside of the ring to provide a mesh
with the planet pinions.
ISOLATION (CLAMPING) DIODES: Diodes positioned in a circuit
to prevent self-induction from damaging electronic components.
IX ROTARY GEAR PUMP: Contains two rotating members, one
shaped with internal gear teeth and the other with external gear teeth. As the
gears separate, the fluid fills the gaps between gear teeth, is pulled across a
crescent-shaped divider, and then is forced to flow through the outlet as the
IX ROTARY LOBE PUMP: Sometimes referred to as a gerotor type
pump. Two rotating members, one shaped with internal lobes and the other with
external lobes, separate and then mesh to cause fluid to flow.
JOURNAL: The bearing surface within which a shaft operates.
JUMPER CABLES: Two heavy duty wires with large alligator
clips used to provide power from a charged battery to a discharged battery
mounted in a vehicle.
JUMPSTART: Utilizing the sufficiently charged battery of one
vehicle to start the engine of another vehicle with a discharged battery by the
use of jumper cables.
KEY: A small block usually fitted in a notch between a shaft
and a hub to prevent slippage of the two parts.
KICKDOWN: Detent downshift system; either linkage, cable, or
KILO: A prefix used in the metric system to indicate one
KNOCK: Noise which results from the spontaneous ignition of
a portion of the air-fuel mixture in the engine cylinder caused by overly
advanced ignition timing or use of incorrectly low octane fuel for that engine.
KNOCK SENSOR: An input device that responds to spark knock,
caused by over advanced ignition timing.
LABOR TIME: A specific amount of time required to perform a
certain repair or diagnostic service as defined by a vehicle or after-market
LACQUER: A quick-drying automotive paint.
LATE: Shift that occurs when engine is at higher than normal
rpm for given amount of throttle.
LIGHT-EMITTING DIODE (LED): A semiconductor diode that emits
light as electrical current flows through it; used in some electronic display
devices to emit a red or other color light.
LIGHT THROTTLE: Approximately one-fourth of accelerator
LIMITED SLIP: A type of differential which transfers
driving force to the wheel with the best traction.
LIMP-IN MODE: Electrical shutdown of the transmission/
transaxle output solenoids, allowing only forward and reverse gears that are
hydraulically energized by the manual valve. This permits the vehicle to be
driven to a service facility for repair.
LIP SEAL: Molded synthetic rubber seal designed with an
outer sealing edge (lip) that points into the fluid containing area to be
sealed. This type of seal is used where rotational and axial forces are
LITHIUM-BASE GREASE: Chassis and wheel bearing grease using
lithium as a base. Not compatible with sodium-base grease.
LOAD DEVICE: A circuit's resistance that converts the
electrical energy into light, sound, heat, or mechanical movement.
RANGE: Indicates the
number of plies at which a tire is rated. Load range B equals four-ply rating;
C equals six-ply rating; and, D equals an eight-ply rating.
LOAD TORQUE: The amount of output torque needed from the
transmission/transaxle to overcome the vehicle load.
LOCKING HUBS: Accessories used on part-time four-wheel drive
systems that allow the front wheels to be disengaged from the drive train when
four-wheel drive is not being used. When four-wheel drive is desired, the hubs
are engaged, locking the wheels to the drive train.
LOCKUP CONVERTER: A torque converter that operates
hydraulically and mechanically. When an internal apply plate (lockup plate)
clamps to the torque converter cover, hydraulic slippage is eliminated.
LOCK RING: See Circlip or Snapring
MAGNET: Any body with the property of attracting iron or
MAGNETIC FIELD: The area surrounding the poles of a magnet
that is affected by its attraction or repulsion forces.
MAIN LINE PRESSURE: Often called control pressure or line
pressure, it refers to the pressure of the oil leaving the pump and is
controlled by the pressure regulator valve.
MALFUNCTION INDICATOR LAMP (MIL): Previously known as a
check engine light, the dash-mounted MIL illuminates and signals the driver
that an emission or driveability problem with the powertrain has been detected
by the ECM/PCM. When this occurs, at least one diagnostic trouble code (DTC)
has been stored into the control module memory.
MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR: Reads the amount of
air pressure (vacuum) in the engine's intake manifold system; its signal is
used to analyze engine load conditions.
MANIFOLD VACUUM: Low pressure in an engine intake manifold
formed just below the throttle plates. Manifold vacuum is highest at idle and
drops under acceleration.
MANIFOLD: A casting of passages or set of pipes which
connect the cylinders to an inlet or outlet source.
MANUAL LEVER POSITION SWITCH (MLPS): A mechanical switching
unit that is typically mounted externally to the transmission/transaxle to
inform the PCM/ECM which gear range the driver has selected.
MANUAL VALVE: Located inside the transmission/transaxle, it
is directly connected to the driver's shift lever. The position of the manual
valve determines which hydraulic circuits will be charged with oil pressure and
the operating mode of the transmission.
MANUAL VALVE LEVER POSITION SENSOR (MVLPS): The input from
this device tells the TCM what gear range was selected.
MASS AIR FLOW (MAF) SENSOR: Measures the airflow into the
MASTER CYLINDER: The primary fluid pressurizing device in a
hydraulic system. In automotive use, it is found in brake and hydraulic clutch
systems and is pedal activated, either directly or, in a power brake system,
through the power booster.
MacPherson STRUT: A suspension component combining a shock
absorber and spring in one unit.
MEDIUM THROTTLE: Approximately one-half of accelerator pedal
MEGA: A metric prefix indicating one million.
MEMBER: An independent component of a hydrodynamic unit such
as an impeller, a stator, or a turbine. It may have one or more elements.
MERCON: A fluid developed by Ford Motor Company in 1988. It
contains a friction modifier and closely resembles operating characteristics of
METAL SEALING RINGS: Made from cast iron or aluminum, their
primary application is with dynamic components involving pressure sealing
circuits of rotating members. These rings are designed with either butt or hook
lock end joints.
