False Abs Activation

zedbear · June 5, 2010

Within the last week or two, the ABS has sometimes been kicking on just as I'm rolling to a stop. Also within the same time frame, my left rear axle seal has been leaking enough that it's seeping down on to the rotor when parked. I've stopped driving it and am replacing the seal in the next few days but could the fluid be causing the ABS issue? Or is it more likely that it's totally unrelated? Thanks.

bronyaur · June 5, 2010

I've seen the low speed ABS activation caused by two things: corrosion between the mating surfaces of the ABS sensor(s) and the hub, and worn front wheel bearings. Probably the most common is the corrosion problem.

Willshire · June 5, 2010

Abs may be engaging rear left wheel as it slips in the gear lube that has run onto the rotor

Metro12 · June 5, 2010

Vehicle Speed Sensor in the wheel hub.

khancock · June 5, 2010

Vehicle Speed Sensor in the wheel hub.

+ 1. This was a very common problem on the GMT-800 series trucks. The Axel seal is totally unrelated; AFAIK there are no ABS sensors in the rear. The hubs come as an assembly w/new ABS sensors. i did both of mine when I had my 2000. Save your self the headache and just get both sides. Its a fairly easy job to do and if one is causing issues the other isn't far behind.

zedbear · June 6, 2010

Yeah, I was under there a little bit ago draining the diff fluid and noticed there wasn't an abs sensor at the back wheels. How does the ABS know the rear wheels are locking up? Sensor in the transmission? Must be a 3 channel system then.

I need to find out for sure what the issue is before I spend hundreds of dollars on new front hubs, lol. The front hubs actually look somewhat newer. Hmm...

bronyaur · June 6, 2010

I need to find out for sure what the issue is before I spend hundreds of dollars on new front hubs, lol. The front hubs actually look somewhat newer. Hmm...

The front wheels are activating the ABS. The following procedure shouldn't cost you anything but time.

Measure AC voltage and clean the wheel speed sensor mounting surfaces.

1. Raise and support the vehicle.

2. Disconnect both the front wheel speed sensor connectors at the frame and harness.

3. Place a Digital Volt Meter (DVM) across the terminals of each wheel speed sensor connector.

4. Rotate the wheel clockwise approximately one revolution per second. The minimum reading should be at least 350 ACmV's. If the reading is less than 350 ACmV's, remove the wheel speed sensor (remove brake caliper and rotor, remove hub to steering knuckle bolts, remove heat shield, use hex wrench to remove wheel speed sensor).

5. Plug the wheel speed sensor bore in order to prevent debris from falling into the hub during service.

6. Clean the wheel speed sensor mounting surface using a wire brush, sand paper, emery cloth, scotch brite, or other suitable material. Be sure to thoroughly clean the wheel speed sensor surface. There should be no rust or corrosion.

7. Check the sensor head to determine if it has been warped/distorted due to the corrosion build up or other causes. Check the mounting surface on the sensor head for flatness by placing it on the edge of a metal machinists scale or other suitable straight edge to measure the flatness. Check the sensor for flatness in multiple (minimum 3) positions/directions. If the sensor head is distorted, replace the sensor.

8. Apply a thin layer of bearing grease to the hub surface and sensor O-ring prior to sensor installation. Use ONLY Wheel Bearing Lubricant, P/N 01051344 (Canadian P/N 993037).

9. Install either the original sensor or a new one in the hub. Ensure that the sensor is seated flush against the hub.

10. Place the DVM across the sensor terminals and recheck the voltage while rotating the wheel. The voltage should now read at least 350 ACmV's.

khancock · June 6, 2010

Yeah, I was under there a little bit ago draining the diff fluid and noticed there wasn't an abs sensor at the back wheels. How does the ABS know the rear wheels are locking up? Sensor in the transmission? Must be a 3 channel system then.

I need to find out for sure what the issue is before I spend hundreds of dollars on new front hubs, lol. The front hubs actually look somewhat newer. Hmm...

I screwed around with this for a while before doing it. Kept putting it off and off. One day I was in the grocery store parking lot, went to hit my brakes and the ABS locked them up on dry pavement while I was starting to park. I nearly smacked a parked van and a person walking to their car. Lucky I was quick enough to slam the E-brake and catch it before anything became an issue. The moral of my story is that this can be a serious safety issue. I think I got two new hubs for under $250 total (about $125ea) ; I don't know what your situation is ($) but my feeling is that is a small price to pay for the level of safety it provides.

zedbear · June 6, 2010

Great info guys. I will do that test as soon as I fix this stupid axle seal leak. Thanks.

Scott02Z71 · June 6, 2010

I had/have a similar situation. My ABS would activate at low speeds all the time. I replaced my hubs with some inexpensive ones (under $250 for both), same problem. I then bought GM replacement hubs and new sensors, now my ABS light is on but the ABS does not activate. All connections are good, I think it is the control module or the ABS pump.

I have an 2002 with 170,000 miles, not sure I want to put more $$$ into fixing it now.

zedbear · June 11, 2010

Ugh. I was rolling to a stop in a fast food drive thru and the ABS "kicked back" at me and I couldn't stop! Luckily I was only doing less than 5 mph and I steered to the side and stopped. Good call, Khancock, lol. I pulled the ABS fuse right there in the parking lot and am going to take a look at things tonight if I can find my multimeter.

zedbear · June 11, 2010

I'm confused. I thought the sensor was part of the hub on 99+ so how can it be removed to clean it??

Scott02Z71 · June 12, 2010

I'm confused. I thought the sensor was part of the hub on 99+ so how can it be removed to clean it??

Take the calipers off and the rotors. There is a opening in the "dust/heat sheild" that you can see the sensor though. The sensor is attached with a small allen head bolt. You can unscrew that and gently pull the sensor out of the hub for cleaning.

I tried cleaning mine, and eventually changed hubs completly. As I said before, no more false activations, but the ABS light is always on and I am pretty sure the ABS doesnt work.

zedbear · June 12, 2010

I'm confused. I thought the sensor was part of the hub on 99+ so how can it be removed to clean it??

Take the calipers off and the rotors. There is a opening in the "dust/heat sheild" that you can see the sensor though. The sensor is attached with a small allen head bolt. You can unscrew that and gently pull the sensor out of the hub for cleaning.

I tried cleaning mine, and eventually changed hubs completly. As I said before, no more false activations, but the ABS light is always on and I am pretty sure the ABS doesnt work.

I took both out to clean them. On one of them, it seemed as though the wire might be pulling out of the sensor body.

I'm just going to replace the sensors. There is no play in the hub bearings so I see no need to replace the whole thing. Mine is 4wd. I think on 2wd models, you HAVE to replace the whole thing to replace the sensor.....

team3-8 · June 12, 2010

I bought a new sensor for my 2003 thinking that's all it needed. Then found out the bearing was bad and they came with sensor. If you need just the sensor let me know. It is a GM part.