ABS Wheel Speed Sensor - front sensor for the four-wheel anti-lock braking system. The sensor mounts in the front axle housing with its pole piece set close to the toothed exciter ring that rotates with the hub. As the teeth pass the tip they induce an alternating voltage whose frequency is proportional to wheel speed. The ABS control unit compares the signals from all four sensors and releases and reapplies brake line pressure at any wheel decelerating faster than the vehicle as a whole. Air gap and tip cleanliness are critical. A build-up of ferrous debris on the sensor tip, a corroded mounting bore or a damaged exciter ring will produce an intermittent or low-amplitude signal, which the control unit logs as a sensor fault and which illuminates the ABS warning lamp.