METER (ANALOG): A linear-style meter representing data as
lengths; a needle-style instrument interfacing with logical numerical
increments. This style of electrical meter uses relatively low impedance
internal resistance and cannot be used for testing electronic circuitry.
METER(DIGITAL): Uses numbers as a direct readout to show
values. Most meters of this style use high impedance internal resistance and
must be used for testing low current electronic circuitry.
MICRO: A metric prefix indicating one-millionth (0.000001).
MILLI: A metric prefix indicating one-thousandth (0.001).
MINIMUM THROTTLE: The least amount of throttle opening
required for upshift; normally close to zero throttle.
MISFIRE: Condition occurring when the fuel mixture in a cylinder
fails to ignite, causing the engine to run roughly.
MODULE: Electronic control unit, amplifier or igniter of
solid state or integrated design which controls the current flow in the
ignition primary circuit based on input from the pick-up coil. When the module
opens the primary circuit, high secondary voltage is induced in the coil.
MODULATED: In an electronic-hydraulic converter clutch
system (or shift valve system), the term modulated refers to the pulsing of a
solenoid, at a variable rate. This action controls the buildup of oil pressure
in the hydraulic circuit to allow a controlled amount of clutch slippage.
MODULATED CONVERTER CLUTCH CONTROL (MCCC): A pulse width
duty cycle valve that controls the converter lockup apply pressure and
maximizes smoother transitions between lock and unlock conditions.
(THROTTLE PRESSURE): A hydraulic signal oil pressure relating to the amount of
engine load, based on either the amount of throttle plate opening or engine
MODULATOR VALVE: A regulator valve that is controlled by
engine vacuum, providing a hydraulic pressure that varies in relation to engine
torque. The hydraulic torque signal functions to delay the shift pattern and provide
a line pressure boost. (See throttle valve.)
MOTOR: An electromagnetic device used to convert electrical
energy into mechanical energy.
MULTIPLE-DISC CLUTCH: A grouping of steel and friction lined
plates that, when compressed together by hydraulic pressure acting upon a
piston, lock or unlock a planetary member.
MULTI-WEIGHT: Type of oil that provides adequate lubrication
at both high and low temperatures.
needed to move one amp through a resistance of one ohm.
MUSHY: Same as soft; slow and drawn out clutch apply with
very little shift feel.
MUTUAL INDUCTION: The generation of Current from one wire
circuit to another by movement of the magnetic field surrounding a
current-carrying circuit as its ampere flow increases or decreases.
NEEDLE BEARING: A bearing which consists of a number
(usually a large number) of long, thin rollers.
NITROGEN OXIDE (NOx): One of the three basic pollutants
found in the exhaust emission of an internal combustion engine. The amount of
NOx usually varies in an inverse proportion to the amount of HC and CO.
NONPOSITIVE SEALING: A sealing method that allows some minor
leakage, which normally assists in lubrication.
O2 SENSOR: Located in the engine's exhaust system, it is an
input device to the ECM/PCM for managing the fuel delivery and ignition system.
A scanner can be used to observe the fluctuating voltage readings produced by
an 02 sensor as the oxygen content of the exhaust is analyzed.
O-RING SEAL: Molded synthetic rubber seal designed with a
circular cross-section. This type of seal is used primarily in static
OBD II (ON-BOARD DIAGNOSTICS, SECOND GENERATION): Refers to
the federal law mandating tighter control of 1996 and newer vehicle emissions,
active monitoring of related devices, and standardization of terminology, data
link connectors, and other technician concerns.
OCTANE RATING: A number, indicating the quality of gasoline
based on its ability to resist knock. The higher the number, the better the
quality. Higher compression engines require higher octane gas.
OEM: Original Equipment Manufactured. OEM equipment is that
furnished standard by the manufacturer.
OFFSET: The distance between the vertical center of the
wheel and the mounting surface at the lugs. Offset is positive if the center
is outside the lug circle; negative offset puts the center line inside the lug
OHM'S LAW: A law of electricity that states the relationship
between voltage, current, and resistance. Volts = amperes x ohms
OHM: The unit used to measure the resistance of conductor-to-electrical
flow. One ohm is the amount of resistance that limits current flow to one
ampere in a circuit with one volt of pressure.
OHMMETER: An instrument used for measuring the resistance,
in ohms, in an electrical circuit.
ONE-WAY CLUTCH: A mechanical clutch of roller or sprag
design that resists torque or transmits power in one direction only. It is used
to either hold or drive a planetary member.
ONE-WAY ROLLER CLUTCH: A mechanical device that transmits or
holds torque in one direction only.
OPENCIRCUIT: A break or lack of contact in an electrical
circuit, either intentional (switch) or unintentional (bad connection or broken
ORIFICE: Located in hydraulic oil circuits, it acts as a
restriction. It slows down fluid flow to either create back pressure or delay
pressure buildup downstream.
OSCILLOSCOPE: A piece of test equipment that shows electric
impulses as a pattern on a screen. Engine performance can be analyzed by
interpreting these patterns.
OUTPUT SHAFT: The shaft which transmits torque from a
device, such as a transmission.
OUTPUT SPEED SENSOR (OSS):
Identifies transmission/transaxle output shaft speed for shift timing and may
be used to calculate TCC slip; often functions as the VSS (vehicle speed
OVERDRIVE: (1.) A device attached to or incorporated in a
transmission/transaxle that allows the engine to turn less than one full revolution
for every complete revolution of the wheels. The net effect is to reduce
engine rpm, thereby using less fuel. A typical overdrive gear ratio would be
.87:1, instead of the normal 1:1 in high gear. (2.) A gear assembly which
produces more shaft revolutions than that transmitted to it.
OVERDRIVE PLANETARY GEARSET: A single planetary gearset
designed to provide a direct drive and overdrive ratio. When coupled to a
three-speed transmission/transaxle configuration, a four-speed/overdrive unit
OVERHEAD CAMSHAFT (OHC): An engine configuration in which
the camshaft is mounted on top of the cylinder head and operates the valve
either directly or by means of rocker arms.
