ANTI-LOCK BRAKING SENSORS (ABS)
Also known as a wheel speed or vehicle speed sensor, the anti-lock braking sensor (ABS) is located behind the wheel and close to the brake pad where it measures both the direction of the wheel's rotation and its speed. This information is fed back to the ECU which can then prevent the wheels from locking under heavy braking by releasing the brake pressure, before re-applying it as soon as the wheel starts to rotate freely again. This also allows the driver to continue to steer the vehicle whilst braking. The main indication of a failed ABS sensor is the ABS warning light illuminating on the vehicle's dashboard and it is often caused by the build-up of dirt on the sensor itself. Sensors should be checked for dirt, corrosion, or damage at service intervals or whenever the brake pads are changed.