HCU (Hydraulic Control Unit)

HYDRAULIC CONTROL UNIT (HCU)

DESCRIPTION

HYDRAULIC CONTROL UNIT
The Hydraulic Control Unit (HCU) contains the valve block assembly, and pump/motor assembly.

Valve Block Assembly: The valve block assembly contains valves with four inlet valves and four outlet valves. The inlet valves are spring-loaded in the open position and the outlet valves are spring loaded in the closed position. During an antilock stop, these valves are cycled to maintain the proper slip ratio for each wheel. The CAB monitors wheel speeds. If the CAB detects a wheel deceleration that is disproportionate to the other wheels, it will close the inlet valve to that wheel. This prevents any increase in fluid pressure. If the wheel continues to decelerate disproportionately, the CAB opens the outlet valve for that wheel to release fluid pressure from that channel. The released fluid is routed to the accumulators. When the wheel speed is no longer disproportionate to the other wheels, the inlet valve will return to its normally open position and the outlet valve will return to the normally closed position. With a traction control system, there are two additional valves that isolate the master cylinder and rear wheels. During a traction control situation the brakes are applied at a slipping drive wheel to reduce wheel slippage.

Pump Motor Assembly: The pump motor assembly provides the extra amount of fluid needed during antilock braking. The pump is supplied fluid that is released to the accumulators when the outlet valve is opened during an antilock stop. The pump is also used to drain the accumulator circuits after the antilock stop is complete. The pump is operated by an integral electric motor. This motor is controlled by the CAB. The CAB may turn on the pump motor when an antilock stop is required. The pump continues to run during the antilock stop and is turned off after the stop is complete. Under some conditions, the pump/motor will run to drain the accumulators during the next drive off. The CAB monitors the pump/motor operation internally.





The Hydraulic Control Unit (HCU) is mounted to the CAB as part of the ICU (Fig. 20). 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 within the HCU 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 (ABS WITHOUT TRACTION CONTROL)





The hydraulic diagram (Fig. 15) 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 which builds pressure in the brake hydraulic system to engage the brakes and stop the vehicle.

ABS HYDRAULIC CIRCUIT AND SOLENOID VALVE FUNCTION (ABS WITHOUT TRACTION CONTROL)





The hydraulic diagram (Fig. 16) 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.

NORMAL BRAKING HYDRAULIC CIRCUIT, SOLENOID VALVE, AND SHUTTLE VALVE FUNCTION (ABS WITH TRACTION CONTROL)





The hydraulic diagram (Fig. 17) shows a vehicle with traction control in the normal braking mode. The diagram shows no wheel spin or slip occurring relative to the speed of the vehicle. The driver is applying the brake pedal which builds pressure in the brake hydraulic system to engage the brakes and stop the vehicle. The hydraulic shuttle valve closes with every brake pedal application so pressure is not created at the inlet to the pump/motor.

ABS BRAKING HYDRAULIC CIRCUIT, SOLENOID VALVE, AND SHUTTLE VALVE FUNCTION (ABS WITH TRACTION CONTROL)





The hydraulic diagram (Fig. 18) 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 hydraulic shuttle valve closes upon brake application so that the pump/motor cannot siphon brake fluid from the master cylinder.
^ 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.

ABS TRACTION CONTROL HYDRAULIC CIRCUIT, SOLENOID VALVE, AND SHUTTLE VALVE FUNCTION (ABS WITH TRACTION CONTROL)





The hydraulic diagram (Fig. 19) shows the vehicle in the Traction Control (TC) mode. The diagram shows a drive wheel is spinning and brake pressure is required to reduce its speed.
^ The normally open TC (ASR) valve is energized to isolate the brake fluid being pumped from the master cylinder and to isolate the driven wheel.
^ The normally open TC (ASR) valve bypasses the pump output back to the master cylinder at a fixed pressure setting.
^ The normally open and normally closed valves modulate (build/decay) the brake pressure as required to the spinning wheel.