ABS Wheel Speed Sensor - wheel rotation sensor in the anti-lock braking and stability control sensing circuit. The sensor mounts in the hub carrier and reads the toothed reluctor ring on the wheel bearing, generating a signal proportional to the rotational speed of that wheel. The ABS modulator uses this signal to detect impending wheel lock and to release and reapply brake pressure at the affected corner. The same signal is shared with the traction control, dynamic stability control and hill descent control functions, and provides a road speed reference to other vehicle systems. A failed sensor, a damaged sensor tip or a contaminated reluctor ring typically illuminates the ABS and stability control warning lamps, disables hill descent control, and logs a fault code identifying the affected corner.