Anti-Lock Control - Traction Control: HCU: Notes
The hydraulic control unit (HCU) is a 4 channel unit that modulates the supply of hydraulic pressure to the brakes under the control of the anti-lock brake system (ABS) module.
The hydraulic control unit (HCU) is attached by three mounting bushes to a bracket in the plenum box on the driver side of the engine compartment. Hydraulic pipes connect the hydraulic control unit (HCU) to the master cylinder and the brakes. Refer to HYDRAULIC BRAKE ACTUATION .
The primary and secondary outlets of the master cylinder are connected to primary and secondary circuits within the hydraulic control unit (HCU). The primary circuit in the hydraulic control unit (HCU) has separate outlet ports to the front brakes. The secondary circuit in the hydraulic control unit (HCU) has separate outlet ports to the rear brakes. Each of the circuits in the hydraulic control unit (HCU) contain 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 anti-lock brake system (ABS) module with a hydraulic pressure signal.
Contact pins on the hydraulic control unit (HCU) mate with contacts on the anti-lock brake system (ABS) module to provide the electrical connections from the anti-lock brake system (ABS) module to the return pump motor and the pressure sensor. The solenoids that operate the valves are installed in the anti-lock brake system (ABS) module.
Replacement hydraulic control unit (HCU) are supplied pre-filled. After installation on the vehicle, T4 must be used to operate the solenoid valves and the return pump to ensure correct bleeding of the hydraulic control unit (HCU) and brake circuits.
| Item Number | Description |
|---|---|
| 1 | Master cylinder |
| 2 | Reservoir |
| 3 | Brake booster |
| 4 | Brake pedal |
| 5 | HCU |
| 6 | Pressure sensor |
| 7 | Priming valve |
| 8 | Pilot valve |
| 9 | Return pump motor |
| 10 | Return pump |
| 11 | Inlet valve |
| 12 | Relief valve |
| 13 | Accumulator |
| 14 | Outlet valve |
| 15 | Left front brake |
| 16 | Right front brake |
| 17 | Right rear brake |
| 18 | Left rear brake |
The HCU has three operating modes: Normal braking/electronic brake force distribution (EBD) mode, anti-lock brake system (ABS) braking and active braking.