Originally Posted by
Teuobk
Hi guys. I posted this write-up in a separate thread, and several people suggested that I replicate it here. Between the time I first posted it (9/06) and now, I've used the procedure yet again to locate a failed speed sensor. For those keeping score, that means I've replaced three of the four sensors on my 540i in the past two years.
The actual sensor replacement is trivial; the tricky part is figuring out which sensor failed. I hope that this can help somebody!
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Replacement of Wheel Speed Sensor
E39 BMW
Synopsis: A malfunctioning wheel speed sensor can cause the anti-lock braking and traction control systems to become disabled, among other things.
Symptoms: Three lights appear simultaneously on the dashboard: ABS, an amber BRAKE, and the triangular "traction lost" icon. Traction control is not operational, nor is antilock braking. One or more of the speedometer, odometer, or cruise control might also be inoperable.
Tools required: Multimeter capable of testing diodes. 5 mm Allen wrench. Long, very narrow probes for multimeter, or equivalent. For rear sensors, a 10 mm socket, 8 mm socket, flat-head screwdriver, and needle-nose pliers might be required to remove trim.
Difficulty: Easy
Diagnosis: A BMW service computer can isolate the source of the problem; standard OBDII code readers cannot pull the requisite information. The Peake Research code reader seems to be capable of reporting whether or not a problem exists, but there do not appear to be any codes that reveal the location of the faulty sensor. If a wheel speed sensor is suspected, and no computer is available, the identification process is one of trial and error. Jack up you car and support appropriately. Access to the sensor is considerably easier with the wheel removed, so pick a corner and remove the wheel.
It is worth noting that certain failure modes are associated with particular corners of the car. If the speedometer and odometer do not work, the driver's side rear sensor is probably bad. If the speedometer works but the cruise control does not, then the passenger side rear senor is probably bad.
Locate the sensor (mounted on the wheel carrier near the hub) and trace its wire back to a blue connector. The connector might be housed in a black plastic box. Open the hinged box if it is present. There might be a second connector next to the blue speed sensor connector; that's for the brake pad wear sensor. Pull the blue speed sensor connector out of the box and disconnect the two sides.
Deep inside the sensor side of the connector, there are two metal prongs. Now comes the tricky part. Turn the mutimeter on and to its diode testing setting (it looks like an arrow with a line in front of it). Connect one probe to one of the prongs, and the other probe to the other prong. The meter should read ~1.4-1.8 V or indicate an "open" condition (e.g., "OL"). Reverse the probes. Once again, the meter should indicate either an open condition or a 1.4-1.8 V drop, but the result should be whichever one you didn't see the first time. If you see any other result, such as 600 mV in each direction or 0.0 V in either direction, you've probably found the bad sensor; congratulations! Note that a 0.0 V drop is possibly indicative of a poor test setup; verify that you are not shorting the multimeter probes. If the sensor tests out okay, try again at a different wheel. Of course, it's always possible that one of your speed sensors has failed in a way that wouldn't present itself in this test. In the worst case, you'd have to go to the dealer anyway for the diagnosis.
You might want to try another wheel even if you think you found the bad sensor in order to confirm that your technique is good. It's sometimes a bit tricky to get a good connection to those prongs. If you think you found two bad sensors, you probably messed up.
Be sure to reconnect all of the sensors that you believe are good. Once you have identified the faulty sensor, obtain a replacement BMW part. One sensor on my 540i was $120; I've read reports of them retailing for as little as $70.
Other possible causes: Reports indicate that the cause of the sensor failure might be a frayed wheel-speed sensor wire in the wheel well; inspect the entire visible run of the cable carefully to confirm the lack of frayed or pinched wires. If frayed wires are found, attempt repair using common wire-repair techniques. Other reports indicate that a bad ABS controller might cause similar symptoms.
Replacement: With the wheel off at the corner with the broken sensor, and the sensor cable disconnected, unsnap the sensor wire from the two mount points on the steering knuckle. Follow the cable to the point where it ends at the sensor. The sensor is attached to the car with one or two small bolts. A 5mm Allen wrench is required to remove the bolts. You might have to do this blind, but it's not that hard. After the bolts are removed, gently pull the sensor straight out of its hole. Put the new sensor where the old one was, and reverse the removal process. There were two bolts holding my old front sensor, but the new front sensor included only one mounting hole. Due to the sensor construction, I don't believe it is a problem to use only one bolt. Be sure to hook the new sensor back to the other connector, and don't forget to snap the cable into the cable brackets.
Commentary: I have had three speed sensors fail on my 1999 540i. The first time, a front sensor failed. The DSC, ABS, and brake warning lights were be off when the car was first started but would turn on after driving a couple hundred feet. A year later, a rear sensor failed. The same three lights shone at startup and never reset. The Peake reader indicated code 0F 78. A few months after that, I had another rear sensor fail. I was able to use the above procedure all three times to successfully locate the failed sensor.
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Jeff
BMWCCA 484984
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