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Simple Fix for ABS Wheel Speed Sensor

185K views 32 replies 18 participants last post by  tokanero 
#1 · (Edited)
Like many, I've endured the ABS kicking in at low speed stops for months. Not feeling like changing the wheel hub bearing, since it seemed fine still (100K miles + another 45K towed behind motorhome), I decided to check the wheel speed sensor output voltage.

I ran across another post not related to Envoys that the sensor should output >450 millivolts AC if you manually spin the tire at 1 revolution per second. To check, disconnect the speed sensor connector attached to the wheel well and connect two small or medium insullated alligator clip leads (Radio Shack) to the pins on the cable going to the wheel. Connect your AC voltmeter (I also used and oscilloscope to look at the pulses) to the other end of the clip leads. Put a large screw driver through two of the lug bolts and spin at a fairly rapid but smooth rate (about one rev / second).

My left sensor was only 200 millivolts. The right sensor was less than 100 mv. If you switch to the ohmmeter function you should see about 970-1000 ohms resistance across the sensor (wheel not moving). This also verifies your clips are making contact.

I removed the right disc brakes and rotor to access the speed sensor mounted on the hub. The sensor is held in place by one screw and is mounted on a stainless steel strip that is riveted to the hub on one end.

To make a long story short, rust builds up under the stainless steel strip, causing the sensor's gap between magnetic pickup and the hub's teeth to increase, which reduces the voltage output.

The fix is simple and costs nothing but time!

1. Use a small wire brush to clear as much rust away from the sensor area as possible.

2. Remove the sensor and tuck it out of the way behind the rotor's shield.

3. Plug the hole in the hub where the sensor mounts with a lint free cloth coated with a little grease to help catch debris. I used a large rifle cleaning patch. This plug should be slightly below the surface of the hub. Make sure no debris falls into the bearings!

3A. Take a small wood chisel and cut the rivet head off. It came off very easily on mine. Remove and clean the stainless steel strip.

4. Use a Dremel grinder to remove all rust down to bare metal in an area slightly larger than the stainless strip. Even out the radius on the corner of the hub casting a little also. It seemed to hold the sensor up. Dont get too carried away with the grinder!

5. Vacuum all the grinding debris. Pull the cloth plug out with needle nose pliers, while keeping the vacuum hose nearby to get any particles sticking to the cloth.

6. Spray some brake cleaner fluid on a lint free cloth and scrub the area you ground down several times until clean.

7.Spray the bare metal with cold galvanizing compound (or primer) and let dry. This is to prevent the rust from returning too quickly.

8. Clean the sensor body, apply a liberal coating of grease to the O-ring area, then insert the sensor through the stainless strip and bolt in place.

9. Measure the AC voltage output again. It should be > 450 millivolts.

10. If you are so inclined, it might be good to drill out the rivet shank and install a screw there, but I didn't.

Using calipers I measured the sensor gap to the toothed wheel in the hub at about 0.065" initially. After doing the rework above, the gap was about 0.040". Don't get too carried away with the grinding. If your voltage output is >600 mv. then your gap is too small and you should add a shim between the sensor and the stainless strip probably.

Driving tests confirmed this fixes the problem with ABS activation at slow speed stops. Now I need to do the left wheel someday....

Total cost for parts was zero. Time required is about an hour, if you have experience removing the brakes, plus a little more to jack up each side and measure the sensor output voltage to see which side is worse. Or just rework both.

Steve
Chapel Hill, NC
 
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#27 ·
replaced hub with sensor, still not working right?

Hello, I have a 2002 envoy xl and have the same low speed abs issue. I replaced the hub but with a cheap china made assembly and the sensor seemed to clip in real good, didn't really bother to look for rust in the wiring connector on the car side, but i still have the same issue? Could I have a bad hub assembly with bad sensor or is it possible the connection from the truck side is rusty? I am about to order a better hub and do this all over again unless someone has any ideas?
 
#29 · (Edited)
My Experience

So I had the same issue with my ABS activating at low speeds. I saw the writeup at it stated that you need to have at least 450mv with the wheel spinning at 1 rev/sec. I don't know if this make a difference, but I used a Fluke DVM that measures true RMS for the AC signal. When I started out, I removed the calipers, pads and rotors on each side and disconnected the ABS sensor from the connector and hooked up the DVM. I did check the resistance of each senor and I was getting around 1050 ohms on each side. For whatever its worth these are AC DELCO parts. At first I was getting a signal of around 60-80mv on each sensor. After removing the shim on each hub and cleaning the rust off the opening, the signal went up to 100-150mv at 1 rev/sec and around 280-300mv with me rotating the wheel as fast as I could. I even checked the signal at very low rotation for spikes and I was getting around 28-30mv from just barely turning the wheel. More important was that the signal was consistent on each wheel. I don't know where the 450mv signal threshold was obtained or what kind of AC signal it is referencing to whether its peak to peak or RMS. I know if you use a "cheap" DVM VS a Fluke with RMS, I am guessing that you will get a different reading. I know that there is a similar issue with the ABS sesnsors on the older Chevy Silverados and that threshold was 350mv. I am curious how these readings were determined, because my meter is reading much lower readings, but the ABS activation on low speeds is no longer an issue on my Trailblazer anymore.
 
#30 ·
Changed the sensor but light is still on.

I changed my driver side abs sensor because O'Reilley's told me it was bad after the diagnostics. But now it is worse. Is it ok to clean the ring with the pick up rings (that is read by the sensor)? it is pretty dirty in there. If so what can I clean it with?
 
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