A wheel speed sensor reads a toothed tone ring at each wheel and reports how fast that wheel is turning. The ABS, traction control and stability control systems compare all four readings to detect a wheel locking or spinning, and many vehicles also use them for the speedometer, cruise control and transmission shift timing. A failing wheel speed sensor usually turns on the ABS and traction control lights.
Bad wheel speed sensor symptoms
- ABS light and traction control light on together; see ABS and traction control light
- ABS activating at low speed on dry pavement, with pedal pulsing as you stop
- Cruise control not working
- Speedometer erratic on vehicles that use wheel speed for the speedometer
- Erratic or delayed shifting, sometimes with a P0500 code
Types of wheel speed sensor
| Type | How it works | How to check it |
|---|---|---|
| Passive (magnetic) | Generates its own AC voltage as the tone ring passes; output rises with speed | Resistance check against specification, or an AC voltage check while spinning the wheel |
| Active (Hall-effect or magnetoresistive) | Powered by the ABS module; sends a square-wave signal that works down to zero speed | Scan tool live data or a scope; resistance checks do not apply |
Where are the wheel speed sensors?
Most vehicles have one at each wheel, mounted in the steering knuckle or axle housing and pointing at a tone ring on the hub, axle shaft or CV joint. Some sensors are built into the wheel bearing hub, so replacing the sensor means replacing the hub assembly. Older rear-wheel-drive trucks sometimes used a single rear sensor in the differential.
How to diagnose a wheel speed sensor
- Read ABS codes with a scan tool that reads the brake module; codes usually name the wheel.
- Watch all four wheel speeds while driving slowly; the bad sensor reads zero, drops out or reads differently from the others.
- Inspect the wiring where it flexes with the suspension and steering, and check the connector for corrosion.
- Look at the tone ring for cracks, missing teeth or rust buildup, and clean metal debris off magnetic sensor tips.
Replacing a wheel speed sensor
Most bolt-in sensors are held by one bolt but can seize in the knuckle from corrosion; penetrating oil and gentle twisting help. Clean the mounting bore so the new sensor sits at the correct gap, route the wire through every original clip, clear the codes and confirm all four wheel speeds match. Wheel bearing or brake work is a common time for sensor wires to be damaged, so check them after any hub or brake job; see how to bleed brakes.
Speed sensors in the transmission
Transmissions have their own input and output speed sensors, which set codes such as P0715 and P0720. If a transmission still shifts badly with healthy speed sensors, see transmission slipping and our transmissions.
More guides: wheel bearing noise · bad transmission speed sensor symptoms.
Trouble codes: U0121 Lost Communication With ABS · P0501 Vehicle Speed Sensor Range · P0503 Vehicle Speed Sensor Erratic · U0415 Invalid Data From ABS Module · P0502 Vehicle Speed Sensor Low Input · all OBD2 codes.
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