Anti-Lock Brakes with Traction Control and Stability Assist

Principles of Operation

Anti-Lock Brake System (ABS)
The ABS module manages anti-lock braking, while the ABS module with stability assist manages anti-lock braking, traction control and engine control systems to maintain vehicle control during deceleration and acceleration.

When the ignition switch is in the RUN position, the ABS module does a preliminary electrical check and the hydraulic pump motor is turned on for approximately 1/2 second. Any malfunction of the ABS disables the stability assist and the ABS warning indicator illuminates. However, the power-assist braking system functions normally.

In extreme cold conditions (temperature less than -25°C [-13°F]), the ABS module will briefly operate the hydraulic pump motor when the accelerator is pressed. This is done to warm the brake fluid.

Electronic Brake Distribution (EBD)
The EBD controls the rear brake pressure and acts as an electronic proportioning valve. It is controlled by the ABS module. When EBD is disabled, the ABS warning indicator illuminates.

Stability Assist
With the ignition switch in the START or RUN positions, the ABS module functions similarly to a conventional anti-lock brake system by monitoring and comparing the rotational speed of each wheel. Wheel speeds are measured by the wheel speed sensor, which electrically senses each tooth of the anti-lock sensor ring as it passes through the sensor's magnetic field. When the ABS module detects an impending wheel lock, wheel spin or vehicle motion that is inconsistent with the driver commands, brake pressure is modulated to the appropriate brake caliper(s). The ABS module triggers the hydraulic control unit (HCU) to open and close the appropriate solenoid valves. Once the affected wheel(s) return to the desired speed, the ABS module returns the solenoid valves to their normal position, and normal base brake operation is restored.

The ABS module communicates with the powertrain control module (PCM) to assist with traction control. At lower speeds, when the drive wheels lose traction and begin to spin, the ABS module requests the PCM to reduce engine torque while simultaneously applying and releasing the appropriate brake caliper(s) to maintain 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. At higher speed, the traction control is accomplished only through the PCM torque control.

The stability assist system constantly monitors the vehicle motion relative to the driver's intended course. This is done by using sensors to compare the driver's 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 ABS and stability assist continue to function as usual.

When the stability assist system activates, any of these can occur:
^ A rumble or grinding sound much like ABS or traction control
^ A small deceleration or a reduction in the acceleration of the vehicle
^ The stability assist indicator flashes
^ If the driver's foot is on the brake pedal, a vibration is felt in the pedal much like ABS

Roll Stability Control (RSC)
The roll stability control (RSC) system monitors the roll-rate sensor and existing stability assist sensors and calculates if the vehicle may be approaching a situation where rollover is probable. If such a situation exists, the RSC system applies preemptive action. When activated, the RSC system adjusts the brake torque at specific wheels in response to direct measurement of the vehicle roll motion. By adjusting brake torque, the system can reduce the cornering forces and, therefore, the total roll moment acting on the vehicle. Systems with RSC have an additional roll-rate sensor located within the sensor cluster and additional programming within the ABS module to help control the vehicle during sudden maneuvers.

Information from the roll-rate sensor is fed into the ABS module. The computer uses information from the ABS wheel speed sensors, the throttle position sensor, the steering wheel angle, the steering wheel rate of change and a yaw rate sensor that measures the change in vehicle direction. If the computer determines from all these inputs that conditions exist for a potential roll over, the system applies 1 or more brakes and reduces engine torque to make the vehicle more stable.

When the RSC system is active, the active/fail lamp in the instrument cluster module will flash 2 times per second. If the RSC system is not yet initialized and is not available, the active/fail lamp will flash once every 2 seconds. If a fault is present that prohibits operation of the RSC system, the active/fail lamp will illuminate and SERVICE RSC NOW is displayed in the odometer display and a warning chime will sound. For additional information on the warning chime, refer to Instrument Panel, Gauges and Warning Indicators.

Sensor Cluster
The sensor cluster (also called the accelerometer) consists of the yaw rate sensor, roll rate sensor, lateral accelerometer and longitudinal accelerometer. The lateral accelerometer measures the acceleration which corresponds to the force involved when the vehicle slides sideways. This acceleration has 2 forms. The first is the centrifugal acceleration which 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.

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's front and rear wheels.

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.

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 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.

Stability/Traction Control Switch
The stability/traction control switch is hard-wired to the instrument cluster module. The instrument cluster module communicates the stability/traction control switch status to the ABS module via the high-speed controller area network (HS-CAN).

The stability/traction control switch allows the driver to control use of the stability assist system. This is independent of the ABS function, which cannot be switched OFF by the driver. The stability assist system status is indicated by a light in the stability/traction control switch. When the stability/traction control switch is set to OFF, the stability assist system is disabled at and below 32 km/h (20 mph). The system will automatically restore to full functionality every time the vehicle speed exceeds 32 km/h (20 mph) or the ignition switch is turned OFF and back ON. However, the stability/traction control switch OFF indicator remains illuminated until the switch is pressed. The anti-lock brake function continues to work as designed unless the yellow anti-lock brake warning indicator is also illuminated. Normal braking function always occurs, unless the red brake warning indicator is illuminated.

ABS Module Configuration
The ABS module must be configured when a new ABS module is installed. The calibration procedure must also be carried out.

ABS Module Calibration

NOTE: When installing a new ABS module or HCU, the ABS module must be calibrated. The ABS module must also be calibrated when a new steering wheel rotation sensor has been installed. The installation of a new sensor cluster does not require ABS module calibration.

If a DTC is logged for any component of the stability assist system, the DTC must be cleared before carrying out the calibration procedure. The need to calibrate the ABS module is also indicated by the active/fail lamp on the instrument cluster module flashing. The lamp flashes after clearing the DTCs that are associated with the RSC system. If a DTC is retrieved after calibration, refer to the Anti-Lock Brake System (ABS) Module Diagnostic Trouble Code (DTC) Index. If installing a new ABS module, the calibration procedures, as well as the configuration procedure, must be carried out. For additional information regarding the module configuration and calibration, follow the diagnostic tool directions for the calibration procedure. Diagnostic Trouble Code Tests and Associated Procedures

When carrying out the lateral or longitudinal accelerometer yaw or roll rate calibration using the diagnostic tool, the vehicle must be on a level surface, must not be moving and the ignition switch must be in the RUN position. When carrying out the steering wheel rotation sensor calibration using the diagnostic tool, the ignition switch must be in the RUN position with the engine running and the steering wheel must be rotated back and forth from lock position to lock position.