Anti-Lock Braking

WARNING: This page is about a different car, the 2012 Ford Pickup. However, it is still accessible from the selected car via links, so may be relevant.

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 pulses 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 position, allowing fluid flow to the affected brake caliper.

The ABS module is self-monitoring and carries out self-tests after the ignition is turned to the RUN position, after an ABS stop and after an AdvanceTrac® system activation. When the ignition is turned to the RUN position, the ABS module carries 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 is 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 deactivates the anti-lock control system and sends a message over the High Speed Controller Area Network (HS-CAN) to the Instrument Panel Cluster (IPC). When the IPC receives this message, it illuminates the yellow ABS warning indicator and the stability/traction control indicator (sliding car icon).