Principles of Operation

Principles of Operation

An anti-lock system (ABS) is intended to prevent wheel lockup during panic stops and in situations where road conditions (wet or icy) may reduce traction. By preventing lockup, the system lessens stopping distance and allows the driver to maintain the steering system (ABS). The ABS prevents wheel lockup by controlling brake fluid pressure.

The ABS module controls fluid pressure by opening and closing four pairs of solenoid operated valves - a dump and isolation valve for each wheel. Under normal braking, fluid from the master cylinder enters the hydraulic control unit (HCU) and exits without pressure system (ABS). If a difference in wheel speed, as measured by the wheel speed sensor, signals an approaching lockup condition, the ABS module will first close off pressure from the master cylinder to the affected wheel. The isolation valve is used for this purpose. If this action does not bring the wheel back from the edge of lockup, the ABS module will open the dump valve, relieving all pressure to the affected wheel. The ABS module will then alternate between the isolation and dump valve to maintain a pressure below lockup. Depending on conditions, the ABS module may pulse the valves up to 18 times per second.

Fluid released from the system by the dump valves accumulates in the reservoirs - one for the front chamber of the master cylinder and one for the rear chamber. This fluid is returned to the master cylinder by the pump. The pump shuts off when wheel speeds return to normal.

The ABS module monitors operation of the system and will turn on the ABS indicator light if a fault is detected. The diagnostic tool is used to display diagnostic test codes recorded by the ABS module. These DTCs are supported by pinpoint tests included in this section.