Control Functions (1998): Abs
When braking, the control module (4) receives a signal from the brake light switch (5) that braking has started. The control module then switches to stand-by mode for ABS control. The signal from the brake light switch is not necessary for ABS control, but makes for gentler adjustment when ABS first cuts in.
A wheel sensor (1) by each wheel sends signals to the ABS control module. The control module uses these signals to calculate the car's reference speed. If there is a risk of the wheels locking, the control module triggers the ABS system hydraulic unit (4) and the hydraulic pressure to the wheel concerned is adjusted to avoid the wheel locking.
The pressure in the circuit is controlled so that the maximum possible braking effect is transmitted to the rod, which occurs when the wheel is rotating at approx. 20% slippage relative to the road surface.
The ABS system does not engage until the car's road speed exceeds approx. 7 km/h (4.4 mph) (i.e. there is nothing to stop the wheels locking under approx. 7 km/h (4.4 mph)).
ABS can be divided into three different stages:
- Maintaining brake pressure. (Constant pressure)
- The wheel is threatening to lock. The control module closes the inlet valve to the circuit to prevent the brake pressure increasing any further. If the wheel still continues to slow down, the brake pressure must be increased.
- Reducing the brake pressure. (Pressure dumping)
- The control module opens the outlet valve to reduce the brake pressure and allow the wheel to speed up. If the wheel starts turning too quickly, the brake pressure must be increased.
- Increasing brake pressure. (Pressure build-up)
- The control module closes the outlet valve and opens the inlet valve, increasing the pressure and slowing the wheel down.
Steps 1, 2 and 3 are then repeated until braking ends or ABS cuts out.