Anti-Lock Braking: Notes

The ABS module continuously monitors and compares the rotational speed of each wheel. This is accomplished through the use of 4 active wheel speed sensors, 1 for each wheel. The wheel speed sensors are connected to the ABS module by 2 circuits. One circuit provides voltage for sensor operation and the other circuit provides sensor input to the ABS module. As the wheels spin, the wheel bearing magnetic strip passes through the magnetic field generated by the active wheel speed sensor. The active wheel speed sensor sends a square wave signal to the ABS module to indicate individual wheel speed.

The ABS module uses a Hydraulic Control Unit (HCU) to isolate and modulate hydraulic pressure to the caliper(s) and wheel cylinder(s). The HCU contains several solenoid-controlled valves, a pump motor, an accumulator and several sensors. The fluid flows through 4 normally open inlet valves inside the HCU, and through the outlet ports of the HCU to the brake calipers and wheel cylinders. If the ABS module senses that a wheel is nearing a lockup condition based on wheel speed sensor data and deceleration algorithms, the module closes the normally open inlet valve for that wheel. This isolates the corresponding caliper or wheel cylinder. The ABS module reads the wheel speed sensor signal from the affected wheel again. If the wheel is still decelerating (locking up), the module opens the normally closed outlet valve allowing a controlled amount of hydraulic pressure in the brake caliper to be relieved into the HCU accumulator. The ABS pump motor runs during an ABS braking event so sufficient brake fluid pressure is consistently available to the HCU valves. This procedure of closing and opening valves is repeated several times per second until the speed of the affected wheel matches the speeds of the other wheels. Once this has been accomplished, the ABS module returns the solenoid valves to their normal position.

The ABS module is self-monitoring. When the ignition is turned to the RUN position, the ABS module carries out a preliminary electrical check of the wheel speed sensor circuitry by sending voltage through the sensor and checking for the voltage to return. At speeds above 20 km/h (12 mph), without the brake pedal being pressed, the pump motor is commanded on for approximately one-half 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. In the event the ABS module detects a concern, the module sets a DTC and sends a message to the Instrument Panel Cluster (IPC) over the High Speed Controller Area Network (HS-CAN) to illuminate the yellow ABS warning indicator and/or the stability/traction control indicator (sliding-car icon). If the concern is severe enough, ABS may be disabled. Normal power-assisted braking, however, remains.