Hydraulic Control Assembly - Antilock Brakes: Description and Operation

Fig. 1 Hydraulic Control Unit:

DESCRIPTION
The ABS Hydraulic Control Unit (HCU) is located outside of the engine compartment on a bracket which attaches to the outer frame rail. It consists of:
^ Brake pressure control valve block assembly.
^ Pump motor.
^ Hydraulic control unit reservoir with fluid level indicator assembly.

OPERATION
Operation of hydraulic control unit is as follows:
^ During normal braking, fluid from the brake master cylinder enters the hydraulic control unit through two inlet ports located at the rear of the HCU.
^ The fluid then passes through four normally open inlet valves, one to each wheel.
^ If the ABS control module senses a wheel is about to lock, the ABS control module closes the appropriate inlet valve which closes that valve. This prevents any more fluid from entering the affected brake.
^ The ABS control module then looks at that wheel again. If it is still decelerating, the ABS control module opens the normally closed outlet valve which decreases the pressure trapped in the line.

SERVICING
The following components are serviced separately:
^ Brake pressure control valve block.
^ Pump motor.
^ Hydraulic control unit reservoir.
^ Other than seals and gaskets, no internal parts can be serviced.