OVERHEAD VALVE (OHV): An engine configuration in which all
of the valves are located in the cylinder head and the camshaft is located in
the cylinder block. The camshaft operates the valves via lifters and pushrods.
OVERRUNCLUTCH: Another name for a one-way mechanical clutch.
Applies to both roller and sprag designs.
OVERSTEER: The tendency of some vehicles, when steering into
a turn, to over-respond or steer more than required, which could result in
excessive slip of the rear wheels. Opposite of under-steer.
OXIDATION STABILIZERS: Absorb and dissipate heat. Automatic
transmission fluid has high resistance to varnish and sludge buildup that
occurs from excessive heat that is generated primarily in the torque converter.
Local temperatures as high as 6000F (3150C) can occur at the clutch plates
during engagement, and this heat must be absorbed and dissipated. If the fluid
cannot withstand the heat, it burns or oxidizes, resulting in an almost immediate
destruction of friction materials, clogged filter screen and hydraulic
passages, and sticky valves.
OXIDES OF NITROGEN: See nitrogen oxide (NOx).
OXYGEN SENSOR: Used with a feedback system to sense the
presence of oxygen in the exhaust gas and signal the computer which can use the
voltage signal to determine engine operating efficiency and adjust the air/fuel
PARALLEL CIRCUIT: (See circuit, parallel.)
PARTS WASHER: A basin or tub, usually with a built-in pump
mechanism and hose used for circulating chemical solvent for the purpose of
cleaning greasy, oily and dirty components.
PART-TIME FOUR WHEEL DRIVE: A system that is normally in the
two wheel drive mode and only runs in four-wheel drive when the system is
manually engaged because more traction is desired. Two or four wheel drive is
normally selected by a lever to engage the front axle, but if locking hubs are
used, these must also be manually engaged in the Lock position. Otherwise, the
front axle will not drive the front wheels.
PASSIVE RESTRAINT: Safety systems such as air bags or
automatic seat belts which operate with no action required on the part of the
driver or passenger. Mandated by Federal regulations on all vehicles sold in
PAYLOAD: The weight the vehicle is capable of carrying in
addition to its own weight. Payload includes weight of the driver, passengers
and cargo, but not coolant, fuel, lubricant, spare tire, etc.
PCM: Powertrain control module.
PCV VALVE: A valve usually located in the rocker cover that
vents crankcase vapors back into the engine to be reburned.
PERCOLATION: A condition in which the fuel actually "boils,"
due to excessive heat. Percolation prevents proper atomization of the fuel causing
PICK-UP COIL: The coil in which voltage is induced in an
PINION GEAR: The smallest gear in a drive gear assembly.
piston: A disc or cup that fits in a cylinder bore and is free to move. In
hydraulics, it provides the means of converting hydraulic pressure into a
usable force. Examples of piston applications are found in servo, clutch, and
PING: A metallic rattling
sound produced by the engine during acceleration. It is usually due to
incorrect ignition timing or a poor grade of gasoline.
PINION: The smaller of two gears. The rear axle pinion
drives the ring gear which transmits motion to the axle shafts.
PISTON RING: An open-ended ring which fits into a groove on
the outer diameter of the piston. Its chief function is to form a seal between
the piston and cylinder wall. Most automotive pistons have three rings: two for
compression sealing; one for oil sealing.
PITMAN ARM: A lever which transmits steering force from the
steering gear to the steering linkage.
PLANET CARRIER: A basic member of a planetary gear assembly
that carries the pinion gears.
PLANET PINIONS: Gears housed in a planet carrier that are in
constant mesh with the sun gear and internal gear. Because they have their own
independent rotating centers, the pinions are capable of rotating around the
sun gear or the inside of the internal gear.
PLANETARY GEAR RATIO: The reduction or overdrive ratio
developed by a planetary gearset.
PLANETARY GEARSET: In its simplest form, it is made up of a
basic assembly group containing a sun gear, internal gear, and planet carrier.
The gears are always in constant mesh and offer a wide range of gear ratio
PLANETARY GEARSET(COMPOUND): Two planetary gearsets combined
PLANETARY GEARSET(SIMPLE): An assembly of gears in constant
mesh consisting of a sun gear, several pinion gears mounted in a carrier, and a
ring gear. It provides gear ratio and direction changes, in addition to a
direct drive and a neutral.
PLY RATING: A. rating given a tire which indicates strength
(but not necessarily actual plies). A two-ply/four-ply rating has only two
plies, but the strength of a four-ply tire.
POLARITY: Indication (positive or negative) of the two poles
of a battery.
PORT: An opening for fluid intake or exhaust.
POSITIVE SEALING: A sealing method that completely prevents
POTENTIAL: Electrical force measured in volts; sometimes
used interchangeably with voltage.
POWER: The ability to do work per unit of time, as expressed
in horsepower; one horsepower equals 33,000 ft. lbs. of work per minute, or 550
ft. lbs. of work per second.
POWER FLOW: The systematic flow or transmission of power
through the gears, from the input shaft to the output shaft.
POWER-TO-WEIGHT RATIO: Ratio of horsepower to weight of car.
POWERTRAIN: See Drivetrain.
POWERTRAIN CONTROL MODULE(PCM): Current designation for the
engine control module (ECM). In many cases, late model vehicle control units
manage the engine as well as the transmission. In other settings, the PCM
controls the engine and is interfaced with a TCM to control transmission
Ppm: Parts per million; unit used to measure exhaust emissions.
PREIGNITION: Early ignition of fuel in the cylinder,
sometimes due to glowing carbon deposits in the combustion chamber. Preignition
can be damaging since combustion takes place prematurely.
PRELOAD: A predetermined load placed on a bearing during
assembly or by adjustment.
PRESS FIT: The mating of two parts under pressure, due to
the inner diameter of one being smaller than the outer diameter of the other,
or vice versa; an interference fit.
PRESSURE: The amount of force exerted upon a surface area.
