Hydraulic Control Unit

WARNING: This page is about a different car, the 2018 Land Rover Discovery and 2017 Land Rover Discovery. However, it is still accessible from the selected car via links, so may be relevant.

The Hydraulic Control Unit (HCU) is a four channel unit that modulates the supply of hydraulic pressure to the brakes, under the control of the Anti-lock Brake System (ABS) control module.

The master cylinder primary and secondary circuit outlets are connected to the HCU primary and secondary circuits.

Each of the HCU circuits 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 dual circuit hydraulic pump during active braking.
A hydraulic 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 internal components from contamination.

A pressure sensor in the primary circuit provides the ABS control module with hydraulic pressure signals. Contact pins on the HCU mate with contacts on the ABS control module to provide the electrical connections from the ABS control module to the dual circuit hydraulic pump motor and the pressure sensors. The solenoids that operate the valves are installed within the ABS control module.

The Hydraulic Control Unit (HCU) features three operating modes:

Normal braking/Electronic Brake Force Distribution (EBD)
Anti-lock Brake System (ABS) braking
Active braking.

Normal Braking/Electronic Brake Force Distribution Mode

Initially, all of the solenoid-operated valves are de-energized. Operating the brake pedal produces a corresponding increase or decrease of pressure in the brakes, through the open pilot valves and inlet valves. If the Anti-lock Brake System (ABS) control module determines that Electronic Brake Force Distribution (EBD) is necessary, it energizes the inlet valves for both the rear brakes, to isolate the brakes from any further increase in hydraulic pressure. Only the rear brakes are controlled by the EBD function.

Anti-lock Brake System Braking Mode

If the Anti-lock Brake System (ABS) control module determines that ABS braking is necessary, it energizes the inlet and outlet valves of the related brake and starts the hydraulic return pump. The inlet valve closes to isolate the brake from pressurized fluid. The outlet valve opens to release pressure from the brake into the accumulator and the return pump circuit. The reduced hydraulic pressure allows the wheel to accelerate. The ABS control module then operates the inlet and outlet valves to modulate the pressure in the brake to apply the maximum braking effort without locking the wheel. Control of the valves for each wheel takes place individually.

Active Braking Mode

The active braking mode is used to generate and control hydraulic pressure to the brakes for functions other than the normal/Electronic Brake Force Distribution (EBD) and Anti-lock Brake System (ABS) braking modes.

For active braking, the ABS control module energizes the pilot valves and priming valves, starts the return pump and energizes all of the inlet valves, brake fluid, drawn from the reservoir through the master cylinder and priming valve, is pressurized by the hydraulic pump and supplied to the inlet valves. The ABS control module then operates the inlet valves and outlet valves, as required, to modulate the pressure in the individual brakes. Some noise may be generated during active braking.

Service Information

The Anti-lock Brake System (ABS) control module and the Hydraulic Control Unit (HCU) form a single component and must not be separated. The ABS control module and HCU assembly is supplied in a prefilled state. After installation, the hydraulic brake system only requires a conventional bleed of the system. There is no requirement to pressure bleed the system.