Story and Photography by Jim Marotta
“A truly stable system expects the unexpected, is prepared to be disrupted, waits to be transformed.” ~ Tom Robbins
It seems that James Bond might drive this car, with a system that unites the Anti-Lock Brake System (ABS) and Traction Control System (TCS) to monitor the vehicle’s motion and make corrections to stabilize the car and prevent skidding. But many vehicles today deliver us to our destinations while practically driving themselves. Eventually, vehicles sold in the U.S. will be required to have such a system.
Electronic Stability Control (ESC) is a refinement of the original
Anti-Lock Brake System (ABS) co-developed by Bosch and Mercedes-Benz, and introduced
on the Mercedes-Benz S-Class in 1978. ESC first appeared on the 1995 Mercedes Benz S-Class.
rollover crashes during magazine testing of the new 1996 Mercedes Benz A-Class
compact car, Mercedes recalled and retrofitted 130,000 A-Class cars with ESC.
This produced a significant reduction in crashes and the number of Mercedes vehicles
fitted with ESC rose significantly. The remaining vehicle
manufacturers quickly followed by developing their own versions. To date, there
are more than a dozen different systems in use.
In September 2007, due to governmental and public demand, the National
Highway Traffic and Safety Administration (NHTSA) proposed a new rule requiring
all new light-duty vehicles sold in the United States to be equipped with ESC
by the 2012 model year. This rule became Federal Motor Vehicle
Safety Standard (FMVSS) No.126. As of 2009, close to 74 percent of new vehicles have
ESC as standard equipment, and an additional 13 percent offer it as an option.
How ESC Works
systems are autonomous units which do not require driver operation. The ESC
control unit receives inputs from several sensors to determine desired versus
actual vehicle trajectory.
ESC compares the
driver's intended vehicle direction to the actual vehicle direction by using
sensors to measure steering angle, lateral acceleration, vehicle rotation
(yaw), and individual road wheel speeds. (Courtesy Robert Bosch Corp.)
Steering Wheel Angle Sensor: commonly a potentiometer or an optical
sensor with two sensing elements positioned at 90 degrees to each other. The
potentiometer provides data to the ESC control unit about the direction the
driver intends to steer. When replaced in service, the sensor must be
calibrated with the wheels in the straight ahead position.
Lateral Acceleration Sensor: a solid state sensor that signals the ECS
control unit when the vehicle is being subjected to g-forces in a turn. The
signal voltage varies according to the amount of g-force. 4WD and AWD vehicles
also incorporate a Longitudinal Acceleration Sensor.
Vehicle Rotation (Yaw) Sensor: a solid-state sensor that mimics a
gyroscope to measure the vehicle’s angular velocity around its vertical axis.
The output is usually in degrees per second. The angle between the vehicle's
heading and its actual movement direction is called slip angle, which is
related to the yaw rate. The Yaw and Lateral Acceleration sensors are usually
combined into one centrally mounted unit.
Wheel Speed Sensor: a magnetic sensor and rotor (tone) or in more
recent systems, an active wheel speed sensor, which provides individual wheel
speed data to the ESC control unit. These sensors are also used for ABS
The ESC control
unit continuously monitors the wheel speed sensors to determine wheel
slippage. At the same time the lateral acceleration sensor determines
acceleration g-force and yaw sensors measure vehicle rotation around its
vertical axis. From this data, the control unit calculates the actual movement
of the vehicle about all three axes, comparing it multiple times per second
with the desired direction.
When the ESC control unit detects loss of steering control, it applies the brakes to individual wheels and brings the vehicle back in line with the driver's commanded direction. Some systems also can reduce engine power or operate the transmission to slow down the vehicle. (Courtesy Robert Bosch Corp.)
