Principles of Operation

Anti-Lock Control

Principles of Operation

Anti-Lock Braking

The ABS module receives wheel speed readings from each wheel speed sensor and processes this information to determine if an ABS event is necessary. The wheel speed sensor electrically senses each tooth of the wheel speed sensor indicators as it passes through the wheel speed sensor magnetic field.

The active wheel speed sensor generates a signal that is sent to the ABS module. The wheel speed sensor circuitry connects to the ABS module through 2 wires and a connector at each wheel speed sensor. When the ignition is turned to the RUN position, the ABS module carries out a self-test by sending a reference voltage to all of the wheel speed sensors through their circuitry to determine if they are functional.

The ABS module continuously monitors and compares the rotational speed of each wheel and when it detects an impending wheel lock, modulates brake pressure to the appropriate brake caliper. This is accomplished by triggering the Hydraulic Control Unit (HCU) to open and close the appropriate solenoid valves. Once the affected wheel returns to normal speed, the ABS module returns the solenoid valves to their normal position, and normal (base) braking resumes.

The HCU has additional internal solenoid valves incorporated to enable control modulation of the wheel brake pressures.

The ABS module is self-monitoring. When the ignition switch is turned to the RUN position, the ABS module will do a preliminary electrical check of the wheel speed sensors by sending voltage through the sensor and checking for the voltage to return. At speeds above 20 km/h (12 mph), without the brake pedal being pressed, the pump motor will be commanded ON for approximately one-half second to check pump motor operation. Also, during all phases of operation the ABS module, with the vehicle in motion, checks for correct operation of the wheel speed sensors. Any malfunction of the ABS will cause the ABS to deactivate. Normal power-assisted braking, however, remains.


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 High Speed Controller Area Network (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 (IC) 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 IC 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 ABS incorporates a strategy called Electronic Brake Distribution (EBD). The EBD strategy uses the HCU as an electronic proportioning valve. On initial application of the brake pedal, full pressure is applied to the rear brakes. The ABS module uses wheel speed input to calculate an estimated rate of deceleration. Once vehicle deceleration exceeds a certain 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.

Some drivers may feel a slight bump sensation in the brake pedal when EBD is active.

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 sensor or HCU faults. When EBD is disabled, the ABS warning indicator will illuminate.


Engine Only Traction Control (EOTC) System Configuration

The Engine Only Traction Control (EOTC) system is contained in the PCM and is configured when the PCM is configured. Refer to Programmable Module Installation (PMI) in Information Bus.