Brake Actuator

Brake Actuator:

The actuator provides ABS fluid pressure control. It modulates wheel circuit hydraulic pressure to prevent wheel lockup. The actuator is made up of three solenoid valves, a return pump, two accumulator chambers, the pump motor relay, and a solenoid valve relay.

SOLENOID VALVES

The solenoid valves are actuated by the ABS computer when the ABS mode is engaged. These three-position valves accomplish critical ABS hydraulic control. They can, depending upon their switching stage, be in one of three modes. In the pressure build stage, the solenoid is in the open position allowing master cylinder pressure to be applied to the wheel cylinder (as with no ABS system). In the pressure hold mode, wheel cylinder pressure is held constant. In the pressure reduction mode, the solenoid valve allows wheel circuit pressure to be reduced by draining some of the hydraulic fluid into an accumulator chamber where it is temporarily stored until it is returned to the master cylinder.


RETURN PUMP

The return pump is a hydraulic pump driven by an electric motor. When wheel circuit pressure is reduced, the return pump moves brake fluid from the accumulator chambers back to the master cylinder. This return of brake fluid sometimes results in a rising brake pedal.


ACCUMULATOR CHAMBERS

The accumulator chambers temporarily store unneeded brake fluid before returning it to the master cylinder. The accumulator chambers provide a place for the brake fluid which flows from the wheel circuits, following a drop in pressure. The accumulators feed the return pump.