ABS Fix

One of the first problems I solved on my '99 XLT 2wd fixer-upper was the ABS light always on. First I replaced the worn out front and rear pads. Then I removed, cleaned and verified resistances in spec on the front wheel speed sensors. I found that the rear wheel sensor mounted on the differential was open/infinite resistance, so I replaced it, thinking that was the root cause.
Wrong, at that point the warning light would clear on startup and then come back on when braking. The brakes also had the pulsing sensation when the light was off, so the pump was engaging. I had a shop pull the codes for me and they indicated a failed rear sensor (old code) and a failed right front sensor. I did some more research and found that if you unplug the sensor and connect an AC voltmeter to the output, you should measure between 100-3500 millivolts. I was getting about 10 millivolts. The Mitchell Manual specified that the sensor air gap to the tone ring should be less than or equal to .070 inch. Using a depth gage, I measured the depth from the sensor mounting surface to the tone ring and then the fixed depth that the sensor was mounted at. There's no adjustment in the design and the gap or difference in depths that I measured was 0.125 inch! Figuring I had nothing to lose, I removed the sensor again and ground 0.0625 inch off the mounting boss that the bolt goes through. This allowed the sensor to mount closer to the tone ring. After reinstalling and doing the voltage test again, I got an output in the 400 millivolt range. Checking the left front sensor revealed the same problem and I applied the same fix. This finally solved the ABS problem and the brakes work great with the light never illuminating again except on startup lamp test. My theory is that the previous owner must have replaced the rotors with aftermarket brand that didn't retain the same tone ring thickness or position. Total cost of repair was $20 for the rear sensor.

had that same problem and i pulled two fuses under the hood the pulsing went away and now i have to tke dash apart to remove the abs bulb.

Was it just the light on or was the braking working improperly? I have a problem with chattering at slow speeds and in reverse and the wife claims to have almost got into an accident yesterday because she had to stop suddenly and the abs braking was somewhat ineffective. I'm heading to the gman cave now to do the DavidMMM69 repair but I don't think I'm pulling apart my dash again.

When the light is on, the computer shuts off the ABS feature. I had no ABS (light always on), then parttime ABS (new rear sensor), now fulltime ABS with smooth braking. The pulsing may be caused a failed speed sensor and the only real valid test is to spin the wheel and observe the AC volts out of the sensor. I did a lot of other continuity checks between the ABS computer plug and all sensors as well as supply voltage, but the primary component that fails is the speed sensor, which is just an electromagnet.

Not the rotors

bustdnukls said:

My theory is that the previous owner must have replaced the rotors with aftermarket brand that didn't retain the same tone ring thickness or position. Total cost of repair was $20 for the rear sensor.

Click to expand...

The tone ring is not in/on the rotor on these vehicles. The tone ring (front) is part of the wheel bearing/hub assembly, and has nothing to do with the rotor. Someone could have installed new front hubs, that were cast/machined out of tolerance at the sensor mounting boss.