By Jim Marotta
Ford introduced 4-Wheel Air Suspension (4WAS) on the 1995
Explorer, and has since made it available on all three generations of the Ford
Expedition and Lincoln Navigator. These big trucks weigh about three tons, so
they already have a relatively comfortable ride using conventional suspension.
Air suspension smooths that ride even further, but it's also used to increase
off-road ride height and for load-leveling, which enhances stability when towing.
Today, air suspension
still costs more than conventional suspension, but thanks to modern materials
and electronics, it is simpler, reliable and offers more than just a
The 4WAS is basically the same as conventional suspension.
Up front, air struts replace the standard strut assemblies. In the rear, air
springs replace the standard suspension's coil springs. All four air units have
a normally-closed solenoid valve operated by the control module for adding or
venting air pressure. Changing pressure not only changes ride height, it also
changes spring rate. You can replace the solenoid valves on the rear air
springs, but not the valves on the front air shocks.
The air suspension system consists of front air
struts (center) and rear air springs (left) with shock absorbers (right).
The electric air compressor
is different from the compressor on the Rear Air Suspension (RAS) system. It
includes a built-in vent solenoid valve and a replaceable air dryer. The
solenoid valve is also a spring-loaded safety valve that opens at 260 psi. A
solid-state relay mounted below the headlight supplies battery voltage to the
In addition to the compressor's vent solenoid and the
solenoid valve on each suspension unit, there are four more solenoid valves in
the system. The front fill solenoid isolates the front air shocks from the rest
of the system, and the rear-fill solenoid isolates the rear air springs. The
front gate solenoid isolates the front air shocks from each other so the
pressure between them does not equalize as the vehicle rolls in a turn. On
smooth roads, this valve opens for a softer ride. Lastly, there is a pressure
relief valve mounted on the frame crossmember near the rear height sensor. The
control unit can use it to vent the rear air springs without having to open the
rear-fill and system-vent solenoids.
On 2006 and earlier models, there are two
linear ride height sensors that have a total stroke of about 3 inches. Sensor
output is 0.25 volts at full extension and 4.75 volts at full compression.
On 2006 and earlier models, there are two linear ride height
sensors, one mounted between the rear suspension panhard rod and the
crossmember just forward of the spare tire, and one between the frame and the
left front upper control arm. Even with only two sensors, the control unit is
still capable of limited left/right height adjustments. The sensors have a
total stroke of about 3 inches, which is how far the panhard rod and control
arm move though the full 10 inches of suspension travel. Sensor output is 0.25
volts at full extension and 4.75 volts at full compression. Adjusting the
sensor or the static ride height of the truck requires a scan tool for storing
the new settings in the 4WAS control unit.
Later models have four identical potentiometer-type sensors,
one at each corner, but they're permanently mounted to left- and right-side
brackets so they can't be interchanged. The bracket is bolted to the frame, and
a small ball stud link connects the sensor arm to the suspension arm.
On earlier models use the same control unit for the 4WAS,
the Rear Air Suspension (RAS) system and the EVO Steering Control Module.
Mounted in the instrument panel above the radio, it supplies reference voltage
to the height sensors and controls the ground circuit of the compressor relay
and the power circuit of each solenoid valve. Inputs come either directly from
sensors or from other modules, including the front Generic Control Module (GEM)
and the PCM. Inputs include vehicle height, vehicle speed, steering wheel
position, brake and accelerator pedal positions, transfer case gear selection,
transmission gear selection and door ajar signals.
On later models the Vehicle Dynamics Module (VDM; mounted
above the parking brake handle) controls air suspension.
A manual air suspension switch turns the system off for
service or long-term storage. On 1997-02 models, the switch is behind the front
passenger side kick panel. On newer models, it's behind an access panel aft of
the third-row seat on the driver side. That switch controls power to the
control unit. When the switch is OFF, a warning light appears on the instrument panel with the
ignition switch ON.
All of the air lines in the system are nylon. Two lines
connect directly to the drier mounted on the compressor, one leading to the
front fill solenoid valve and the other to the rear fill valve. The system uses
quick-connect fittings throughout. You cannot repair or splice the air lines.