When the control
unit detects loss of steering control, such as when the vehicle understeers,
oversteers, skids or hydroplanes, and the input values do not correspond
to the actual vehicle direction, the system reacts without any action on the
part of the driver, by reducing engine power and/or applying the brakes on only
the appropriate wheel to restore vehicle stability. The resulting rotary
movement of the vehicle counteracts the skidding condition within the limits of
the vehicle and keeps it on the desired course.
a) Understeer b) Oversteer
a) During an understeer condition, when the front wheels do not follow the trajectory the driver is trying to impose while taking the corner, ESC applies the inside rear brake to prevent the front of the vehicle from plowing forward. b) During an oversteer condition, when the rear wheels do not
track behind the front wheels but instead slide out toward the outside of the
turn, ESC applies the outside front brake to prevent the rear of the vehicle
from spinning out. (Courtesy Insurance Institute for Highway Safety)
Although most ESC systems use similar sensors and hardware, the way systems are programmed to respond varies once a loss of
control is detected. Some ESC systems activate sooner than others or slow a
vehicle more quickly when a driver begins to lose control.
Manufacturers calibrate stability control systems to provide
different safety and handling characteristics in the same way each car has a
unique chassis balance: no two systems are identical. Manufacturers safety calibrate some stability control
systems and provide an early intervention while others allow a subtle, more progressive degree of control that only
intervenes at high cornering forces.
The Plot Thickens
Recently, system co-developer Robert Bosch Corporation
started production of a new version of its Electronic Stability Program (ESP)®
which integrates the lateral acceleration and yaw sensors inside the electronic
control unit, eliminating the need for separate wiring harnesses to connect the
Previously, Bosch placed the electronic control unit of the ESP in the engine compartment, attached directly to the ABS system's
hydraulic modulator. In order to integrate the sensors in the control unit, Bosch adapted the
sensors to the considerably higher ambient temperatures and to the
vibrations that occur when the brake control system intervenes. The result reduced
vibration and optimized balance in a three-point installation of the hydraulic
unit. This installation also ensured that driving on extremely poor roads would
have no influence on system function.
A new version of Bosch’s Electronic Stability Program (ESP)®
integrates the lateral acceleration and yaw sensors inside the electronic
control unit, eliminating the need for separate wiring harnesses to connect the
sensors. (Courtesy of Robert Bosch Corp.)
Another challenge was the lateral-acceleration sensor’s
orientation. Bosch placed existing sensors installed in the vehicle interior precisely at right angles to the direction of travel. By integrating two
acceleration sensors that measure precisely at right angles to each other, the
hydraulic modulator, which must still be installed horizontally, can be
positioned around its vertical axis as desired. Now the ECU calculates the vehicle’s acceleration precisely, based on the known installation location and the
signals from both acceleration sensors. With the precise vehicle acceleration information, the ECU can measure the vehicle's longitudinal acceleration.
Even something simple like a wheel alignment may
require special equipment when
servicing vehicles with ESC. For example, a scan tool may be necessary to perform
the steering angle sensor calibration. If not performed correctly, it can
result in trouble codes being set because the steering angle sensor is reading
that the vehicle is turning even though the front wheels are pointed straight
While ABS is not a new invention, technicians should be aware
that to properly perform a brake fluid flush (either as preventative
maintenance, or as part of a brake service) a scan tool must be used to open
and close the solenoid valves to flush all the old fluid. This is particularly
important in ESC-equipped vehicles as the ESC solenoids operate independently
of brake pedal application.
In a related situation, during routine brake service, carefully
examine wiring harnesses and connectors to determine their condition and proper
routing. If a wiring harness is pulled too tight during suspension movement it
can result in a damaged wire or connector, thus setting a wheel speed sensor
For those still using dynamometers to perform IM 240
emissions tests, it should be noted that vehicles equipped with ESC systems
will not function on a 2-wheel dynamometer. When only the drive wheels spin,
the logic circuit in the traction control module interprets this as
"slippage" (similar to accelerating on ice). The traction control
module will reduce throttle position, making testing impossible.
You don’t need 007’s connections (or his car) to obtain more intelligence on specific ESC systems. You can become a ChiltonPRO or ChiltonDIY subscriber and receive full ESC system service instructions.
A muscle car enthusiast and drag racer, Jim Marotta is a freelance automotive writer with more than 20 years experience in the automotive industry.