Hydraulic Control Unit (HCU)

The HCU is located in engine compartment. See Figure . HCU contains 2 fluid accumulators and a valve block which consists of inlet and outlet valves. Fluid accumulators, used for primary and secondary brake hydraulic circuits, store brake fluid released when outlet valves are opened during an ABS cycle. Valve block is located on bottom of HCU, just below pump/motor. Valve block consists of input and outlet valves.

If wheel locks, inlet valve is closed to prevent any further brake pressure increase. Outlet valve is then opened to release brake fluid back into fluid accumulators until wheel is released. Once wheel is no longer locked, outlet valve is closed and inlet valve is opened to reapply brake pressure. If wheel is decelerating within predetermined limits, both outlet valve and inlet valve are closed to maintain constant brake pressure.

Pump/motor is mounted on HCU and consists of an electrical motor and a dual piston pump. Pump/motor obtains brake fluid from fluid accumulators to supply pressure during an ABS stop. Pump/motor only operates during an ABS stop and is controlled by the CAB through pump/motor relay.