BMW E39 ABS and DSC lights on: wheel sensor, wiring and ABS unit
E39 ABS and DSC lights are on, what is the usual cause?
The most common cause is a wheel speed sensor, its wiring or a dirty sensor bore. Read the fault codes first, clean the bore and check the sensor fitting and wiring. A weak battery or a poor battery terminal can turn on the same lights, so check the battery too.
How do I fit a wheel speed sensor so that it works?
The sensor is sensitive to its fitting depth: if it is too far from the ABS ring, the signal is too weak and the lights come on. Brush the corrosion off from under the sensor lug and clean the bore with brake cleaner and compressed air. Do not use oil-based rust protection, it collects dirt.
Can the fault be in the bearing or the ABS ring?
Yes. The ABS ring is part of the wheel bearing, so a loose bearing or a small dent in the ring can turn the lights on. Check through the sensor hole whether the ring runs out. Even after a bearing replacement the lights can come on if the ring was damaged or the air gap is wrong.
How do I tell if the fault is in the ABS unit and not the sensor?
The fault codes cannot tell you which one is faulty. If the lights are on when the car is warm but go out when it is cold, the fault is probably in the unit, whose internal wiring can come loose. Measure the sensors and wiring before replacing the unit.