Principles of Operation
Anti-Lock Control - Traction Control and Stability Assist
Principles of Operation
ABS with Traction Control, Electronic Stability Control (ESC) and Roll Stability Control (RSC(R))
The ABS module with traction control and Roll Stability Control (RSC(R)) manages anti-lock braking to maintain vehicle control during deceleration and performs additional functions specific to traction control and RSC(R).
ABS Operation
When the ignition switch is in the RUN position, the ABS module does a preliminary electrical check and, at approximately 20 km/h (12 mph), the hydraulic pump motor is turned on for approximately one-half second. During this time a buzzing or humming noise may be heard and a vibration maybe felt in the brake pedal. This is a normal condition. Any malfunction of the ABS disables the stability assist and the ABS warning indicator illuminates. However, the power-assist braking system functions normally.
The ABS module monitors and compares the rotational speed of each wheel. Wheel speeds are measured by the wheel speed sensor, which electrically senses each tooth of the sensor ring as it passes through the magnetic field of the sensor. When the ABS module detects an impending wheel lock, the ABS module commands the hydraulic pump motor on and commands the Hydraulic Control Unit (HCU) to open and close the appropriate solenoid valves to modulate the brake pressure to the individual brake caliper(s). Once the affected wheel(s) return to the desired speed, the ABS module commands the HCU to return the solenoid valves to their normal position and normal base brake operation is restored.
The ABS module must be configured when a new ABS module is installed.
Traction Control
When the drive wheels lose traction and begin to spin, with vehicle speed under 100 km/h (62 mph), the ABS module commands the hydraulic pump motor on and commands the HCU to open and close the appropriate solenoid valves to modulate the brake pressure to the brake caliper(s) while simultaneously sending a request to the PCM over the High Speed Controller Area Network (HS-CAN) bus to reduce engine torque to maintain vehicle traction. The PCM accomplishes this by minor incremental timing changes and fewer fuel injector pulses until the ABS module ends the request. The request ends when the driven wheel speed returns to the desired speed. After the vehicle speed exceeds 100 km/h (62 mph), the traction control is accomplished only through the PCM torque control. The traction control system can be deactivated by pressing the traction control switch in the instrument panel.
When the traction control system activates, any of the following can occur:
- A rumble or grinding sound much like ABS
- A small deceleration or a reduction in the acceleration of the vehicle
- The stability/traction control indicator "sliding-car icon" in the Instrument Panel Cluster (IPC) flashes
- If the driver's foot is on the brake pedal, a vibration can be felt in the pedal much like ABS
Electronic Stability Control (ESC)
The Electronic Stability Control (ESC) system constantly monitors the vehicle motion relative to the intended course. This is done by using sensors to compare the steering input and the yaw rate sensor with that of the actual vehicle motion. The ESC system monitors information from the Throttle Position (TP) sensor, the steering wheel rotation sensor (steering wheel angle and rate of change) and a yaw rate sensor which is integral to the Restraints Control Module (RCM) and measures changes in vehicle direction. If the ESC system determines from all these inputs that the vehicle is unable to travel in the intended direction, the system adjusts the brake torque at specific wheels in response to direct measurement of the vehicle motion and reduces engine torque to allow the vehicle to follow the intended course.
When the ESC system activates, any of the following can occur:
- A rumble or grinding sound much like ABS
- A small deceleration or a reduction in the acceleration of the vehicle
- The stability/traction control indicator "sliding-car icon" in the IPC flashes
- If the driver's foot is on the brake pedal, a vibration can be felt in the pedal much like ABS
Roll Stability Control (RSC(R))
The RSC(R) system constantly monitors the vehicle motion relative to the intended course. This is done by using sensors to compare the steering input and brake application with that of the actual vehicle motion. The system does not activate when the vehicle is traveling in reverse; however, the standard ABS and traction control continue to function as usual. The RSC(R) system monitors information from the wheel speed sensors, the TP sensor, the steering wheel rotation sensor (steering wheel angle and rate of change) and a roll-rate sensor which is integral to the RCM and measures changes in vehicle motion. If the RSC(R) system determines from all these inputs that conditions exist for a potential roll-over event, the system adjusts the brake torque at specific wheels in response to direct measurement of the vehicle roll motion and reduces engine torque to make the vehicle more stable. By adjusting brake torque, the system can reduce the cornering forces and, therefore, the total roll moment acting on the vehicle.
