Hydraulic Control Unit (HCU)

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

The HCU is installed in the engine compartment. Hydraulic pipes connect the HCU to the master cylinder and the brake calipers. For additional information, refer to: Hydraulic Brake Actuation .

The primary and secondary outlets of the master cylinder are connected to the 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 contains 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.

A replacement HCU is supplied pre-filled. After installation on the vehicle, the Land Rover approved diagnostic system must be used to operate the solenoid valves and the return pump to ensure correct bleeding of the HCU and brake circuits.