Hydraulic Control Unit

The HCU (hydraulic control unit) is attached to the ABS module in the LH (left-hand) front wheel arch. The HCU is a four channel unit that modulates the supply of hydraulic pressure to the brakes under control of the ABS module.

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 RH (right-hand) front and LH rear brakes. The secondary circuit in the HCU has separate outlet ports to the LH front and RH rear brakes.

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 return pump during active braking.
• A dual circuit return pump, driven by an electric motor, 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.

Hydraulic Control Unit Schematic