Principles of Operation: Notes

The anti-lock brake system (ABS) module manages anti-lock braking.

When the ignition switch is placed in the RUN position, the ABS module carries out a preliminary electrical check. At approximately 20 km/h (12 mph), the hydraulic pump motor is turned on for approximately 0.5 second. Any malfunction of the ABS disables the ABS and the yellow ABS warning indicator illuminates; however, the power-assist braking system still functions normally.

The ABS module receives wheel speed readings from each wheel speed sensor and processes this information to determine if an ABS event is necessary. The wheel speed sensor electrically senses each tooth of the wheel speed sensor indicators as they pass through the wheel speed sensor magnetic field.

The active wheel speed sensor generates a signal that is sent to the ABS module. The wheel speed sensor circuitry connects to the ABS module through 2 wires and a connector at each wheel speed sensor. When the ignition is turned to the RUN position, the ABS module carries out a self-test by sending a reference voltage to all of the wheel speed sensors through their circuitry to determine if they are functional.

The ABS module continuously monitors and compares the rotational speed of each wheel and when it detects an impending wheel lock, modulates brake pressure to the appropriate brake caliper. This is accomplished by triggering the hydraulic control unit (HCU) to open and close the appropriate solenoid valves. Once the affected wheel returns to normal speed, the ABS module returns the solenoid valves to their normal positions, and normal (base) braking resumes.

The HCU has additional internal solenoid valves incorporated to enable control modulation of the wheel brake pressures.

The ABS module is self-monitoring. When the ignition switch is turned to the RUN position, the ABS module will do a preliminary electrical check, and at above 20 km/h (12 mph) the pump motor is turned ON for approximately 1/2 second. Also, during all phases of operation the ABS module, with the vehicle in motion, checks for correct operation of the wheel speed sensors. Any malfunction of the ABS will cause the ABS to shut off. Normal power assisted braking, however, remains.