When the RSC(R) system activates, any of the following can occur:
- A rumble or grinding sound much like ABS
- A small deceleration or a reduction in the acceleration of the vehicle
- The stability/traction control indicator "sliding-car icon" in the IPC flashes
- If the driver's foot is on the brake pedal, a vibration can be felt in the pedal much like ABS
Some drivers may notice a slight movement of the brake pedal when the system checks itself. The brake pedal moves when an active test of the brake booster is run. During this test a small amount of pressure is generated at the master cylinder, but no pressure is generated in the brake calipers.
Traction Control Switch
The traction control switch is hardwired to the IPC. The IPC communicates switch status to the ABS module via the HS-CAN bus. The traction control switch allows the driver to control use of the traction control system. This is independent of the ABS, ESC and RSC(R) functions, which cannot be switched off by the driver. There are 2 separate sliding car icons; the stability/traction control indicator "sliding car icon" and the stability/traction control indicator "sliding-car OFF icon". Traction control system status is indicated by the stability/traction control indicator "sliding-car icon" in the IPC. The stability/traction control indicator "sliding car OFF icon" is used to alert the driver that the traction control portion of the AdvanceTrac(R) system has been disabled by the driver. This icon only illuminates solidly and does not flash. The system automatically restores to full functionality when the ignition switch is cycled from OFF to RUN.
Electronic Brake Distribution (EBD)
On initial application of the brake pedal, full pressure is applied to the rear brakes. The ABS module then uses wheel speed input to calculate an estimated rate of deceleration. Once vehicle deceleration exceeds a predetermined threshold, the ABS module closes the appropriate isolation valves in the HCU to hold the rear brake pressure constant while allowing the front brake pressure to build. This creates a balanced braking condition between the front and rear wheels and minimizes the chance of rear wheel lockup during hard braking. As the vehicle decelerates, the valves are opened to increase the rear brake pressure in proportion to the front brake pressure.
A slight bump sensation may be felt in the brake pedal when Electronic Brake Distribution (EBD) is active.
EBD is disabled if DTCs are present for 2 or more wheel speed sensors. When EBD is disabled, the red brake warning indicator, the yellow ABS warning indicator and the stability/traction control indicator "sliding-car icon" are illuminated.
Stability Control Sensors
The stability control sensors for the ESC and RSC(R) systems consist of the yaw rate sensor, roll rate sensor, lateral accelerometer and longitudinal accelerometer. The sensors are housed in the RCM which sends sensor information to the ABS module over a dedicated Controller Area Network (CAN) bus. If any of the sensors are defective, a new RCM must be installed.
- The yaw rate sensor measures the relative vehicle motion about the vertical axis through its center of gravity.
- The roll rate sensor measures the rate of rotation along the front to rear horizontal axis.
- The lateral accelerometer measures the acceleration that corresponds to the force involved when the vehicle moves sideways.
- The longitudinal accelerometer measures the acceleration corresponding to the force involved when the vehicle moves forward and rearward in the horizontal plane, along the centerline of the vehicle front and rear wheels.
Lateral acceleration has 2 forms. The first is the centrifugal acceleration that is generated when the vehicle travels around in a circle. The second is the acceleration due to gravity. On level ground there is no contribution from this acceleration. However, if the vehicle is parked sideways on a bank or incline, the sensor measures some lateral acceleration due to gravity, even though the vehicle is not moving.
Steering Wheel Rotation Sensor
The steering wheel rotation sensor measures the rate of rotation of the steering wheel by monitoring the steering wheel rotation ring as it passes through the sensor gap. The steering wheel rotation sensor uses 2 signal lines to transmit information to the ABS module about whether the steering wheel is being turned left or right and how far it is being turned. The steering wheel rotation sensor does not indicate the absolute position of the steering wheel relative to straight-ahead. The stability assist system learns this position by comparing the steering wheel position with other signals and storing the position it has learned. The stability assist system confirms this position and modifies it as necessary during every new driving cycle.
ABS Module Calibration
When a new ABS module, HCU, RCM or steering wheel rotation sensor is installed, the ABS module must be calibrated. The calibration procedure is required for the ABS module to learn the "zero-position" of the various ESC/RSC(R) sensors and components.
If a DTC is logged for any component of the ESC/RSC(R) system, the DTC must be cleared or repaired before carrying out the calibration procedure. The need to calibrate the ABS module is also indicated by the stability/traction control indicator "sliding-car icon" in the IPC flashing once every 2 seconds. The indicator flashes after clearing the DTCs that are associated with the ESC/RSC(R) systems.
To calibrate the ABS module, carry out the IVD Initialization sequence using the scan tool. When carrying out the calibration procedure, the vehicle must be on a level surface and must not be moving. If a DTC is retrieved after calibration, refer to the ABS Module DTC Chart. Anti-Lock Control - Traction Control and Stability Assist