Hydraulic Control Unit
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.