PRESSURE CONTROL SOLENOID (PCS): An output device that
provides a boost oil pressure to the mainline regulator valve to control line
pressure. Its operation is determined by the amount of current sent from the
PRESSURE GAUGE: An instrument used for measuring the fluid
pressure in a hydraulic circuit.
PRESSURE REGULATOR VALVE: In automatic transmissions, its
purpose is to regulate the pressure of the pump output and supply the basic
fluid pressure necessary to operate the transmission. The regulated fluid
pressure may be referred to as mainline pressure, line pressure, or control
PRESSURE SWITCH ASSEMBLY (PSA): Mounted inside the
transmission, it is a grouping of oil pressure switches that inputs to the PCM
when certain hydraulic passages are charged with oil pressure.
PRESSURE PLATE: A spring-loaded plate (part of the clutch)
that transmits power to the driven (friction) plate when the clutch is engaged.
PRIMARY CIRCUIT: The low voltage side of the ignition system
which consists of the ignition switch, ballast resistor or resistance wire,
bypass, coil, electronic control unit and pick-up coil as well as the connecting
wires and harnesses.
PROFILE: Term used for tire measurement (tire series), which
is the ratio of tire height to tread width.
PROM (PROGRAMMABLE READ-ONLY MEMORY): The heart of the
computer that compares input data and makes the engineered program or strategy
decisions about when to trigger the appropriate output based on stored computer
instructions. Pulse generator: A two-wire pickup sensor used to produce a
fluctuating electrical signal. This changing signal is read by the controller
to determine the speed of the object and can be used to measure
transmission/transaxle input speed, output speed, and vehicle speed.
PSI: Pounds per square inch; a measurement of pressure.
PULSE WIDTH DUTY CYCLE SOLENOID (PULSE WIDTH MODULATED
SOLENOID): A computer-controlled solenoid that turns on and off at a variable
rate producing a modulated oil pressure; often referred to as a pulse width
modulated (PWM) solenoid. Employed in many electronic automatic transmissions
and transaxles, these solenoids are used to manage shift control and converter
clutch hydraulic circuits.
PUSHROD: A steel rod between the hydraulic valve lifter and
the valve rocker arm in overhead valve (OHV) engines.
PUMP: A mechanical device designed to create fluid flow and
pressure buildup in a hydraulic system.
QUARTER PANEL: General term used to refer to a rear fender.
Quarter panel is the area from the rear door opening to the tail light area and
from rear wheel well to the base of the trunk and roof-line.
RACE: The surface on the inner or outer ring of a bearing on
which the balls, needles or rollers move.
RACK AND PINION: A type of automotive steering system using
a pinion gear attached to the end of the steering shaft. The pinion meshes with
a long rack attached to the steering linkage.
RADIAL TIRE: Tire design which uses body cords running at
right angles to the center line of the tire. Two or more belts are used to give
tread strength. Radials can be identified by their characteristic sidewall
RADIATOR: Part of the cooling system for a water-cooled
engine, mounted in the front of the vehicle and connected to the engine with
rubber hoses. Through the radiator, excess combustion heat is dissipated into
the atmosphere through forced convection using a water and glycol based mixture
that circulates through, and cools, the engine.
RANGE REFERENCE AND CLUTCH/BAND APPLY CHART: A guide that
shows the application of clutches and bands for each gear, within the selector
range positions. These charts are extremely useful for understanding how the
unit operates and for diagnosing malfunctions.
RAVIGNEAUX GEARSET: A compound planetary gearset that
features matched dual planetary pinions (sets of two) mounted in a single
planet carrier. Two sun gears and one ring mesh with the carrier pinions.
REACTION MEMBER: The stationary planetary member, in a
planetary gearset, that is grounded to the transmission/transaxle case through
the use of friction and wedging devices known as bands, disc clutches, and
REACTION PRESSURE: The fluid pressure that moves a spool
valve against an opposing force or forces; the area on which the opposing force
acts. The opposing force can be a spring or a combination of spring force and
auxiliary hydraulic force.
REACTOR, TORQUE CONVERTER: The reaction member of a fluid
torque converter, more commonly called a stator. (See stator.)
REAR MAIN OIL SEAL: A synthetic or rope-type seal that
prevents oil from leaking out of the engine past the rear main crankshaft
RECIRCULATING BALL: Type of steering system in which
recirculating steel balls occupy the area between the nut and worm wheel,
causing a reduction in friction.
RECTIFIER: A device (used primarily in alternators) that
permits electrical current to flow in one direction only.
REDUCTION: (See gear reduction.) regulator valve: A valve
that changes the pressure of the oil in a hydraulic circuit as the oil passes
through the valve by bleeding off (or exhausting) some of the volume of oil
supplied to the valve.
REFRIGERANT 12 (R-12) or 134 (R-134): The generic name of
the refrigerant used in automotive air conditioning systems.
REGULATOR: A device which maintains the amperage and/or
voltage levels of a circuit at predetermined values.
RELAY: A switch which automatically opens and/or closes a
RELAY VALVE: A valve that directs flow and pressure. Relay
valves simply connect or disconnect interrelated passages without restricting
the fluid flow or changing the pressure.
RELIEF VALVE: A spring-loaded, pressure-operated valve that limits
oil pressure buildup in a hydraulic circuit to a predetermined maximum value.
RELUCTOR: A wheel that rotates inside the distributor and
triggers the release of voltage in an electronic ignition.
RESERVOIR: The storage area for fluid in a hydraulic system;
often called a sump.
RESIN: A liquid plastic used in body work.
RESIDUAL MAGNETISM: The magnetic strength stored in a
material after a magnetizing field has been removed.
RESISTANCE: The opposition to the flow of current through a
circuit or electrical device, and is measured in ohms. Resistance is equal to
the voltage divided by the amperage.
RESISTOR SPARK PLUG: A spark plug using a resistor to shorten
the spark duration. This suppresses radio interference and lengthens plug life.
RESISTOR: A device, usually made of wire, which offers a
preset amount of resistance in an electrical circuit.
RESULTANT FORCE: The single effective directional thrust of
the fluid force on the turbine produced by the vortex and rotary forces acting
in different planes.
