Anti-Lock Control: Notes

WARNING: This page is about a different car, the 2008 Mercury Mariner and 2008 Ford Escape. However, it is still accessible from the selected car via links, so may be relevant.

The anti-lock control system is controlled by the ABS module which continuously monitors and compares the rotational speed of 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 speed sensor tone rings pass through the magnetic field generated by the active wheel speed sensor. This causes the strength of the magnetic field to increase (as a tooth passes the sensor) or decrease (as a gap passes the sensor) and generates a square-wave signal that is sent to the ABS module to indicate individual wheel speed.

When the ABS module detects an impending wheel lock-up, brake fluid pressure to the appropriate brake caliper will be modulated (pulsated). This is accomplished by opening and closing the appropriate solenoid valves inside the Hydraulic Control Unit (HCU) while the hydraulic pump motor is also actuated. Once the affected wheel returns to normal speed, the ABS module returns the solenoid valves to their normal position and the hydraulic pump motor is deactivated.

The ABS module is self-monitoring and will carry out 4 self-tests at pre-determined times. 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. At the same time, the hydraulic pump motor is turned on for approximately 0.5 second and the HCU valves are actuated to determine operational status of the HCU. When the vehicle is traveling at speeds above 20 km/h (13 mph), and the brake pedal is not being pressed, the pump motor will be commanded ON again for approximately 0.5 second to check pump motor operation and control circuits. The fourth test occurs at 71 km/h (44 mph) and is used to condition the brake fluid inside the HCU. During these tests a small amount of vibration and noise is generated by the pump motor and transmitted to the brake and accelerator pedals, but no pressure is generated at the brake calipers. Some drivers may notice a little noise and slight movement of the brake and accelerator pedals during these tests. This is the normal operation of the system and no service is required if these symptoms are reported.

Additionally, 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. If a malfunction is detected (during normal operation or during a self-test), the ABS module will deactivate the anti-lock control system and will also 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 the stability/traction control indicator "sliding-car icon".