Anti-Lock Braking System (ABS) Function

When braking, the control module (1) receives a signal from the brake light switch (2) telling it that braking is in progress. The control module then puts the ABS system on standby. The brake light switch signal is not essential to start ABS working, but it makes it start working more gently.

A wheel sensor (3) on each wheel sends signals to the ABS control module, which uses these signals to calculate the car's reference speed. If there is a risk of a wheel locking, the control module uses the ABS system hydraulic unit (4), adjusting the hydraulic pressure to that wheel to avoid it locking.

The pressure in the circuit is controlled so that the maximum possible braking effect is transmitted to the road which happens when the wheel is turning with 20% slippage relative to the road surface.

ABS only works once the car's speed is greater than approx. 7 km/h (4 mph), which means that the wheels may lock at speeds below approx. 7 km/h (4 mph). The control module also gets signals from the pedal sensor (5) on the brake pedal position. This information is used to control the hydraulic pump so as not to let ABS affect the brake pedal position too much.

ABS works in three stages:

1. Maintaining brake pressure. (Constant pressure)

The wheel is threatening to lock. The control module shuts the inlet valve to the circuit to prevent the brake pressure increasing any further. If the wheel's speed of rotation continues to fall, brake pressure must be reduced still further.

2. Reducing brake pressure. (Pressure dumping)

The control module opens the outlet valve, reducing the brake pressure and allowing the wheel's speed of rotation to increase. If the wheel speeds up too much, brake pressure must be increased.

3. Increasing brake pressure. (Pressure buildup)

The control module shuts the outlet valve and opens the inlet valve, increasing the brake pressure and slowing the wheel down.

The system then repeats steps 1, 2 or 3 until braking finishes or the ABS system stops.