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RE: ABS problems revisited

I had an ABS wheel sensor problem once.  It had similar behavior.

The sensor can be perfectly good and still cause these problems. The critical
parameter is the spacing between the ABS sensor and the toothed rotor (in the

When the sensors are new, they have a calibrated coating on the tip. You insert
the sensor into the hole until it touches the rotor. The coating wears off, and
the spacing is perfect.

I had a bearing job done once and the mechanic got caca all over the wheel
sensor. We (he didn't "get" ABS) removed each wheel sensor, checked that the
tip was clean, then reinserted them. We pushed the sensor in until it touched
the rotor, then backed off just enough (2-3-4 mils) until the sensor didn't
touch when the wheel was rotated. This was real easy on a lift, but you
should be able to do this with jack stands if you're careful.

The ABS computer needs to know that all wheels are turning at the same speed.

Good luck,
Maynard, MA

<I'm still trying to diagnose an ABS problem in my `91 V8 5-speed (without
<the benefit of a Bosch ABS tester).  As I mentioned before, a solenoid will 
<start operating at very low speeds as I'm approaching a stop.  Lately, the 
<system will switch itself off when driving at highway speeds (no braking).  
<I'm reasonably sure it's a wheel sensor problem, but I Ohm'ed  each one and 
<they all came up within reasonable values (1.26-1.28 kOhm).  I'll next check 
<if one is shorted to ground.  I have a couple of questions:
<1.  Is there any easy way I can figure out which wheel the system thinks is 
<locking up?
<2.  What does the wheel sensor itself output (i.e., a voltage or a frequency)?
<I plan to take home a portable battery oscilloscope this weekend, if I can
<figure out what to try to measure with it.  I'd appreciate any advice you
<can give me.
<Dick Meyer
<Applied Research Laboratory
<Penn State University
<State College, PA 16804