HCU (Hydraulic Control Unit): Description: Pump/Motor

There are two pump assemblies in the HCU: one for the primary hydraulic circuit, and one for the secondary hydraulic circuit. Both pumps are driven by a common electric motor (Figure ). This DC-type motor is integral to the HCU and is controlled by the CAB.

The pump/motor provides the extra amount of brake fluid needed during anti-lock braking. Brake fluid is released to the accumulators when the outlet valve is opened during an anti-lock stop. The pump mechanism consists of two opposing pistons operated by an eccentric camshaft. In operation, these pistons are used to purge fluid from the accumulators back into the master cylinder circuits. When the anti-lock stop is complete, the pump/motor drains the accumulators.

The pump motor is also used to build pressure when the system goes into traction control mode. For more information, (Refer to TRACTION CONTROL SYSTEM  ).

The CAB may turn on the pump/motor when an anti-lock stop is detected. The pump/motor continues to run during the anti-lock stop and is turned off after the stop is complete. Under some conditions, the pump/motor runs to drain the accumulators during the next drive-off.

The pump/motor is not a serviceable item; if it requires replacement, the HCU must be replaced.