Principles of Operation

Principles of Operation

ABS
The ABS module manages anti-lock braking to maintain vehicle control during deceleration.

When the ignition switch is turned to the RUN position, the ABS module does a preliminary electrical check of the wheel speed sensors and their circuits. At approximately 20 km/h (12 mph), the hydraulic pump motor is turned on for approximately 1/2 second. During this time a buzzing or humming noise may be heard and a vibration may be felt in the brake pedal. This is a normal condition. If a malfunction is detected, the system is disabled and the ABS warning indicator illuminates. However, the base 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. This generates a square-wave signal that is sent to the ABS module. The frequency of the square-wave signal will increase or decrease as wheel speed increases or decreases. The ABS module compares all 4 wheel speed sensor signals to each other. The vehicle speed signal it receives over the high-speed controller area network (HS-CAN) bus and the brake pedal input to determine if a wheel lock event is occurring. 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.

Engine Only Traction Control (EOTC)
Some vehicles may be equipped with an engine only traction control (EOTC) system. In order to achieve maximum traction, the EOTC system is designed to limit wheel spin by modulating engine torque when driving on slippery or loose surfaces. Engine torque modulation is achieved by regulating fuel and spark to the cylinders. This means that the traction control system is controlled by the PCM and not the ABS module. The only function of the ABS module in an EOTC system is to provide wheel speed information to the PCM along the HS-CAN bus. The PCM uses the wheel speed information to determine when traction control is needed. The traction control system indicator in the instrument cluster will flash during a traction control event and will illuminate without flashing if the system is inoperative due to DTCs or some other concern.

The traction control system can be disabled by pressing the traction control switch. Traction control system status is indicated by the traction control indicator in the instrument cluster being illuminated and the message center (if equipped) displaying Traction Control Off. The traction control system will reset and return to normal operation when the ignition switch is cycled or when the traction control switch is pressed and released a second time during the same ignition cycle.

Electronic Brake Distribution (EBD)
The HCU is used as an electronic proportioning valve for electronic brake distribution (EBD). The HCU controls the brake fluid pressure to the rear brakes to prevent them from locking up during vehicle braking. If anti-lock braking is disabled due to DTCs being present in the ABS module, EBD will continue to function unless the DTCs are for wheel speed sensors or the HCU. Wheel speed sensor DTCs and HCU DTCs will disable the EBD function of the HCU. When EBD is disabled, the red brake warning indicator and the ABS warning indicator will illuminate and appropriate DTCs will be stored in the ABS module.

ABS Module Configuration
ABS module configuration is not required on this vehicle.

Traction Control System Configuration
The traction control system is contained in the PCM and is configured when the PCM is configured. Refer to Programmable Module Installation in Information Bus (Module Configuration).