RETARD: Set the ignition timing so that spark occurs later
(fewer degrees before TDC).
RHEOSTAT: A device for regulating a current by means of a
RING GEAR: The name given to a ring-shaped gear attached to
a differential case, or affixed to a flywheel or as part of a planetary gear
ROCKER ARM: A lever which rotates around a shaft pushing
down (opening) the valve with an end when the other end is pushed up by the
pushrod. Spring pressure will later close the valve.
ROCKER PANEL: The body panel below the doors between the
ROLLER BEARING: A bearing made up of hardened inner and
outer races between which hardened steel rollers move.
ROLLER CLUTCH: A type of one-way clutch design using rollers
and springs mounted within an inner and outer cam race assembly.
ROTARY FLOW: The path of the fluid trapped between the
blades of the members as they revolve with the rotation of the torque converter
cover (rotational inertia).
ROTOR: (1.) The disc-shaped part of a disc brake assembly,
upon which the brake pads bear; also called, brake disc. (2.) The device
mounted atop the distributor shaft, which passes current to the distributor cap
ROTARY ENGINE: See Wankel engine.
RPM: Revolutions per minute (usually indicates engine
RTV: A gasket making compound that cures as it is exposed to
the atmosphere. It is used between surfaces that are not perfectly machined to
one another, leaving a slight gap that the RTV fills and in which it hardens.
The letters RTV represent room temperature vulcanizing.
RUN-ON: Condition when the engine continues to run, even
when the key is turned off. See dieseling.
SEALED BEAM: A automotive headlight. The lens, reflector
and filament from a single unit.
SEATBELT INTERLOCK: A system whereby the car cannot be
started unless the seatbelt is buckled.
SECONDARY CIRCUIT: The high voltage side of the ignition
system, usually above 20,000 volts. The secondary includes the ignition coil,
coil wire, distributor cap and rotor, spark plug wires and spark plugs.
SELF-INDUCTION: The generation of voltage in a
current-carrying wire by changing the amount of current flowing within that
SEMI-CONDUCTOR: A material (silicon or germanium) that is
neither a good conductor nor an insulator; used in diodes and transistors.
SEMI-FLOATING AXLE: In this design, a wheel is attached to
the axle shaft, which takes both drive and cornering loads. Almost all solid
axle passenger cars and light trucks use this design.
SENDING UNIT: A mechanical, electrical, hydraulic or electromagnetic
device which transmits information to a gauge.
SENSOR: Any device designed to measure engine operating
conditions or ambient pressures and temperatures. Usually electronic in nature
and designed to send a voltage signal to an on-board computer, some sensors may
operate as a simple on/off switch or they may provide a variable voltage signal
(like a potentiometer) as conditions or measured parameters change.
SERIES CIRCUIT: (See circuit, series.)
SERPENTINE BELT: An accessory drive belt, with small
multiple v-ribs, routed around most or all of the engine-powered accessories
such as the alternator and power steering pump. Usually both the front and the
back side of the belt comes into contact with various pulleys.
SERVO: In an automatic transmission, it is a piston in a
cylinder assembly that converts hydraulic pressure into mechanical force and
movement; used for the application of the bands and clutches.
SHIFT BUSYNESS: When referring to a torque converter clutch,
it is the frequent apply and release of the clutch plate due to uncommon
SHIFT VALVE: Classified as a relay valve, it triggers the
automatic shift in response to a governor and a throttle signal by directing
fluid to the appropriate band and clutch apply combination to cause the shift
SHIM: Spacers of precise, predetermined thickness used between
parts to establish a proper working relationship.
SHIMMY: Vibration (sometimes violent) in the front end
caused by misaligned front end, out of balance tires or worn suspension
SHORT CIRCUIT: An electrical malfunction where current
takes the path of least resistance to ground (usually through damaged
insulation). Current flow is excessive from low resistance resulting in a
SHUDDER: Repeated jerking or stick-slip sensation, similar
to chuggle but more severe and rapid in nature, that may be most noticeable
during certain ranges of vehicle speed; also used to define condition after
converter clutch engagement.
SIMPSON GEARSET: A compound planetary gear train that
integrates two simple planetary gearsets referred to as the front planetary and
the rear planetary.
SINGLE OVERHEAD CAMSHAFT: See overhead camshaft.
SKIDPLATE: A metal plate attached to the underside of the
body to protect the fuel tank, transfer case or other vulnerable parts from
SLAVE CYLINDER: In automotive use, a device in the hydraulic
clutch system which is activated by hydraulic force, disengaging the clutch.
SLIPPING: Noticeable increase in engine rpm without vehicle
speed increase; usually occurs during or after initial clutch or band
SLUDGE: Thick, black deposits in engine formed from dirt,
oil, water, etc. It is usually formed in engines when oil changes are
SNAP RING: A circular retaining clip used inside or outside
a shaft or part to secure a shaft, such as a floating wrist pin.
SOFT: Slow, almost unnoticeable clutch apply with very
little shift feel.
SOFTCODES: DTCs that have been set into the PCM memory but
are not present at the time of testing; often referred to as history or
SOHC: Single overhead camshaft.
SOLENOID: An electrically operated, magnetic switching device.
SPALLING: A wear pattern identified by metal chips flaking
off the hardened surface. This condition is caused by foreign particles,
overloading situations, and/or normal wear.
SPARK PLUG: A device screwed into the combustion chamber of
a spark ignition engine. The basic construction is a conductive core inside of
a ceramic insulator, mounted in an outer conductive base. An electrical charge
from the spark plug wire travels along the conductive core and jumps a preset
air gap to a grounding point or points at the end of the conductive base. The
resultant spark ignites the fuel/air mixture in the combustion chamber.
SPECIFIC GRAVITY (BATTERY):
The relative weight of liquid (battery electrolyte) as compared to the weight
of an equal volume of water.
SPLINES: Ridges machined or cast onto the outer diameter of
a shaft or inner diameter of a bore to enable parts to mate without rotation.
