Hydraulic Control Unit: 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 master cylinder primary and secondary circuit outlets are connected to the HCU primary and secondary circuits. Refer to HYDRAULIC BRAKE ACTUATION . 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.
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 dual circuit hydraulic pump motor, and the pressure sensor. The solenoids that operate the valves are installed within the ABS module.
HCU Schematic Diagram
| Item Number | Description |
|---|---|
| 1 | Brake booster |
| 2 | Primary hydraulic circuit |
| 3 | Secondary hydraulic circuit |
| 4 | Pulsation damper |
| 5 | Pulsation damper |
| 6 | HCU |
| 7 | Damping chamber |
| 8 | Dual circuit hydraulic pump |
| 9 | D.C. motor |
| 10 | Solenoid-operated pilot valve (2 off) |
| 11 | Solenoid-operated priming valve (2 off) |
| 12 | Check valve |
| 13 | Low-pressure accumulator (2 off) |
| 14 | Solenoid-operated inlet valve (RH rear brake) |
| 15 | Solenoid-operated inlet valve (LH front brake) |
| 16 | Solenoid-operated outlet valve (LH front brake) |
| 17 | Solenoid-operated outlet valve (RH rear brake) |
| 18 | Pressure sensor (5 off) |
| 19 | LH front brake (secondary circuit) |
| 20 | RH rear brake (secondary circuit) |
| 21 | LH rear brake (primary circuit) |
| 22 | RH front brake (primary circuit) |
| 23 | Solenoid-operated outlet valve (RH front brake) |
| 24 | Solenoid-operated outlet valve (LH rear brake) |
| 25 | Solenoid-operated inlet valve (LH rear brake) |
| 26 | Solenoid-operated inlet valve (RH front brake) |
The HCU features 3 operating modes:
- Normal braking/ EBD
- ABS braking
- Active braking.