General Information

Hydraulic Control Unit:

PURPOSE
The Hydraulic Control Unit (HCU) is located on the right side of the vehicle, mounted to the front suspension. The HCU is mounted to the front suspension crossmember using a mounting bracket attached to the crossmember, which the HCU is mounted to using 3 isolation bushings. The HCU contains the following components for controlling brakes during Antilock Brake System (ABS) braking: 4 Decay Valves, 4 Shuttle Valves, 2 Fluid Sumps, a Pump/Motor and a relay box. Also attached to the HCU are the rear brake Proportioning Valves and the vehicles 6 hydraulic brake tubes.

Decay Solenoids
There are 4 decay solenoids, one for each wheel. In the released position they provide a fluid path from the master cylinder to the wheel brakes of the vehicle. In the actuated (decay) position, they provide a fluid path from wheel brakes of the vehicle to the sumps. The Decay solenoids are spring loaded in the released (build) position during normal braking.

Shuttle Valves
There are 4 Shuttle Valves, one for each wheel. The Shuttle Valve is a hydraulically actuated valve which shuttles when the decay solenoid and pump are energized. This places an orifice (restriction) in the line between the pump and the decay solenoid. This restriction provides a controlled build rate to each wheel brake during an ABS stop. The Shuttle Valve will remain in the orificed position until the ABS cycle is complete. When the ABS cycle has been completed the decay solenoids will return to their released position which will equalize the pressure across the Shuttle Valves. When the pressure equalizes, the spring loaded Shuttle Orifice valves will return to the unrestricted position.

Fluid Sumps
There are two fluid sumps in the HCU, one each for the primary and secondary hydraulic circuits. The fluid sumps temporarily store brake fluid that is decayed from the wheel brakes during an ABS cycle. This fluid is then delivered to the pump to provide build pressure. The typical pressure in the sumps is 50 psi, during ABS operation only.

Pump Motor Assembly
The HCU contains 2 Pump Assemblies, one each for the primary and secondary hydraulic circuits. Both pumps are driven by a common electric motor which is part of the HCU. The pumps pick up fluid from the sumps to supply build pressure to the brakes during an ABS stop. The motor only runs during an ABS stop and is controlled by the Controller Antilock Brake (CAB) via the Pump/Motor Relay. The Pump/Motor Assembly is not a serviceable item. If it requires service the HCU must be replaced.

Relay Box
ABX-4 utilizes two relays contained in a relay box mounted to the HCU. The relay box contains a System Relay and a Pump/Motor Relay. A single 10-way connector provides the electrical interface. The relay box is serviceable as an assembly.