How to test an ABS wheel speed sensor: passive and active sensors
The ABS module uses the wheel speed sensors for ABS, traction and stability control, and often shares the speeds with the engine, the gearbox and the speedometer. A faulty sensor, a damaged tone ring or a damaged magnetic encoder (often built into the wheel bearing seal) brings on the ABS, ESP and traction lights, and can disable cruise control or cause odd gear shifts.
Updated · written by ipacen; typical values, not manufacturer data
Find out the type. A passive (inductive) wheel speed sensor typically measures about 800–2,000 Ω and makes an AC voltage when the wheel turns, rising with speed. An active sensor (Hall or magnetoresistive, used on most cars since the 2000s) gets a supply from the ABS module and sends a current signal that switches between about 7 and 14 mA; it cannot be judged by resistance. The fastest check is live data: all four wheel speeds should agree within about 1–2 km/h when driving straight.
Typical values
| Check | Typical good | Suspect |
|---|---|---|
| Passive: resistance | About 800–2,000 Ω (check the spec) | Open or close to 0 Ω |
| Passive: AC output, wheel turned by hand | Some hundreds of millivolts, rising with speed | Nothing, or far less than the other side |
| Active: supply at the connector, ignition on | Present | Missing: module or wiring |
| Active: signal current | Switching about 7 ↔ 14 mA | Stuck, or with missing pulses |
| Live data, driving straight | All wheels within about 1–2 km/h | One wheel reads zero, low, or drops out |
Typical ranges, not manufacturer data: check the specification for the engine.
Which type is it?
Wiring: what you should measure
Passive (2-wire)
| Wire | What you should see |
|---|---|
| Signal + and − | AC voltage when the wheel turns; about 800–2,000 Ω between them |
| Either pin to ground | Open circuit |
Active (2-wire)
| Wire | What you should see |
|---|---|
| Supply from the ABS module | Present with the ignition on (the voltage depends on the system) |
| Signal / return | A current switching between about 7 and 14 mA as the wheel turns |
Before you start
- Read the ABS fault codes (usually manufacturer-specific C-codes) and check that all tyres are the same size.
- Inspect the sensor tip for metal debris and the wiring along the suspension, where movement breaks wires.
- Check the wheel bearing for play and the tone ring for cracks or missing teeth.
Scan tool
- Compare all wheel speeds on a road test, including very low speed, and watch for a wheel that drops out.
Multimeter
- Passive: measure resistance and the AC output while turning the wheel by hand.
- Active: measure the supply at the connector with the ignition on; measure the current in series on the mA range while turning the wheel slowly.
Oscilloscope
- Passive: an AC wave that grows with speed. Active: a low-current clamp on one wire (or a small resistor in series) shows a square wave between two current levels.
- Turn the wheel slowly through a full turn and look for missing or weak pulses: they point to a damaged ring or encoder.
Magnetic encoder
- Hold a magnetic field viewer card against the bearing seal while turning the wheel: an even pattern of poles should show. Keep magnets and magnetic tools away from these seals.
What it looks like on a scope
Typical shapes drawn by ipacen, not a capture from one car: amplitude and timing vary with the engine, the sensor and the rpm.
What the readings tell you
| You find | It usually means |
|---|---|
| One wheel reads zero from the start | Sensor, connector or wiring open, or no supply on an active sensor |
| Wheel speed drops out at a certain speed | Damaged tone ring or encoder, too big an air gap, or a failing sensor |
| All four sensors faulty at once | ABS module supply or ground, or a network (CAN) problem |
| Faults after a wheel bearing job | Bearing fitted the wrong way round (encoder facing away from the sensor), or the encoder damaged by a magnet |
Questions mechanics ask
Can you test an active ABS sensor with a multimeter?
Not by resistance. Check the supply at the connector, then measure the current in series on the mA range while turning the wheel slowly: it should switch between two levels, typically about 7 and 14 mA. A scope with a low-current clamp shows it best.
Why do ABS lights come on after a wheel bearing replacement?
Often the bearing has a magnetic encoder in one seal. Fitted the wrong way round, or damaged by a magnetic tool, the sensor cannot read it. Check the encoder with a field viewer card.
Is it the sensor or the tone ring?
Scope it while turning the wheel slowly. A sensor that works but shows missing or weak pulses once per turn points to the ring or encoder; no signal at all, or no supply, points to the sensor or its wiring.
Values are typical, not manufacturer data. Always follow the service information for the vehicle and work safely.