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BMW ABS Warning Light – Wheel Speed Sensors

April 7, 2010

This answer is applicable for many BMWs in addition to the one listed below.

Q
2001 525i
The other day the abs light came on, and keeps coming on and going off. My speedometer works fine, what are my odds of it being one of the wheel sensors? I’ve been told to check resistance on each of them and compare, but is there any way to know without taking to BMW dealership. I don’t think there is a code reader that will read abs stuff. Thank you.

A
Yes, your ABS fault certainly could be due to a failing wheel speed sensor. Unfortunately, there are no code reader tools that will access the ABS system ….. that would be applicable to a home mechanic. Therefore, there are only a few things you can do in attempting to track down an ABS fault.

If the speedometer never faults, then we can be reasonably sure that the left-rear sensor is OK (this one give the input to the speedometer, on the E39 5-series chassis). So, if the fault is a wheel speed sensor, it would be either one of the front ones or the right-rear. You could swap the two rear sensors and see if the speedometer goes out when the ABS light comes on. This would indicate that the sensor that is now installed at the left-rear is at fault. If there is no change, then one of the front sensors may be at fault. If you were to purchase one front sensor, you could install it in either position as a test sensor.

Of course, you can measure the resistance of the sensors to see if one is obviously different than the others. Of the three contacts in the plug housing, two are for the sensor’s pick-up coil and one is the shield/grounding wire. You should have a resistance figure between the two pick-up coil wires and an open circuit between either of the two pick-up coil wires and the shield wire. Note that on the late model wheel speed sensors that the plug contact pins are quite small and are rather deep in the plug housing. You will have to devise a way to effectively contact the pins with your multi-meter probes.

Finally, the later model BMWs (mid-90s-on) do have a history of faults in the ABS control units, especially the E39 (5 series 97 through 03) and the E38 (7 series 95 through 01) chassis. If the fault is not due to a wheel speed sensor, you will have to have the internal ABS fault codes checked in order to determine where the fault originates.

Click below for ABS wheel speed sensors:

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