E. Hydraulics

If there is still a tendency toward locking because the constant pressure is still too high, the outlet valve in the solenoid valve is opened to lower the fluid pressure. Simultaneously, the brake fluid still in storage is pumped back into the tandem brake master cylinder by the return pump. If the pressure is so low that the wheel wants to accelerate again, there will be no further pressure reduction and the fluid pressure will again be held constant.

When the acceleration of the wheel again passes a threshold value, the pressure is again increased by opening the input valve in the solenoid valve.

By means of pertinent signals from the ABS control unit, the hydraulic unit can actuate the three following control stages of

pressure build-up

pressure holding and

pressure reduction.

The control sequence is continually repeated under controlled braking, until the brake pedal is released or shortly before the vehicle stops.

Fig 1: Hydraulics Break System - Circuit Diagram

Irrespective of the pressure in the tandem brake master cylinder, the hydraulic unit can change the fluid pressure for the brake calipers during regulation.

However, a pressure increase as compared with pressure coming from the master cylinder is not possible.

Fig 2: Identifying Hydraulics Break Unit Components Location

The hydraulic unit comprises three fast-switching solenoid valves. Of these valves, one each is associated with the left-hand and right-hand front wheel brake and the third with the rear wheel brake.

By activating the valves with varying voltages, the brake fluid pressure of the individual brake calipers can be increased = pressure build-up stage (no current)

held = pressure holding stage (half max. current)

or reduced = pressure reduction stage (max. current).

Fig 3: Identifying Front Axle Solenoid Valves

Pressure build-up stage

In the pressure build-up stage the pressure can be increased via the open input valve in the solenoid valve up to the pressure activated by the tandem brake master cylinder.

Fig 4: Hydraulics Break System - Circuit Diagram

Pressure holding stage

In the pressure holding stage which precedes each pressure reduction stage, the fluid pressure from the hydraulic unit to the wheel brakes is held constant because the output and input in the solenoid valve is closed.

Fig 5: Hydraulics Break System - Circuit Diagram

Pressure reduction stage

During the pressure reduction stage the brake fluid flows via a reservoir (6) into return pump (7). To maintain the fluid volume of the master cylinder, the return pump returns the brake fluid into the master cylinder against the prevailing pressure. To dampen the delivery noise, each circuit is provided with a silencer (10).

Fig 6: Hydraulics Break System - Circuit Diagram

On plug socket (13a) of the hydraulic unit there is a relay (A7K1) for solenoid valves and relay (A7K2) for the return pump. The hydraulic unit is connected to the vehicle ground at the hexagon nut (10) via a grounding cable.

Fig 7: Identifying Plug Socket, Hydraulic Unit Relay And Solenoid Valves