Late model (90's?) GM/Ford ABS

Have a late model (I have a '97 Chevy) GM or Ford 4WD truck with ABS? Ever have the ABS kick as you slowly come to a stop? I just googled it. It's a known issue with the ABS sensors. GM and Ford used a Kelsey Hayes hub with a built in ABS trigger wheel. The hub itself isn't the issue, how the sensor is mounted is. Rust builds up on the knuckle and lifts the sensor away from the ring. This makes the signal very weak at low speeds. The signal looks like the wheel just stopped and you get an ABS event.

Fix (for now) is to remove the sensors, clean the mounting surface, and reassemble. Supposedly NHSTA is looking into it for a recall. Couldn't verify that. In the meantime, thought I'd share here, it can be spooky when you're towing, luckily it hasn't gotten REAL spooky for me.

Tim.

Yeah, we fixed it on my friend's '99 (I think) Silverado. Pretty un-fun, and not trivial to diagnose. In fact, even knowing what to look for, I had to borrow a Tech II (dealer service tool) and watch the individual wheel speed sensor signals as he came to a stop, reproducing the ABS misactivation, to understand and diagnose it. I could clearly see one or both of the front wheel speed signals dropping out - go right to zero. The ABS controller naturally thinks the wheel has just locked (as if it just hit a patch of ice), so reduces and modulates pressure in an attempt to recover the wheel.

Fortunately, in this case you CAN hit the brakes much harder and use the other 2 or 3 wheels to stop the truck in time, without issue. Once you're aware of the problem and expect it.

It's a bummer of a problem, not too easy to detect. Clearly a moisture sealing problem, due to all the rust in there. On his truck, he still had the problem after replacing the sensors; he had to spend some quality time in there cleaning and removing all the rust before he got rid of the problem. Been fine ever since.

So I can more or less verify everything you say, except I feel it's more of a sealing issue - could be a manufacturing/tolerance issue with the knuckle as much as the sensor; the pair together are not sealing properly, allowing the rust. Air gaps on those sensors are indeed pretty important!

Added - we stuffed paper towel in the sensor hole, then wire-wheeled the corrosion off.

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Vaughan Scott
Detroit Region #280052
'79 924 #77 ITB/GTS1
www.vaughanscott.com

Just wondering, as I recently replaced a front hub on my '01 Yukon XL 2500 4wd, is the hub different on the "early" trucks?
Because my replacement hub came with a new ABS sensor, wiring, wheel studs, you name it.
While the new hubs having sealed bearings makes it a more expensive part, replacing the entire front hub can be done in 30 minutes or so.
Michael

Could well be; some cars and maybe trucks have the sensors built into the wheel bearings/hubs (like the GM W-cars).

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Vaughan Scott
Detroit Region #280052
'79 924 #77 ITB/GTS1
www.vaughanscott.com

This article has pictures of the hub on my truck.

http://www.mightyautoparts.com/pdf/articles/gb0804.pdf

The ABS ring is inside the hub, but that's not the issue, it's where the sensor mounts outside on the knuckle. GM had a recall on '99-'02 trucks in Eastern Canada, good chance you're could could be affected by the issue someday.

[This message has been edited by tderonne (edited July 28, 2005).]

Good pic.
I see now where some terminology had me confused. To me, the "knuckle" is what I'd say everything is bolted to: the hub/bearing, control arms, brake caliper, etc. The ABS sensor is actually mounted to the hub/bearing, just like most vehicles these days.
That's some crazy corrosion on there. I guess with me being in South Florida, mine don't get that bad :-)
At least with this info we know a solution that doesn't cost anything but a little bit of time.
Michael

From what I understand, this is a common problem with newer trucks parked on grass, dirt, gravel, etc. The moisture does a number on the underside. I always wonder why my trailers rusted like hell underneith!!

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Jeff L
#74 ITB GTi