ABS wheel speed sensor

The anti-lock braking system needs to know how quickly each road wheel is turning. A sensor at the hub reads a rotating toothed or magnetic feature and sends a signal to the control unit. If a wheel slows towards locking during braking, the system can adjust pressure at that wheel. On many cars the same speed information feeds other stability functions, which is why one failed signal can bring several warnings at once.

The sensor is only one part of that chain. Its cable and connector must carry the signal, the ring must rotate cleanly past it, and the hub or bearing must hold the correct relationship between them. Delphi's diagnostic guidance points to damaged rings, contamination, wiring and bearing issues as reasons a sensor fault may appear. A diagnostic code naming a corner identifies where the signal is missing; it does not prove the small sensor itself has failed.

The normal hydraulic brakes may still slow the vehicle when the ABS warning is on. You should not assume anti-lock protection is available, however, or treat the lamp as decorative. Arrange diagnosis promptly and leave more stopping room, especially on a slippery road. If there is also a change in pedal feel or a brake fluid warning, describe both symptoms immediately to the garage because that may be a separate braking fault.

A persistent ABS lamp after the normal start-up check is the obvious clue. Some cars also display a stability control warning or stop using a function that relies on wheel speed. The fault may begin only in wet weather, after work around a wheel, or while turning, because a damaged lead moves or a poor connector admits water. A lamp that appears intermittently still deserves a scan; the stored information can be useful before it becomes permanent.

Ask the workshop to note which wheel the diagnostic system identifies and what live wheel-speed readings show during a suitable test. A sensor that reports nothing while the others report movement is a strong lead, but its wiring and the rotating target still need checking. A damaged magnetic strip built into a bearing can demand a bearing repair rather than another sensor. A dirty or cracked ring may create a signal that drops out at a particular point of each rotation.

Do not replace a sensor solely because a generic code reader prints its name. An incorrect part, a loose plug or a cable damaged during installation can leave the light on and add cost. A proper diagnosis should separate an electrical break from a mechanical signal problem and confirm that the warning clears after the repair.

DVSA section 1.6 requires testers to inspect fitted ABS systems. A warning device showing a system malfunction is a major defect, so a lit ABS fault lamp fails the test. A warning device that does not work is also major. A missing or damaged wheel speed sensor, damaged wiring, or other missing or damaged ABS component is listed as major too. The MOT record may name the warning device rather than the sensor if the lamp is what the tester can establish.

The manual also deals with deliberately removing ABS. It says removing or disabling the system is not permitted on a vehicle first used on or after 1 January 2010 where the system was fitted. Pulling a bulb, disconnecting a control unit or asking a garage to code out the fault is therefore no cure for a failing system. It can also leave the driver without the warning that should appear during the lamp check.

A major MOT defect goes on the electronic failure record. If the last MOT is still valid and no dangerous defect is recorded, GOV.UK's rule for taking the vehicle away still requires roadworthiness. An advisory about nearby wiring may appear before a full failure, but an illuminated fault lamp is not merely advisory under section 1.6. Read the precise wording and ask for a diagnosis tied to it.

The job starts with a scan of the ABS control unit, not an order for a part. The technician records the relevant fault, checks whether it returns and compares live signals from the wheels. They inspect the lead from the hub to the body, the plug, corrosion at the connector and the surface that the sensor reads. If the signal disappears only as the suspension moves, a flexing wire may be the fault. If it drops at the same point of rotation, the ring or bearing deserves attention.

When the sensor itself is defective, the garage removes it from the hub, cleans the mounting area as required, fits the correct replacement, reconnects the lead safely and checks that the system recognises wheel speed again. A seized sensor can break during removal; corrosion in the mounting hole can make a simple looking swap slower. Some designs put the sensing feature inside a wheel bearing, so diagnosis must establish whether the replaceable sensor or that bearing is the failed component.

After fitting, ask for confirmation that the ABS lamp completes its normal check and that the original fault no longer returns. Clearing the code without curing the cause is not a repair. A workshop may need a controlled road test to confirm a signal that cannot be checked fully with a stationary wheel. The invoice should identify the corner and part replaced, which helps if the warning reappears later.

The repair data gives a fitted range of £70 to £350 and about 1.5 hours for the sensor entry. The low end suits straightforward access and an inexpensive separate sensor. The high end reflects more difficult diagnosis, an awkward or corroded installation and a dearer vehicle-specific part. The range is a guide for this repair, not a cap on every related ABS fault.

A cable integrated into the sensor may be routed through several clips and behind a wheel arch liner. A sensor seized in an alloy or corroded hub can take much longer to extract without harming the mounting. A premium marque part can cost more than an equivalent on a common hatchback. Some garages quote diagnosis separately, while others include it in the fitted price. Ask which arrangement applies before authorising the work.

If a scan shows a damaged ring, bearing, wiring loom or ABS controller, that is no longer just a sensor replacement. A sensible estimate should name the failed item and describe how the garage ruled out the cheaper possibilities. Paying to replace the sensor on the wrong corner, then paying again for the true fault, is the expensive version of skipping diagnosis.

A separately bolted sensor can look like an easy home job, but reaching it safely and proving it is faulty are different tasks. The car must be supported securely, the replacement lead routed away from the tyre and suspension, and any connector sealed and clipped as designed. A broken retaining bolt or sensor body may turn the job into drilling and careful cleaning at the hub. Forcing a sensor against its magnetic target can damage the new part.

You also need a way to read ABS specific codes and confirm a live signal. A basic engine code reader may not show the relevant information. If the warning persists after a home replacement, have the ring, bearing and loom checked before buying another sensor. Do not defeat the dashboard lamp to obtain an MOT pass; the manual tests that device in its own right.

Tell the garage when it came back: immediately at start-up, only once the car moves, after rain or after a particular turn. Those details help separate a permanent electrical fault from a speed signal that fails under movement. Ask for the new fault code and live readings, and compare them with the original diagnosis. A return visit should check the connection and routing of the replacement as well as the ring it reads.

For an MOT failure, GOV.UK says a dangerous defect prevents driving away; section 1.6 itself lists the ABS defects above as major. Retest terms depend on whether the vehicle remains at the station or is taken away and returned. Agree that arrangement when booking repair. If the MOT record names a sensor but a garage finds only a dashboard lamp fault, ask both parties to explain the evidence before buying a part that will not address the recorded problem.