Hydraulic Control Unit (HCU)

The 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.

Valve Block Assembly: The valve block assembly contains eight or ten valve/solenoids 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 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 reap ply 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 two traction control (isolation) valves and a pressure switch (1993 only). 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 DRB, 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.

Relays: The main relay is attached to the outside of the power distribution center (PDC). The main relay has two functions. First, when the relay is energized, it supplies power to the valve/solenoids in the valve block assembly and the pump/motor relay coil. Second, when the relay is deenergized, it supplies a ground to turn on the ANTILOCK warning lamp. The main relay is controlled by the CAB. The CAB energizes the main relay when it receives an ignition input signal.

The pump/motor relay supplies power to the pump/motor. It is attached to the outside of the power distribution center (PDC). The relay coil is powered by the main relay and the coil ground is controlled by the CAB. The pump/motor relay is a special five-terminal relay and should not be replaced with a standard relay. If either is defective, both relays must be replaced. They are serviced as an assembly.

Reservoir and Fluid Level Switch - FLS2: The reservoir at the HCU includes a fluid level switch (FLS-2). This reservoir is connected to the master cylinder reservoir by a low pressure hydraulic tube. The CAB monitors the fluid level via the fluid level switch (FLS-2). When the reservoir is properly filled the fluid level switch is closed. If the fluid level falls below an acceptable level, the fluid level switch will open. When the CAB detects a low fluid condition (FLS-2 OPEN), it will inhibit the antilock and traction control function, and turn on the ABS and LTCS warning lamps.

Traction Control Indicator Lamps: The green traction control function lamp is located in the message center. It is used to inform the driver that the system has entered the traction control mode. The lamp is controlled by the CAB. The CAB turns on the lamp by directly grounding the circuit.

The amber TRAC OFF lamp (warning lamp) is also located in the message center. It is used to inform the driver that the traction control function has been turned off by the CAB. This lamp will also come on if the traction control switch is used to turn off the system. The traction control warning lamp is also illuminated for approximately 2 seconds after the ignition is turned on. With the ignition in crank and the traction control system in thermal protection mode, the traction control warning and function lamps will be illuminated.

There are five fuses used by the antilock and traction control system. Three of these are located in the fuse block and two are in the power distribution center (PDC).

Vacuum Booster/Master Cylinder/Pedal Travel Sensor: The Teves Mark IV antilock brake system uses a special vacuum booster and master cylinder.

The vacuum booster used by the Teves Mark IV antilock brake system is the new "0" lost travel reaction disc type booster with the addition of a pedal travel sensor.

The pedal travel sensor (PTS) is a linear variable displacement transducer with seven steps that provide brake pedal position information to the CAB.

The master cylinder is a standard tandem design except that the compensation ports have been replaced by special center valves. The center valve is a spring-loaded ball and seat design that is unseated by a stop in the master cylinder when the brakes are released.

A fluid level switch is located in the master cylinder fluid reservoir. The switch closes when a low fluid level is detected. The FLS-1 turns on the brake warning lamp grounding the lamp circuit. This switch does not disable the ABS system.

Wheel Speed Sensors and Tone Wheels: One wheel speed sensor (WSS) is located at each wheel and sends a small AC signal to the control module (CAB). This voltage is generated by magnetic induction when a toothed sensor ring (tone wheel) passes by a stationary magnetic sensor (wheel speed sensor). The CAB converts the AC signals into digital signals for each wheel.

The front wheel sensor is attached to a boss in the steering knuckle. The tone wheel is an integral part of the front axle shaft. The rear speed sensor is mounted in the caliper adapter plate and the rear tone wheel is an integral part of the rear rotor hub. The wheel speed sensor air gap is not adjustable. The correct wheel speed sensor air gap is from 0.52mm to 0.64mm. All wheel speed sensors have a resistance between 950 ohms and 1250 ohms.

The four wheel speed sensors are serviced individually. The front tone wheels are serviced as an assembly with the outer C.V. joint housing. The rear tone wheels are serviced as an assembly with the rotor.

Correct antilock and traction control system operation is dependent on tone wheel speed signals from the wheel speed sensors. The vehicle's wheels and tires should all be the same size and type to generate accurate signals. In addition, the tires should be inflated to the recommended pressures for optimum system operation. Variations in wheel and tire size or significant variations in inflation pressure can produce inaccurate wheel speed signals; however, the system will continue to function when using the mini-spare.