Hydraulic System: Description and Operation

HYDRAULIC CONTROL UNIT
The HCU on the EBC 125 has an integral valve body for controlling the rear Brakes. The HCU on the EBC 325 has an integral valve body for controlling the front and rear Brakes.

Within the HCU are solenoids, valves, check valves, and a reset switch necessary to apply and release brake pressure as required to avoid wheel lockup, keep the wheels rolling, and maintain optimum deceleration.
The Isolation Valve(s) is/are normally open, allowing unrestricted flow from the master cylinder to the wheels. When the CAB determines antilock intervention is required, the valve(s) close to isolate the master cylinder hydraulic circuit(s). Fluid is trapped in the circuit(s) and then prevented from reaching the wheels.

The Dump Valve(s) is/are pulsed on and off by the CAB. This valve(s) cycles only if the isolation valve is closed. When the dump valve is on, it allows fluid to the low-pressure accumulator for temporary storage.

This causes the pressure to the wheel to decrease. When the valve is off, fluid is allowed to the wheel.
The Brake Return Check Valve allows the HCU to drain faster after the antilock activation, when the brake is released.

The Reset Switch is used in the EBC 125 (RWAL) system only. The switch is positioned in the HCU to monitor the master cylinder (input), rear brake (output) and accumulator pressure. During normal braking, pressure is equal for input and output and the switch remains open.

During an antilock stop the switch will close as the isolation valve is cycled and pressure becomes unequal between input and output. At the end of an antilock stop the isolation valve will open as master cylinder pressure and rear pressure equalize. The switch is used to monitor correct operation of the HCU and to set a DTC if incorrect pressure is detected on the EBC 125 (RWAL) antilock system when not in an ABS event.

When the brakes are applied, fluid is forced from the master cylinder outlet port(s) to the HCU inlet port(s). This pressure is transmitted through normally open isolation valve(s) inside the HCU, then through the outlet port(s) of the HCU to the wheels. If the CAB senses that a wheel is about to lock based on wheel speed sensor data, it pulls the normally open isolation valve closed for that circuit. This prevents any more fluid from entering that circuit. The CAB continues to look at the wheel speed sensor(s) signal to determine if the wheel is still decelerating.

If deceleration is still taking place, the normally closed dump valve for that circuit is opened. This action dumps any pressure that is trapped between the normally open isolation valve and the brake back into an accumulator. Once the affected wheel comes back up to speed, the CAB returns the valve(s) to their normal condition allowing the affected brake to be reapplied. On the EBC 325 system, there is a pump/motor unit, two accumulators, three isolation valves, and three dump valves which are used to provide rapid response during the reapply sequence and to minimize pedal feedback due to the increased hydraulic circuits.