DRIVE: In a torque converter, it refers to
parallel paths of torque transmission, one of which is mechanical and the other
SPONGY PEDAL: A soft or spongy feeling when the brake pedal
is depressed. It is usually due to air in the brake lines.
SPOOLVALVE: A precision-machined, cylindrically shaped valve
made up of lands and grooves. Depending on its position in the valve bore,
various interconnecting hydraulic circuit passages are either opened or closed.
SPRAG CLUTCH: A type of one-way clutch design using cams or
contoured-shaped sprags between inner and outer races. (See one-way clutch.)
SPRUNG WEIGHT: The weight of a car supported by the springs.
SQUARE-CUT SEAL: Molded synthetic rubber seal designed with
a square- or rectangular-shaped cross-section. This type of seal is used for
both dynamic and static applications.
SRS: Supplemental restraint system
STABILIZER (SWAY) BAR: A bar linking both sides of the suspension.
It resists sway on turns by taking some of added load from one wheel and putting
it on the other.
STAGE: The number of turbine sets separated by a stator. A
turbine set may be made up of one or more turbine members. A three-element
converter is classified as a single stage.
STALL: In fluid drive transmission/transaxle applications,
stall refers to engine rpm with the transmission/transaxle engaged and the
vehicle stationary; throttle valve can be in any position between closed and
STALL SPEED: In fluid drive transmission/transaxle
applications, stall speed refers to the maximum engine rpm with the
transmission/transaxle engaged and vehicle stationary, when the throttle valve
is wide open. (See stall; stall test.)
STALL TEST: A procedure recommended by many manufacturers to
help determine the integrity of an engine, the torque converter stator, and
certain clutch and band combinations. With the shift lever in each of the
forward and reverse positions and with the brakes firmly applied, the
accelerator pedal is momentarily pressed to the wide open throttle (WOT)
position. The engine rpm reading at full throttle can provide clues for
diagnosing the condition of the items listed above.
STALL TORQUE: The maximum design or engineered torque ratio
of a fluid torque converter, produced under stall speed conditions. (See stall
STARTER: A high-torque electric motor used for the purpose
of starting the engine, typically through a high ratio geared drive connected
to the flywheel ring gear.
STATIC: A sealing application in which the parts being
sealed do not move in relation to each other.
STATOR (REACTOR): The reaction member of a fluid torque
converter that changes the direction of the fluid as it leaves the turbine to
enter the impeller vanes. During the torque multiplication phase, this action
assists the impeller's rotary force and results in an increase in torque.
STEERING GEOMETRY: Combination of various angles of suspension
components (caster, camber, toe-in); roughly equivalent to front end alignment.
STRAIGHT WEIGHT: Term designating motor oil as suitable for
use within a narrow range of temperatures. Outside the narrow temperature range
its flow characteristics will not adequately lubricate.
STROKE: The distance the piston travels from bottom dead
center to top dead center.
SUBSTITUTION: Replacing one part suspected of a defect with
a like part of known quality.
SUMP: The storage vessel or reservoir that provides a ready
source of fluid to the pump. In an automatic transmission, the sump is the oil
pan. All fluid eventually returns to the sump for recycling into the hydraulic
SUN GEAR: In a planetary gearset, it is the center gear that
meshes with a cluster of planet pinions.
SUPERCHARGER: An air pump driven mechanically by the engine
through belts, chains, shafts or gears from the crankshaft. Two general types
of supercharger are the positive displacement and centrifugal type, which pump
air in direct relationship to the speed of the engine.
SUPPLEMENTAL RESTRAINT SYSTEM: See air bag.
SURGE: Repeating engine-related feeling of acceleration and
deceleration that is less intense than chuggle.
SWITCH: A device used to open, close, or redirect the
current in an electrical circuit.
SYNCHROMESH: A manual transmission/transaxle that is
equipped with devices (synchronizers) that match the gear speeds so that the
transmission/transaxle can be downshifted without clashing gears.
SYNTHETIC OIL: Non-petroleum based oil.
TACHOMETER: A device used to measure the rotary speed of an
engine, shaft, gear, etc., usually in rotations per minute.
TDC: Top dead center. The exact top of the piston's stroke.
TEFLON SEALING RINGS: Teflon is a soft, durable,
plastic-like material that is resistant to heat and provides excellent sealing.
These rings are designed with either scarf-cut joints or as one-piece rings.
Teflon sealing rings have replaced many metal ring applications.
TERMINAL: A device attached to the end of a wire or cable to
make an electrical connection.
TEST LIGHT, CIRCUIT-POWERED: Uses available circuit voltage
to test circuit continuity.
TEST LIGHT, SELF-POWERED: Uses its own battery source to
test circuit continuity.
THERMISTOR: A special resistor used to measure fluid
temperature; it decreases its resistance with increases in temperature.
THERMOSTAT: A valve, located in the cooling system of an
engine, which is closed when cold and opens gradually in response to engine
heating, controlling the temperature of the coolant and rate of coolant flow.
THERMOSTATIC ELEMENT: A heat-sensitive, spring-type device
that controls a drain port from the upper sump area to the lower sump. When the
transaxle fluid reaches operating temperature, the port is closed and the upper
sump fills, thus reducing the fluid level in the lower sump.
THROTTLE POSITION (TP) SENSOR: Reads the degree of throttle
opening; its signal is used to analyze engine load conditions. The ECM/PCM
decides to apply the TCC, or to disengage it for coast or load conditions that
need a converter torque boost.
THROTTLE PRESSURE/MODULATOR PRESSURE: A hydraulic signal oil
pressure relating to the amount of engine load, based on either the amount of
throttle plate opening or engine vacuum.
THROTTLE VALVE: A regulating or balanced valve that is
controlled mechanically by throttle linkage or engine vacuum. It sends a
hydraulic signal to the shift valve body to control shift timing and shift
quality. (See balanced valve; modulator valve.)
