ABS operation

The ABS control unit receives a signal from the brake-light switch and the AC voltage signals from the four speed sensors. These signals are converted into digital wheel-speed signals independently of each other by two microprocessors. The wheel slip (approximately proportional to the computed vehicle reference speed) is formed from these wheel-speed signals.

If vehicle deceleration and excess slip are detected at a wheel, the pressure-holding phase is initiated, i.e. the inlet valve for the relevant wheel is closed to prevent a further increase in pressure. If in spite of the pressure being held constant the wheel tends to continue to lock up, the pressure in the wheel cylinder is reduced. For this purpose, the outlet valve is opened and the brake fluid is pumped back to the brake master cylinder via the return pump (pressure-reduction phase) until the wheel turns again. Then, depending on the control cycle, further appropriate cycles are initiated.

This function and the input signals are continuously monitored. If a fault is detected, the control unit switches the ABS function off, switches on the ABS warning light and stores the fault in a non-volatile memory in the control unit.

In addition, whenever a journey is begun and a speed of 6 km/h is exceeded, a test programme is started. The solenoid valves and the pump motor are electrically actuated and checked. If a fault is detected, the control unit switches the ABS function off, the ABS warning light is switched on and the fault is stored.