HCU (Hydraulic Control Unit)

HCU (HYDRAULIC CONTROL UNIT)

DESCRIPTION





The Hydraulic Control Unit (HCU) is mounted to the CAB as part of the ICU. The HCU controls the flow of brake fluid to the brakes using a series of valves and accumulators. A pump/motor is mounted on the HCU to supply build pressure to the brakes during an ABS stop.

OPERATION - HYDRAULIC CIRCUITS AND VALVES
The hydraulic fluid control valves control the flow of pressurized brake fluid to the wheel brakes during the different modes of ABS braking. The following paragraphs explain how this works. For purposes of explanation only, it is assumed that only the right front wheel is experiencing antilock braking; the following diagrams show only the right front wheel in an antilock braking operation.

NORMAL BRAKING HYDRAULIC CIRCUIT AND SOLENOID VALVE FUNCTION





The hydraulic diagram shows the vehicle in the normal braking mode of the base brake hydraulic system. The diagram shows no wheel spin or slip occurring relative to the speed of the vehicle. The driver is applying the brake pedal; this builds pressure in the brake hydraulic system to engage the brakes and stop the vehicle.

ABS HYDRAULIC CIRCUIT AND SOLENOID VALVE FUNCTION





The hydraulic diagram shows the vehicle in the ABS braking mode. The diagram shows one wheel is slipping because the driver is attempting to stop the vehicle at a faster rate than is allowed by the surface on which the tires are riding.
^ The normally open and normally closed valves modulate (build/decay) the brake hydraulic pressure as required.
^ The pump/motor is switched on so that the brake fluid from the low pressure accumulators is returned to the master cylinder circuits.
^ The brake fluid is routed to either the master cylinder or the wheel brake depending on the position of the normally open valve.