THROW-OUT BEARING: As the clutch pedal is depressed, the
throwout bearing moves against the spring fingers of the pressure plate,
forcing the pressure plate to disengage from the driven disc.
TIE ROD: A rod connecting the steering arms. Tie rods have
threaded ends that are used to adjust toe-in.
TIE-UP: Condition where two opposing clutches are attempting
to apply at same time, causing engine to labor with noticeable loss of engine
TIMING BELT: A square-toothed, reinforced rubber belt that
is driven by the crankshaft and operates the camshaft.
TIMING CHAIN: A roller chain that is driven by the
crankshaft and operates the camshaft.
TIRE ROTATION: Moving the tires from one position to another
to make the tires wear evenly.
TOE-IN (OUT): A term comparing the extreme front and rear of
the front tires. Closer together at the front is toe-in; farther apart at the
front is toe-out.
TOP DEAD CENTER (TDC): The point at which the piston reaches
the top of its travel on the compression stroke.
TORQUE: Measurement of turning or twisting force, expressed
as foot-pounds or inch-pounds.
TORQUE CONVERTER: A turbine used to transmit power from a
driving member to a driven member via hydraulic action, providing changes in
drive ratio and torque. In automotive use, it links the driveplate at the rear
of the engine to the automatic transmission.
TORQUE CONVERTER CLUTCH: The apply plate (lockup plate)
assembly used for mechanical power flow through the converter.
TORQUE PHASE: Sometimes referred to as slip phase or stall
phase, torque multiplication occurs when the turbine is turning at a slower
speed than the impeller, and the stator is reactionary (stationary). This
sequence generates a boost in output torque.
TORQUE RATING (STALL TORQUE): The maximum torque
multiplication that occurs during stall conditions, with the engine at wide
open throttle (WOT) and zero turbine speed.
TORQUE RATIO: An expression of the gear ratio factor on
torque effect. A 3:1 gear ratio or 3:1 torque ratio increases the torque input
by the ratio factor of 3. Input torque (100 ft. lbs.)x 3 = output torque (300
TRACTION: The amount of usable tractive effort before the
drive wheels slip on the road contact surface.
TORSION BAR SUSPENSION: Long rods of spring steel which take
the place of springs. One end of the bar is anchored and the other arm
(attached to the suspension) is free to twist. The bars' resistance to
twisting causes springing action.
TRACK: Distance between the centers of the tires where they
contact the ground.
TRACTION CONTROL: A control system that prevents the
spinning of a vehicle's drive wheels when excess power is applied.
TRACTIVE EFFORT: The amount of force available to the drive
wheels, to move the vehicle.
TRANSAXLE: A single housing containing the transmission and
differential. Transaxles are usually found on front engine/front wheel drive or
rear engine/rear wheel drive cars.
TRANSDUCER: A device that changes energy from one form to
another. For example, a transducer in a microphone changes sound energy to
electrical energy. In automotive air-conditioning controls used in automatic
temperature systems, a transducer changes an electrical signal to a vacuum
signal, which operates mechanical doors.
TRANSMISSION: A powertrain component designed to modify
torque and speed developed by the engine; also provides direct drive, reverse,
TRANSMISSION CONTROL MODULE (TCM): Manages transmission
functions. These vary according to the manufacturer's product design but may
include converter clutch operation, electronic shift scheduling, and mainline
TRANSMISSION FLUID TEMPERATURE (TFT)SENSOR: Originally
called a transmission oil temperature (TOT) sensor, this input device to the
ECM/PCM senses the fluid temperature and provides a resistance value. It
operates on the thermistor principle.
TRANSMISSION INPUT SPEED (TIS) SENSOR: Measures turbine
shaft (input shaft) rpm's and compares to engine rpm's to determine torque
converter slip. When compared to the transmission output speed sensor or VSS,
gear ratio and clutch engagement timing can be determined.
TRANSMISSION OIL TEMPERATURE (TOT) SENSOR: (See transmission
fluid temperature (TFT) sensor.)
TRANSMISSION RANGE SELECTOR (TRS) SWITCH: Tells the module
which gear shift position the driver has chosen. turbine: The output (driven)
member of a fluid coupling or fluid torque converter. It is splined to the
input (turbine) shaft of the transmission.
TRANSFER CASE: A gearbox driven from the transmission that
delivers power to both front and rear driveshafts in a four-wheel drive system.
Transfer cases usually have a high and low range set of gears, used depending
on how much pulling power is needed.
TRANSISTOR: A semi-conductor component which can be actuated
by a small voltage to perform an electrical switching function.
TREAD WEAR INDICATOR: Bars molded into the tire at right
angles to the tread that appear as horizontal bars when
<frac>1<over>16</frac>in. of tread remains.
TREAD WEAR PATTERN: The pattern of wear on tires which can
be "read" to diagnose problems in the front suspension.
TUNE-UP: A regular maintenance function, usually associated
with the replacement and adjustment of parts and components in the electrical
and fuel systems of a vehicle for the purpose of attaining optimum performance.
TURBOCHARGER: An exhaust driven pump which compresses
intake air and forces it into the combustion chambers at higher than
atmospheric pressures. The increased air pressure allows more fuel to be
burned and results in increased horsepower being produced.
TURBULENCE: The interference of molecules of a fluid (or
vapor) with each other in a fluid flow.
TYPE F: Transmission fluid developed and used by Ford Motor
Company up to 1982. This fluid type provides a high coefficient of friction.
TYPE 7176: The preferred choice of transmission fluid for
Chrysler automatic transmissions and transaxles. Developed in 1986, it closely
resembles Dexron and Mercon. Type 7176 is the recommended service fill fluid
for all Chrysler products utilizing a lockup torque converter dating back to 1978.
U-JOINT (UNIVERSAL JOINT): A flexible coupling in the drive
train that allows the driveshafts or axle shafts to operate at different
angles and still transmit rotary power.
UNDERSTEER: The tendency of a car to continue straight ahead
while negotiating a turn.
UNIT BODY: Design in which the car body acts as the frame.