When the ignition
switch is first turned ON, the control module runs a
self-diagnostic test, storing or displaying any electrical or electronic
problems detected as B or C codes.Any time a door or
the rear hatch opens, the system will remember the height sensor readings. If you
add or remove weight, the system will operate as needed to maintain that
height. Once the doors are all closed and vehicle speed exceeds 10 mph, ride
height will readjust as needed. The system's first priority is load leveling,
but that's not all it can do.
The control unit can
select from three different programmed suspension heights. "Trim" height is the
normal ride height for paved road. Defined as trim plus 1 inch, "off-road"
height engages automatically when the ignition switch is ON, the transfer case is in 4X4 LOW and the
vehicle speed is less than 25 mph. Defined as trim minus 1 inch, "kneel" engages
when vehicle speed is zero, all doors are closed and the ignition switch is OFF.
Obviously the kneel height is to ease vehicle entry, and while one inch doesn't
seem like much, it can make a difference. If the system detects
a malfunction, the system will default to trim height, either by operating the
compressor or by venting pressure as needed.
As noted earlier,
there is a switch to turn the system off for service. It should be turned off whenever
you lift the wheels off the ground. Failure to do so can cause expensive
damage, especially to the rear air springs. The only service required by the
4WAS itself is a visual inspection. Look for physical damage to the air
springs, cut or crimped air lines or evidence of something rubbing the lines.
There is no pressure sensor in this system, so the self-diagnostic test can't
detect a leak. Symptoms of a leak include excessive compressor operation,
failure to reach or maintain commanded height, and most commonly, a sagging
Leaking, dry-rotted air bags
will cause excessive compressor operation, failure to reach or maintain
commanded height, and most commonly, a sagging suspension.
A quick visual check for
physical damage, like this torn airbag can make quick work of diagnosis.
Use soapy water to
leak-check the fittings, and check the whole system with a smoke machine. Smoke
is especially handy for finding a solenoid valve that's open when it shouldn't
be, and vice-versa. Do not, under any circumstances, disconnect any fittings
without venting the system first.
Check height sensors
for damage and to make sure they're securely mounted. With the vehicle at trim
height, sensor output should be 2.66 volts. Repairing, adjusting or replacing a
sensor requires a scan tool to recalibrate the system.
Historically the most
troublesome part of the system is the compressor. Failures tend to be gradual
rather than catastrophic, and tell-tales signs are noise, rough running and
excessive current draw. The sagging rear end noted earlier is what commonly
drives owners to seek help. When replacing a compressor, techs often replace
the solid-state relay too.
After replacing a
sensor or the control unit, calibrate
the control unit. This involves setting the mechanical ride height (with the
system vented), and then using a scan tool to command specific functions
through the control unit. The Ford NGS and the new IDS tools are best for the
job, but there are some purpose-built aftermarket tools available from
companies that sell air suspension parts.
This brings us to the
final point. Since the compressor is the part that fails most often, techs
often just install a new part and "get lucky." However if something else is
wrong, the lack of proper tools, equipment, information, and parts can turn a
potentially profitable job into an ugly situation. Should you decide to make
the investment, there are several companies that offer everything you need to
become an air suspension specialist.
While compressors (shown) are the part that
fails the most frequently, a thorough diagnosis can save you money on replacing
parts in the long run.
When you need to maintain or service a vehicle, get a valuable subscription to Chilton’s comprehensive service
procedures, specifications, technical service bulletins, graphics, and wiring diagrams at www.ChiltonDIY.com or www.ChiltonPRO.com. A subscription, designed for professional technicians and for do-it-yourselfers, can save you many hours of frustration and pay for itself as soon as you use it.
A muscle car enthusiast and drag racer, Jim Marotta is a freelance automotive writer with more than 20 years experience in the automotive industry.
10 Nov 2010 9:26 PM
Filed under: air suspension, Lincoln, air compressorr suspension switch, ride height sensor, 2009 Lincoln Navigator air suspension component locations, suspension, 1995 Ford Explorer, Ford Expedition, 4-Wheel Air Suspension (4WAS), Lincoln Navigator, Ford VCM IDS tool, Rear Air Suspension (RAS), calibrate the suspension control unit, Ford NGS tool