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Old 03-16-2009, 05:10 PM   #6
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Re: Wheel Bearing & ABS Light

Originally Posted by frehleycomet View Post
I replaced the driver's side wheel bearing on my 2001 venture and now the ABS light goes on about 5 to 10 seconds after starting the car. It stays on throughtout the drive. Before the light goes on I get a sound as if passing your fingers up and down an old fashioned washboard. I have read throughout these threads to replace the bearing but I don't forsee that happening. How can this bearing be bad when the car feels great driving now. Any help would be appreciated. Thanks.
OK, I've never seen what I'm about to say on any forum before, and I'm ignoring your comment about the washboard sound, but consider this carefully.
I replaced my driver's side wheel bearing on a 99 olds a couple years back. I purchased a lifetime bearing from NAPA for this job. Once the bearing was replaced, the ABS light came on and stayed on every time I reset it and drove down the road. This was not a problem before the replacement so clearly the new bearing wasn't delivering a signal to the ABS system for some reason--or, and this is the point--it wasn't sending a signal the ABS system could use.
I used a fluke 179 meter to measure the output from the sensor in the bearing. The voltage looked to be almost (ALMOST) the same as the passenger side bearing sensor. I destroyed the old bearing taking it out so couldn't compare the sensor in it to the new one.
I used an oscilloscope to view the output pulse coming from the new bearing and compared it to the pulse from the passenger side. Both bearings gave the same pulses per turn (sorry don't recall how many there were) but the replacement bearing gave shorter duration pulses than the original on the other side. I called Napa and they told me it happens, bring it back and exchange for another.
The 2nd bearing worked at once and I've never had an ABS problem since. The pulse from the 2nd bearing looked much closer to the original but still a bit shorter.
Its my view that the magnet or gap spacing in some of the replacement bearings are just not close enough to tolerance for the ABS system to work--or possibly the ABS system is too picky about pulse width. In any event, you need to change out the new bearing again if you have determined there is no cable problem.
You can best test the cable harness by removing the large connector from the computer module and testing the pins directly at that location instead of trying to measure down under the wheel. Better scan tools will show you the pulse count directly as you spin the wheel as well.
I don't have access to that good of a scan tool but my very affordable fluke voltmeter can easily show the pulses from the wheel sensor when set to DC volts and measured at the connector end of the ECM inside the air scoop. You need a book or pin numbers to know where to measure.
Let me know if you are interested and I'll look up the pin numbers and post 'em. That gets tricky due to orientation confusion so I'll leave it for another post if desired.
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