Anti-Lock Control - Traction Control: HCU: Notes

The HCU is a 4 channel unit that modulates the supply of hydraulic pressure to the brakes under the control of the ABS module.

The HCU is attached by three mounting bushes to a bracket in the plenum box on the driver side of the engine compartment. Hydraulic pipes connect the HCU to the master cylinder and the brakes. For additional information, refer to Hydraulic Brake Actuation .

The primary and secondary outlets of the master cylinder are connected to primary and secondary circuits within the HCU. The primary circuit in the HCU has separate outlet ports to the front brakes. The secondary circuit in the HCU has separate outlet ports to the rear brakes. Each of the circuits in the HCU contain the following components to control the supply of hydraulic pressure to the brakes:

• A normally open, solenoid operated, pilot valve, to enable active braking.
• A normally closed, solenoid operated, priming valve, to connect the brake fluid reservoir to the return pump during active braking.
• A return pump, to generate hydraulic pressure for active braking and return brake fluid to the reservoir.
• Normally open, solenoid operated, inlet valves and normally closed, solenoid operated, outlet valves, to modulate the hydraulic pressure in the individual brakes.
• An accumulator and a relief valve, to allow the fast release of pressure from the brakes.
• Filters, to protect the components from contamination.

The primary circuit also incorporates a pressure sensor to provide the ABS module with a hydraulic pressure signal.

Contact pins on the HCU mate with contacts on the ABS module to provide the electrical connections from the ABS module to the return pump motor and the pressure sensor. The solenoids that operate the valves are installed in the ABS module.

Replacement HCU are supplied pre-filled. After installation on the vehicle, T4 must be used to operate the solenoid valves and the return pump to ensure correct bleeding of the HCU and brake circuits.

Fig 1: Schematic Of Hydraulic Control Unit (HCU)

Courtesy of JAGUAR CARS, INC.

The HCU has three operating modes: Normal braking/EBD mode, ABS braking and active braking.