Antilock Brakes / Traction Control Systems: Description and Operation

The anti-lock system (ABS) controls each brake separately. The brake pedal force required to engage the system function may vary with road surface conditions. A dry surface will require a higher force, while a slippery surface will require much less force. During system operation, the driver will sense a slight pulsation in brake pedal accompanied by a rise in brake pedal height and a clicking sound. The pedal effort and pedal feel during normal braking are similar to a conventional power brake system.

When brakes are applied, fluid is forced from the master cylinder outlet ports to the hydraulic control unit (HCU) inlet ports. This pressure is transmitted through four normally open solenoid valves contained inside the HCU, then through outlet ports of the HCU to each wheel. The primary (rear) circuit of the master cylinder feeds the right front and left rear brakes. The secondary (front) circuit feeds the left front and right rear brakes.
When the electronic control unit ECU senses wheel lock conditions, based on wheel speed sensor data, it pulses the normally open solenoid valve closed for that circuit. This prevents any more fluid from entering that circuit. The ECU senses the wheel again, if the wheel is still decelerating the ECU then pulses open the normally closed valve which decreases pressure trapped in line.
The ECU monitors the electromechanical components of the system. Malfunction of the anti-lock brake system will cause the ECU to shut off or inhibit the anti-lock function, while retaining normal power assisted braking. Malfunctions are indicated by one or two warning lamps inside the vehicle.
Loss of hydraulic fluid in the HCU reservoir will disable the anti-lock brake system.
The four wheel anti-lock brake system is self monitoring. When the ignition switch is turned to the RUN position, the ECU will perform a preliminary self check on the anti-lock electrical system indicated by a three or four second illumination of the amber "Check Anti-Lock Brakes" lamp on the instrument cluster, During vehicle operation, includeing normal and anti-lock braking, the ECU monitors all electrical anti-lock functions and some hydraulic system operation.
For most malfunctions of the anti-lock brake system, the amber "Check Anti-Lock Brake" and/or the red "Brake" lamp will be illuminated. The sequence of illumination of these warning lamps, combined with the problem symptoms, can determine the appropriate diagnostic test to perform. Most malfunctions are recored as a coded number in the controller memory, pinpointing the exact component requiring service.