Principles of Operation

Principles of Operation Anti-lock Brake System (ABS)

The ABS module executes control of the anti-lock brakes and full speed traction control functions to enhance driver control of the vehicle. The ABS module manages the interactions between the anti-lock, traction control, and engine control systems to optimize the vehicle traction during deceleration and acceleration.

The ABS module continuously monitors and compares the rotational speed of each wheel. The rotational wheel speed is measured by the wheel speed sensor which electrically senses the pole pairs of the magnetic encoder ring passing the sensor head.

The ABS module is self-monitoring. When the ignition switch is turned to the RUN position, the ABS module carries out a preliminary electrical check. At approximately 20 km/h (12 mph) the pump motor is turned on for approximately 0.5 second. Any concern with the ABS causes the ABS module to shut off, the ABS warning indicator to illuminate, and the power assist braking system to function normally.

Traction Control
The traction control system helps maintain vehicle traction at the limits of tire adhesion. System effectiveness varies with vehicle speed, road conditions, and steering inputs.

The ABS module defaults to ON when the engine is started.

The Traction Control Switch (TCS) allows the driver to control the ON/OFF operation of the traction control system by pressing the switch for a minimum of 1 second, independent of the ABS function. The ABS control system cannot be switched off by the driver. The traction control system status is indicated by a traction control system warning light. An illuminated traction control warning light in the instrument cluster indicates that the system has been switched off by the traction control switch, or a system error has been detected and the system function is disabled. A flashing warning light indicates that wheel slip has occurred. The warning light remains flashing until traction control intervention is no longer required.

The ABS module communicates with the Powertrain Control Module (PCM) requesting assistance with traction control. At speeds under 40 km/h (25 mph) the ABS module requests the PCM to reduce engine torque, while simultaneously applying and releasing the appropriate brake to restore traction when one or both drive wheels lose traction and begin to spin. The PCM accomplishes this by minor incremental ignition timing changes and fewer fuel injector pulses until the driven wheel speed returns to normal and the traction control module ends the request. After the vehicle speed exceeds 40 km/h (25 mph), the traction control is accomplished only by the PCM controlling the torque.

During a traction control event you may experience any of the following normal behaviors:
- a rumble or grinding sound much like ABS
- a small deceleration of the vehicle
- the traction control indicator will flash

The ABS module continually monitors all sensors and actuators used to improve the traction control of the vehicle. Some drivers may notice a slight movement of the brake pedal when the system checks itself. If the brake system has not been bled correctly, the brake pedal movement may become more significant. The ABS function continues to work as designed unless the yellow ABS warning indicator is also illuminated. The normal brake function should always occur, unless the red brake warning indicator is illuminated.

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