Principles of Operation Anti-Lock Control

Principles of Operation Anti-Lock Control
The ABS module receives wheel speed information from each wheel speed sensor and processes this information to determine if a wheel is approaching a lockup condition. If the ABS module detects an impending wheel lockup, the module will trigger the hydraulic control unit (HCU) to open and close the appropriate solenoid valves to modulate brake pressure to the appropriate brake caliper or wheel cylinder. Once the affected wheel returns to normal speed, the ABS module returns the solenoid valves to their normal position and normal (base) braking resumes.

ABS Module

NOTE: The ABS module is rear wheel drive (RWD) and four wheel drive (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 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 HCU 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 HCU 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 ABS module will be stored as a DTC in the keep-alive memory. The DTCs can be retrieved by following the on-board diagnostic procedures.

Hydraulic Control Unit (HCU)
The HCU has internal solenoid valves incorporated to enable modulation of the wheel brake pressures. The HCU consists of the following components:
^ Brake pressure control valve block
^ Pump motor

The components are not serviced separately.

Electronic Brake Distribution (EBD)
The HCU is used as an electronic proportioning valve for electronic brake distribution (EBD). The HCU controls the brake fluid pressure to the rear brakes to prevent them from locking up during vehicle braking. 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 sensors or the HCU. Wheel speed sensor DTCs and HCU DTCs will disable the EBD function of the HCU. When EBD is disabled, the red brake warning indicator and the ABS warning indicator will illuminate.

Wheel Speed Sensors

NOTE: Any time a wheel speed sensor is removed, thoroughly clean the mounting surfaces. Apply the specified grease to the front wheel speed sensor mounting surfaces when installing.

The ABS 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. 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.

ABS Warning Indicator
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 and, at speeds above 10 km/h (6 mph), the pump motor is turned on for approximately 1/2 second. 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 module, HCU, wheel speed sensors or their related wiring harnesses and the ABS module will set DTCs and illuminate the yellow ABS warning indicator.
The ABS module uses the yellow ABS warning indicator to alert the driver of malfunctions in the ABS module. The ABS module uses the red brake warning indicator when there is a failure of the HCU that will affect the EBD.