UNLEADED FUEL: Fuel which contains no lead (a common gasoline
additive). The presence of lead in fuel will destroy the functioning elements
of a catalytic converter, making it useless.
UNSPRUNG WEIGHT: The weight of car components not supported
by the springs (wheels, tires, brakes, rear axle, control arms, etc.).
UPSHIFT: A shift that results in a decrease in torque ratio
and an increase in speed.
VACUUM: A negative pressure; any pressure less than
VACUUM ADVANCE: A device which advances the ignition timing
in response to increased engine vacuum.
VACUUM GAUGE: An instrument used for measuring the existing
vacuum in a vacuum circuit or chamber. The unit of measure is inches (of
mercury in a barometer).
VACUUM MODULATOR: Generates a hydraulic oil pressure in
response to the amount of engine vacuum.
VALVES: Devices that can open or close fluid passages in a
hydraulic system and are used for directing fluid flow and controlling
VALVE BODY ASSEMBLY: The main hydraulic control assembly of
the transmission/transaxle that contains numerous valves, check balls, and
other components to control the distribution of pressurized oil throughout the
VALVE CLEARANCE: The measured gap between the end of the
valve stem and the rocker arm, cam lobe or follower that activates the valve.
VALVE GUIDES: The guide through which the stem of the valve
passes. The guide is designed to keep the valve in proper alignment.
VALVE LASH (clearance): The operating clearance in the valve
VALVE TRAIN: The system that operates intake and exhaust
valves, consisting of camshaft, valves and springs, lifters, pushrods and
VAPOR LOCK: Boiling of the fuel in the fuel lines due to
excess heat. This will interfere with the flow of fuel in the lines and can
completely stop the flow. Vapor lock normally only occurs in hot weather.
VARIABLE DISPLACEMENT (VARIABLE CAPACITY) VANE PUMP:
Slipper-type vanes, mounted in a revolving rotor and contained within the bore
of a movable slide, capture and then force fluid to flow. Movement of the slide
to various positions changes the size of the vane chambers and the amount of
fluid flow. Note: GM refers to this pump design as variable displacement, and
Ford terms it variable capacity.
VARIABLE FORCE SOLENOID (VFS): Commonly referred to as the
electronic pressure control (EPC) solenoid, it replaces the cable/linkage style
of TV system control and is integrated with a spool valve and spring assembly
to control pressure. A variable computer-controlled current flow varies the
internal force of the solenoid on the spool valve and resulting control
VARIABLE ORIFICE THERMAL VALVE: Temperature-sensitive hydraulic
oil control device that adjusts the size of a circuit path opening. By altering
the size of the opening, the oil flow rate is adapted for cold to hot oil
VARNISH: Term applied to the residue formed when gasoline
gets old and stale.
VCM: See Electronic Control Unit (ECU).
VEHICLE SPEED SENSOR (VSS): Provides an electrical signal to
the computer module, measuring vehicle speed, and affects the torque converter
clutch engagement and release.
VESPEL SEALING RINGS: Hard plastic material that produces
excellent sealing in dynamic settings. These rings are found in late versions
of the 4T60 and in all 4T60-E and 4T80-E transaxles.
VISCOSITY: The ability of a fluid to flow. The lower the
viscosity rating, the easier the fluid will flow. 10 weight motor oil will
flow much easier than 40 weight motor oil.
VISCOSITY INDEX IMPROVERS: Keeps the viscosity nearly
constant with changes in temperature. This is especially important at low
temperatures, when the oil needs to be thin to aid in shifting and for
cold-weather starting. Yet it must not be so thin that at high temperatures it
will cause excessive hydraulic leakage so that pumps are unable to maintain the
VISCOUS CLUTCH: A specially designed torque converter clutch
apply plate that, through the use of a silicon fluid, clamps smoothly and
absorbs torsional vibrations.
VOLT: Unit used to measure the force or pressure of
electricity. It is defined as the pressure
VOLTAGE: The electrical pressure that causes current to flow.
Voltage is measured in volts (V).
VOLTAGE, APPLIED: The actual voltage read at a given point
in a circuit. It equals the available voltage of the power supply minus the
losses in the circuit up to that point.
VOLTAGE DROP: The voltage lost or used in a circuit by
normal loads such as a motor or lamp or by abnormal loads such as a poor
(high-resistance) lead or terminal connection.
VOLTAGE REGULATOR: A device that controls the current output
of the alternator or generator.
VOLTMETER: An instrument used for measuring electrical force
in units called volts. Voltmeters are always connected parallel with the
circuit being tested.
VORTEX FLOW: The crosswise or circulatory flow of oil
between the blades of the members caused by the centrifugal pumping action of
WANKEL ENGINE: An engine which uses no pistons. In place of
pistons, triangular-shaped rotors revolve in specially shaped housings.
WATER PUMP: A belt driven component of the cooling system
that mounts on the engine, circulating the coolant under pressure.
WATT: The unit for measuring electrical power. One watt is
the product of one ampere and one volt (watts equals amps times volts). Wattage
is the horsepower of electricity (746 watts equal one horsepower).
WHEEL ALIGNMENT: Inclusive term to describe the front end
geometry (caster, camber, toe-in/out).
WHEEL CYLINDER: Found in the automotive drum brake assembly,
it is a device, actuated by hydraulic pressure, which, through internal
pistons, pushes the brake shoes outward against the drums.
WHEEL WEIGHT: Small weights attached to the wheel to balance
the wheel and tire assembly. Out-of-balance tires quickly wear out and also
give erratic handling when installed on the front.
WHEELBASE: Distance between the center of front wheels and
the center of rear wheels.
WIDE OPEN THROTTLE (WOT): Full travel of accelerator pedal.
WORK: The force exerted to move a mass or object. Work
involves motion; if a force is exerted and no motion takes place, no work is
done. Work per unit of time is called power. Work = force x distance = ft. lbs.
33,000 ft. lbs. in one minute = 1 horsepower
ZERO-THROTTLE COAST DOWN: A full release of accelerator
pedal while vehicle is in motion and in drive range.