Principles of Operation

Principles of Operation
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 three normally open solenoid valves contained inside the HCU and then through the outlet ports of the HCU to each wheel.
^ If the ABS control module senses a wheel is about to 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 control 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 control module returns the valves to their normal condition, allowing fluid to flow to the affected brake.
^ The ABS control 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 control module will carry out a preliminary self-check on the ABS electrical system indicated by a three-second illumination of the yellow ABS warning indicator in the instrument cluster.
^ During vehicle operation, including normal and anti-lock braking, the ABS control 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 one-half 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.

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

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

Anti-Lock Brake System (ABS) Control Module

NOTE: The ABS control module is 4x2 and 4x4 specific. Do not interchange modules.

The ABS control module is an on-board diagnostic, non-repairable unit consisting of the microprocessors and the necessary circuitry for their operation, mounted to the HCU. It communicates with the diagnostic tool over the International Standards Organization (ISO) communication bus, circuit 70 (LB/WH). The ABS control module monitors system operation during normal driving as well as during anti-lock braking.

ABS control 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 control module that open and close the appropriate solenoid valves. This results in moderate pulsations in the brake pedal.
^ The ABS control brake module used in 4x4 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 ABS will be stored as a Diagnostic Trouble Code (DTC) in the keep-alive memory of the ABS control module. The DTCs can be retrieved by 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 ABS uses three 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 indicator rotate past the stationary sensor, a signal proportional to the speed of the rotation is generated and sent to the ABS control module through a twisted cable.