ABS Brakes

64FJ40 · Nov 14, 2009

I understand the concept of antilock braking systems, but I've never had a problem with one so I'm looking for a little guidance in figuring this out before I dive headlong into the wrong diagnosis.

A friend's truck - 1998 Chevy K1500. He was complaining that the brakes felt weird sometimes when he came to a stop. I felt it once, but I wasn't driving and couldn't tell where "it" came from - just that there was a little shudder a few feet before completely stopping.

The rotors looked fine, the pads were fine, but the calipers weren't in the best shape. The piston dust covers were cracked and partly missing. I thought maybe one of the pistons was hanging up coming to a stop. New calipers, pads, turned rotors. I drove it for a day and I can't re-create the problem.

I gave it back, and he says it's still doing it.

I know the brakes themselves work smooth as butter at this point. I'm guessing the weird feeling is the ABS system modulating line pressure? And since it only does it occasionally, my first thought is that maybe there's an issue with one of the wheel speed sensors? Or a broken tooth on the reluctor ring (not sure if that's even possible)?

Close? Not even close? Other thoughts or things to check?

Thanks.

zukkev · Nov 14, 2009

There have been issues with wheel speed sensors causing the very symptom you are describing...if you have access to a way to test each one, you should be able to isolate the problem sensor---it is also possible the ring came loose or has a dent on the tone ring bad enough to cause erroneous readings on one of the front shafts too...

64FJ40 · Nov 14, 2009

zukkev said:
There have been issues with wheel speed sensors causing the very symptom you are describing...if you have access to a way to test each one, you should be able to isolate the problem sensor---it is also possible the ring came loose or has a dent on the tone ring bad enough to cause erroneous readings on one of the front shafts too...

Awesome. Well, not so much, but at least I'm not searching in vain.

Is the wheel speed sensor just an open/close when the tone ring is within range? Or is it more complicated than that?

zukkev · Nov 14, 2009

64FJ40 said:
Awesome. Well, not so much, but at least I'm not searching in vain.

Is the wheel speed sensor just an open/close when the tone ring is within range? Or is it more complicated than that?

No, not square wave, speed sensors produce a sign wave (a/c signal)--- to really catch the glitch in the signal, a lab scope is the best tool, unless it has a long enough glitch to catch it using a DMM (not very likely given the intermittance of the problem!)---WSS's produce their own a/c voltage as the ring spins, the abs controller then converts this a/c voltage into a digital reading so the PCM recognizes it...

64FJ40 · Nov 14, 2009

zukkev said:
No, not square wave, speed sensors produce a sign wave (a/c signal)--- to really catch the glitch in the signal, a lab scope is the best tool, unless it has a long enough glitch to catch it using a DMM (not very likely given the intermittance of the problem!)---WSS's produce their own a/c voltage as the ring spins, the abs controller then converts this a/c voltage into a digital reading so the PCM recognizes it...

Ah. I figured that would be the case. Thanks for the info. Time to send this one on to someone with a scope.