Anti-Lock Braking

The ABS module uses a Hydraulic Control Unit (HCU) to prevent the wheels from locking up during a braking event. The HCU contains several solenoid-controlled valves, a pump motor, an accumulator and several sensors. The fluid pressure is transmitted through 4 normally open inlet valves inside the HCU, through the outlet ports of the HCU and finally to the brake calipers. 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 inlet 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 outlet valve for that circuit is opened. This allows a controlled amount of hydraulic pressure in the brake caliper to be relieved into the HCU accumulator. Once the speed of the affected wheel matches the speeds of the other wheels, the ABS module returns the valves to their normal condition, allowing fluid flow to the affected brake caliper.

The ABS module is self-monitoring and will carry out self-tests after the ignition key is turned to the RUN position, after an ABS stop and after an AdvanceTrac® system activation. When the ignition switch is turned to the RUN position, the ABS module will carry out a preliminary electrical check of the wheel speed sensors and their circuits by sending voltage through the sensor and checking for the voltage to return. When the vehicle is traveling at speeds above 21 km/h (13 mph) and the brake pedal is not being pressed, the HCU pump motor will be commanded ON for approximately 0.5 second to check pump motor operation. Also, during all phases of operation while the vehicle is in motion, the ABS module checks for correct operation of the wheel speed sensors by comparing wheel speed input to other sensor input. Depending on the nature of the malfunction detected, the ABS module will deactivate the anti-lock control system and send a message over the High Speed Controller Area Network (HS-CAN) bus to the Instrument Cluster (IC). When the IC receives this message, it will illuminate the yellow ABS warning indicator and/or (if equipped) the stability/traction control message center indicator (sliding-car icon).