The Anti-Lock Brake System (ABS) prevents wheel lockup by automatically modulating the brake pressure during an emergency stop. By not locking the wheels, the driver can improve steering control during hard braking and stop the vehicle in the shortest possible distance under most conditions.
The ABS controls both front and rear brakes separately. The brake pedal force required to engage the ABS function may vary with the road surface conditions. A dry surface requires a higher force, while a slippery surface requires much less force.
During the ABS operation, the driver will sense a slight pulsation in the brake pedal. In addition, a mechanical noise from the engine compartment may be heard. The pedal effort and pedal feel during normal braking are similar to that of a conventional power brake system.
Hydraulic Control Unit (HCU)
The Hydraulic Control Unit (HCU) consists of a valve body assembly, pump and a motor assembly. During normal braking, fluid from the brake master cylinder enters the HCU through two inlet ports located on the bottom of the HCU. The fluid then passes through four normally open Isolation (ISO) valves, one to each of the four wheels. If the anti-lock brake control module senses that a wheel is about to lock, the anti-lock brake control module pulses the appropriate isolation valve, which in turn closes that valve. This prevents any more fluid from entering the effected brake. The anti-lock brake control module then looks at that wheel again. If it is still decelerating, the anti-lock brake control module pulses the normally closed dump valve, which decreases the pressure trapped in the line.
Anti-Lock Brake Control Module
The anti-lock brake control module is a non-repairable, on-board diagnostic unit consisting of one microprocessor and the necessary circuitry for its operation. The two primary functions of the anti-lock brake control module are to control the HCU during ABS operation and to monitor the system during normal driving, as well as during anti-lock braking.
Under normal braking conditions, the microprocessor produces short test pulses to check the electrical system without any mechanical reaction. The anti-lock brake control module continuously monitors the speed of each wheel to determine if any wheel is beginning to lock. When a wheel locking tendency is detected, the anti-lock brake control module signals the appropriate isolation valve and/or dump valve in the HCU to open or close as well as the pump motor to turn on and return brake fluid back to the foundation braking system.
Most faults that occur in the ABS will be stored as a Diagnostic Trouble Code (DTC) in the anti-lock brake control module. Once a DTC is stored, it will be retained by the anti-lock brake control module even with the key in the OFF position. DTCs can be retrieved by following the on-board diagnostics procedures.
Anti-Lock Brake Sensor
The ABS uses four variable-reluctance sensors and toothed anti-lock sensor indicators to determine the rotational speed of each wheel. The sensors operate on the magnetic induction principle. As the teeth on the indicator rotate past the stationary sensor, a signal proportional to the speed of the rotation is generated and sent to the anti-lock brake control module through a twisted pair wire cable and wiring harness.
The anti-lock brake control module monitors the electromechanical components of the system. Malfunction of the ABS will cause the anti-lock brake control module to shut off or inhibit the system. However, normal power assisted braking remains. Malfunctions are indicated by the yellow ABS warning indicator inside the vehicle.
The ABS is self-monitoring. When the ignition switch is placed in the RUN position, the anti-lock brake control module will perform a preliminary self-check on the anti-lock electrical system indicated by a two to three second illumination of the yellow ABS warning indicator in the instrument cluster. At 6.4 km/h (4 mph) , the valves and motor will be powered briefly to verify proper function. During vehicle operation, including normal and anti-lock braking, the anti-lock brake control module monitors all electrical anti-lock functions and some hydraulic operations.