Principles of Operation

Principles of Operation
The anti-lock brake system (ABS) electronic control module manages the anti-lock braking, roll stability control (RSC) and traction assist systems to help maintain vehicle control during acceleration, deceleration and certain vehicle maneuvers. The ABS module also communicates with the powertrain control module (PCM) along the high speed communication area network (HS-CAN) bus for traction assist operations.

When the ignition switch is in the RUN position, the ABS module initializes and carries out a preliminary electrical check and the hydraulic pump motor is turned on for approximately 1/2-second. Any malfunction observed during this test disables the traction assist and roll stability control system and illuminates the ABS warning indicator. However, the power-assist braking system functions normally.

The ABS warning indicator will also illuminate during an ABS event and during a stability control event.

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 electronic brake distribution (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.

Traction Assist
With the ignition switch in the RUN position, 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 tone ring as it passes through the magnetic field produced by the wheel speed sensor. When the ABS module detects an impending wheel spin 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 maintaining vehicle traction. 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.

Roll Stability Control (RSC)
The ABS module monitors the roll-rate sensor and existing stability control sensors and calculates if the vehicle may be approaching a situation where rollover is probable. If such a situation exists, the ABS module activates the roll stability control (RSC) system for preemptive action. When activated, the ABS module 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 motion acting on the vehicle. Vehicles 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 module uses information from the wheel speed sensors, throttle position sensor, steering wheel angle, the steering wheel rate of change and a yaw rate sensor that measures the change in vehicle direction. If the ABS module determines from all these inputs that conditions exist for a potential rollover, the system applies the brakes to one or more wheels and reduces engine torque to make the vehicle more stable.

When the ABS module activates the RSC system, the stability lamp in the instrument cluster will flash 2 times per second. If the RSC system is not yet initialized and is not available, the stability lamp will flash once every 2 seconds. If a fault is present in the ABS module that prohibits operation of the RSC system, the stability lamp will illuminate and SERVICE RSC NOW is displayed in the odometer display (if equipped) and a warning chime will sound.

The ABS module also 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 RSC system does not activate when the vehicle is traveling in reverse; however, the ABS and traction assist systems continue to function as usual.

When the ABS module activates the RSC system, any of these can occur:
^ A rumble or grinding sound much like the ABS or traction assist.
^ A small reduction in the acceleration or speed 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 the ABS.

Stability Control Sensor Cluster
The stability control sensor cluster 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 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 the high speed communication area network (HS-CAN) bus 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.

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 HS-CAN bus.

The stability/traction control switch allows the driver to control use of the stability control/traction assist system. This is independent of the ABS function, which cannot be switched OFF by the driver. The stability control/traction 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 system is disabled below 60 km/h (37 mph). The system will automatically restore to full functionality every time the vehicle speed exceeds 70 km/h (43 mph) or when the ignition switch is turned to the OFF position and back to the ON position. 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 installation of a new stability control sensor cluster does not require ABS module calibration.

If a diagnostic trouble code (DTC) is set for any component of the stability control system, the DTC must be cleared before carrying out the calibration procedure. The need to calibrate the ABS module is also indicated by the ABS warning indicator on the instrument cluster module flashing once every 2 seconds. The warning indicator 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) Chart. If installing a new ABS module, the calibration procedures, as well as the configuration procedure, must be carried out. For information regarding the module configuration and calibration, follow the scan tool directions for the calibration procedure. Anti-Lock Brake System (ABS) Module Diagnostic Trouble Code (DTC) Chart

Stability Control Sensor Cluster Calibration
When carrying out the stability control sensor cluster calibration using the scan tool, the vehicle must be on a level surface, must not be moving and the ignition switch must be in the RUN position.