Wheel speed sensor (ABS sensor)
Definition
Each wheel has a speed sensor that reads a toothed tone ring or a magnetic encoder built into the wheel bearing seal and sends the ABS module a signal proportional to wheel speed; the module uses the four signals for ABS, ESP, traction control, speedometer, hill hold, tyre pressure estimation and the transmission. Older passive (inductive) sensors generate an AC voltage that disappears at very low speed; modern active (Hall or magneto-resistive) sensors are supplied with 12 V and output a square wave or current pulses from zero speed and can also report direction. Failures are internal breaks from heat and vibration, cables chafed or snapped by suspension movement, water in the connector, a cracked or rusted tone ring on the CV joint or hub that gives an erratic signal, and excessive hub rust that increases the air gap. Symptoms are ABS and ESP lamps, ABS kicking in at walking pace on a dry road, loss of speedometer on some vehicles, and no cruise control; compare the four wheel speeds in live data while driving to find the one that drops out. Codes: C0035/C0040/C0045/C0050 (LF/RF/LR/RR sensor circuit), C0031-C0034 and manufacturer variants (e.g. VAG 00283-00290, BMW 5DF0-5E20, Ford C1145-C1175).