Hydraulic Control Unit
Hydraulic Control UnitThe hydraulic control unit (HCU) contains the valve block assembly, the pump/motor assembly, and a fluid reservoir with a fluid level switch. On the LH the HCU is located on the left frame rail, forward of the master cylinder.
There are two hydraulic control units. One is used for ABS only. The other is used for ABS and LTCS combined. Each has a different part number. If the wrong HCU is installed, the system operation will be disabled and a trouble code will be stored.
Mark IV (LTCS) System
Valve Block Assembly: The valve block assembly contains 10 valve/solenoids with 4 inlet valves and 4 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 channel. If a wheel locks, the inlet valve is closed to prevent any further pressure increase. Then the outlet valve is opened to release the pressure back to the reservoir until the wheel is no longer slipping. Once the wheel is no longer slipping, the outlet valve is closed and the inlet valve is opened to reapply pressure. If the wheel is decelerating within its predetermined limits (proper slip ratio), both valves will close to hold the pressure constant. During the stop, the build pressure is initially supplied by the master cylinder. When the build pressure is reapplied, fluid comes from the master cylinder, which causes the brake pedal to drop. The pedal continues to drop until the pedal travel sensor signals the CAB to turn on the pump/motor. The pump/motor then pumps fluid from the reservoir into the brake system. The master cylinder pistons and brake pedal are then moved back until the pedal travel sensor signals the CAB to turn off the pump/motor.
On vehicles equipped with traction control, the valve block assembly also contains the 2 traction control (isolation) valves. The valves are spring-loaded in the open position. During traction control operation, the traction control valves are closed by the CAB. This hydraulically isolates the front brake channels from the master cylinder. The CAB activates the motor/pump by grounding the motor/pump relay coil. By opening and closing the inlet and outlet valves, the CAB can apply and release the drive wheel calipers as needed to control wheel spin. Two pressure relief valves allow unused fluid volume/pressure to return to the reservoir. The relief valves are needed because the pump can supply more volume than the system requires.
NOTE: A unique brake bleeding procedure, using the DRBIII, must be performed whenever any of the brake tube connections to the HCU are opened.
Proportioning Valves: The proportioning valves used in this system are attached to the rear outlet ports of the hydraulic control unit (HCU). These valves limit pressure to the rear brakes to improve front to rear brake balance during normal braking.
Pump/Motor Assembly: The pump/motor assembly consists of an electric motor with a rotation sensor and two pumps. The motor shaft inside the pump is designed as a cam in order to drive both pumps. The rotation sensor is an inductive pick-up used by the CAB to monitor pump/motor operation.
Mark IVG System
The hydraulic control unit (HCU) contains the valve block assembly, 4 accumulators, and the pump/motor assembly.
Valve Block Assembly: The valve block assembly contains valve/solenoids with 4 inlet valves and 4 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 channel. If a wheel locks, the inlet valve is closed to prevent any further pressure increase. Then the outlet valve is opened to release the pressure to the accumulators until the wheel is no longer slipping. Once the wheel is no longer slipping, the outlet valve is closed and the inlet valve is opened to reapply pressure. If the wheel is decelerating within its predetermined limits (proper slip ratio), both valves will close to hold the pressure constant.
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 DC-type motor is controlled by the CAB. The CAB turns on the motor through the pump/motor relay when an antilock stop is detected. The pump continues to run during the antilock stop and is turned off approximately 3-5 seconds after the stop is complete. The pump mechanism consists of two opposing pistons operated by an eccentric cam. One piston supplies the primary hydraulic circuit. The opposing piston supplies the secondary hydraulic circuit. In operation, one piston draws fluid from the accumulators. The opposing piston pumps fluid to the valve body solenoids. The CAB monitors the pump/motor operation using a rotation sensor.
Accumulators: The accumulators provide temporary fluid storage during an antilock stop and are drained by the pump/motor. The 6mm accumulator is used for the front brakes, the 3mm accumulator is used for the rear brakes, the 1mm accumulator is used for the lip seal saver, and the 0.Smm accumulator is used for noise reduction.