Overall Function

The ABS is operational when the ignition is switched on and a speed of 5-7 km/h has been attained. All braking operations in the locking range are controlled starting at the so-called control speed of 8 km/h (from 12 km/h on 1st version up to 02/84).

The following describes the control cycle on one wheel. The control sequence on the other wheels is the same. The wheel speed measured by the wheel speed sensors provide the wheel deceleration and the wheel acceleration signals for the electronic control unit. Linking of the individual wheel speeds provides the so-called reference speed, which is the approximate vehicle speed. A comparison of the wheel speed with the reference speed supplies the slip signals.

If as a result of excessive brake fluid pressure in a brake caliper the wheel slows a tendency toward locking, a condition which is recognized by means of the wheel speed sequence (wheel slip), the fluid pressure will be hold constant, that is an additional pressure increase is not possible.

I 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
pressure reduction.

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