Principles of Operation

Principles of Operation

Anti-Lock Brake System (ABS)
The anti-lock brake system (ABS) operates as follows:
^ When the brakes are applied, fluid is forced from the brake master cylinder outlet ports to the hydraulic control unit (HCU) inlet ports. This pressure is transmitted through 3 normally open solenoid valves contained inside the HCU and then through the outlet ports of the HCU to each wheel.
^ If the ABS module senses a pending wheel lock, based on wheel speed sensor data, it closes the normally open solenoid valve for that circuit. This prevents any more fluid from entering that circuit.
^ The ABS module then checks the wheel speed sensor signal from the affected wheel(s) again.
^ If that wheel(s) is still decelerating, it opens the closed solenoid valve for that circuit.
^ Once the affected wheel comes back up to speed, the ABS module returns the valves to their normal condition, allowing fluid to flow to the affected brake.
^ The ABS module monitors the electromechanical components of the system.
^ A malfunction in the ABS will cause the ABS control module to shut off or inhibit the system. However, normal power-assisted braking remains.
^ Malfunctions are indicated by the yellow ABS warning indicator, the red BRAKE warning indicator, or both warning indicators which are located in the instrument cluster.
^ The ABS is self-monitoring, When the ignition switch is turned to the RUN position, the ABS module will carry out a preliminary self-check on the ABS electrical system indicated by a 3 second illumination of the yellow ABS warning indicator in the instrument cluster.
^ During vehicle operation, including normal and antilock braking, the ABS module monitors all electrical anti-lock functions and some hydraulic operations.
^ The first time the ignition switch is turned to the ON position and the vehicle is driven to approximately 10 km/h (6.25 mph), the ABS control module turns on the pump motor for approximately 1/2 second. At this time, a mechanical noise may be heard. This is a normal function of the self-check by the ABS control module.
^ Pedal pulsation coupled with noise while braking on loose gravel, bumps, wet or snowy roads is normal and indicates correct functioning of the ABS control system.

Electronic Brake Distribution (EBD)
The electronic brake distribution (EBD) controls rear brake pressure and acts as an electronic proportioning valve. The EBD is integral to the ABS module and is not repaired separately. Depending on the coherency of the sensor inputs received and the existence of internal faults, the ABS module may set a DTC(s) and disable EBD illuminating the red brake warning indicator.

Warning Lamp Indicators
The anti-lock brake system (ABS) uses the yellow ABS warning indicator to, alert the driver of malfunctions in the anti-lock brake system.

The yellow ABS warning indicator will come on for numerous reasons. It warns the driver that the anti-lock brake system has been deactivated due to a symptom that exists in the system. Normal power assist braking remains, but wheels can lock during a panic stop while the yellow ABS warning indicator is illuminated.

The diagnostic procedures must be followed step-by-step in order to correct the condition.

Hydraulic Control Unit (HCU)
The HCU consists of the following components:
^ Brake pressure control valve block
^ Pump motor

A new brake pressure control valve block and pump motor are installed as an assembly.

Anti-Lock Brake System (ABS) Module

Note:
^ The ABS module is 2WD and 4WD specific. Do not interchange modules.

The ABS module is an on-board diagnostic, non-repairable unit consisting of the microprocessors and the necessary circuitry for their operation and is mounted to the HCU. The ABS module communicates with the NGS Tester over the international standards organization (ISO) communication bus, circuit (LB/W). The ABS module monitors system operation during normal driving as well as during anti-lock braking.

ABS module operation is as follows:
^ Under normal driving conditions, the microprocessor produces short test pulses to the solenoid valves that check the electrical system without any mechanical reaction.
^ Impending wheel lock conditions trigger signals from the ABS module that open and close the appropriate solenoid valves. This results in moderate pulsations in the brake pedal.
^ The ABS module used in 4WD application includes a G-sensor. It detects vehicle movements during a brake lockup event that is transferred to other wheels through the powertrain.

During normal braking, the brake pedal feel will be identical to a standard brake system.

Most faults that occur in the anti-lock brake system will be stored as a diagnostic trouble code (DTC) in the keep-alive memory of the ABS module. The DTCs can be retrieved by following the on-board diagnostic procedures.

Wheel Speed Sensor

Note:
^ Any time a wheel speed sensor is removed, thoroughly clean the mounting surfaces. Apply high -temperature grease to the front wheel speed sensors when installing.

The anti-lock brake system uses 3 variable-reluctance sensors to determine vehicle speed. The wheel speed sensors operate on a magnetic induction principle. As the teeth on the wheel speed sensor ring rotate past the stationary sensor, a signal proportional to the speed of the rotation is generated and sent to the ABS module.