Principles of Operation

Principles of Operation

The anti-lock brake system (ABS) prevents wheel lockup by automatically modulating the brake pressure during an emergency stop. By not locking the wheels, the driver can improve steering control during hard braking and stop the vehicle in the shortest possible distance under most conditions.

The ABS controls both front and rear brakes separately. The brake pedal force required to engage the ABS function may vary with the road surface condition. A dry surface requires greater force than a slippery surface.

During the ABS operation, the driver may feel a pulsation in the brake pedal, accompanied by a slight up and down movement in the pedal height. In addition, a mechanical noise from the engine compartment may be heard. The pedal effort and pedal feel during normal braking are similar to that of a conventional power brake system.

When the brakes are applied, brake fluid is forced from the brake master cylinder outlet ports to the hydraulic control unit (HCU) inlet ports. The fluid pressure is transmitted through 3 normally open solenoid valves inside the HCU, through the outlet ports of the HCU to the brakes. One circuit of the brake master cylinder feeds the front brakes while the other circuit feeds the rear brakes. If the ABS module senses that a wheel is about to lock, based on wheel speed sensor data, the solenoid valve will pulse closed, preventing more fluid from entering that circuit. The ABS module then reads the sensor signal from the affected wheel again. If the wheel is still decelerating, the normally closed solenoid valve for that circuit is opened. A controlled amount of hydraulic pressure between the normally open valve and the brake is relieved into the HCU accumulator. Once the affected wheel returns to vehicle speed, the ABS module returns the solenoid valves to their normal condition, allowing fluid flow to the affected brake.

The ABS module monitors the electromechanical components of the system. Malfunction of the ABS will cause the ABS module to shut off or inhibit the system. However, normal power-assisted braking remains. Malfunctions are indicated by the yellow ABS warning indicator in the instrument cluster.

The ABS is self-monitoring. When the ignition switch is placed in the RUN position, the ABS module carries out a preliminary self-check on the anti-lock electrical system indicated by a 3-second illumination of the yellow ABS warning indicator in the instrument cluster.

A self-test of the solenoid valve coils and the pump motor is carried out when the vehicle is started. During starting, the brake pedal must be released to run the self-test. If the brake pedal is applied or a failed brake switch occurs, the self-test is run at 6 km/h (4 mph) regardless of brake pedal position.

When installing a new ABS module, the following "as built data" items must be configured into the new module:
^ Transfer case (if equipped)
- 2WD
- manual shift on the fly
- torque on demand
- electronic shift on the fly
^ Rear axle
- light duty
- heavy duty
- wheel base
- tire size

Engine Only Traction Control
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 powertrain control module (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 communication area network (HS-CAN) bus. The PCM uses the wheel speed information to determine when traction control is needed.

The traction assist system can be disabled by pressing the traction assist switch and is indicated by the traction assist indicator light on the switch. The traction assist system will reset and return to normal operation when the ignition switch is cycled or when the traction assist switch is pressed and released a second time during the same ignition cycle.

The traction assist system is contained in the powertrain control module (PCM) and is configured when the PCM is configured. Refer to Information Bus (Module Configuration).