Antilock Brakes kicking in at Low Speed on Dry Pavement?!

[chemist]

Here's an odd-ball for you. My wife is telling me that while driving in our community (all roads 25 MPH or lower) on perfectly dry pavement, the vans antilock brakes gave significant kick-back in the brake pedal upon approaching a stop sign. There was nothing abnormal about the stop--it was NOT a sudden or abrupt stop. She told me that she also felt it on putting the van in reverse to back up once it was stopped... This reminds me that over the past couple months my van has done this twice to a much lesser degree on backing out of the driveway. At the time I thought nothing of it, but now, not so sure...

What do you make of this? What should I look at? Could it be a wheel sensor, or would the antilock system just revert to standard brakes like on my older Buick once? I have NEVER had the brake lines open, so I wouldn't think air--but could it be? Anyone ever experience this one?

[HPbyStan]

There is a thread on that here somewhere. I was one of those that thought the ABS just defaulted to std brakes when there was a problem BUT, sometimes you end up with NO BRAKES at all. It happened to me and I'm too old for that much excitement. The problem started like what you are describing but got worse. I unpluged all the wires going to the ABS unit and NOW I have std brakes. Not as good as ABS that works but a lot better than a system that won't let fluid pressure get anywhere.

[LiftedAWDAstro]

Pull the ABS fuse for now and you should see the problem go away. The brakes will act just like standard brakes. I would suspect a wheel sensor getting dirty. The front speed sensors are in the hub bearings and the rear is done via the transfer case VSS sensor. No real easy way to clean the front sensors other than probably pulling the hub assemblies and removing the sensors. You could check to see if one of the hubs is getting worn or is already bad.

[chemist]

Thanks! Hey, for now, where is the ABS fuse located? I'm assuming it's not in the standard fusebox on the driver's side, or is it?

[peter]

Ditto on the sensor. In my case, it caused an accident. And in my case, the sensor was falsing due to worn bearings in the thingamajiggy knuckle where my halfshaft goes thru.

PS: Yes! "thingamajiggy" is indeed a technical term

[LiftedAWDAstro]

The fuse should be large 30 or 60 amp one located under the hood in the fuse box.

[chemist]

Pulled the fuse. Once pulled both brake and ABS lights came on. Replaced, both went out. Pulled and test drove, drove fine. Left it out.

That said, wife gave me another usefull bit of information. She said that when this first happened to her today prior to the kickback, pedal went straight to the floor without resistance. Then next time brake applied she got the antilock kickback...WTF?! That one just doesn't make sense to me. Anyone make sense of that one?

[kings-x59]

No sense here. But ABS systems will do weird things. I had a '99 cavalier that had a parking brake cable freeze up and it made the abs system clatter brakes like a jack hammer.

[ScottS]

PLEASE GET IT FIXED IMMEDIATELY.

Don't fool aroudn with t.

Yes it is the wheel speed sensors and while 99% of the time you can stop ok , the other 1% -

Park it until you fix it. Pull the fuse if you have to drive it but get it fixed.

Ask me how I know

[HPbyStan]

That is something that should have gotten GM to extend the warranty. A lot more risk envolved there than just pealing paint for an example.

[chemist]

That is something that should have gotten GM to extend the warranty. A lot more risk envolved there than just pealing paint for an example.

Warranty?! Hell, GM SHOULD have recalled it, and given a chip that told the system to go off-line when this happened. It's criminal not to have! Their 1995 Buicks LeSabres and 1990 Corsicas had ABS systems that reverted to standard braking when the ABS sensors failed, so we know damn well they HAD the technology to make this happen. Given some of the other posts here, I wonder how many people have died from this! :-k

[chemist]

You know what, screw the ABS system given what you folks are telling me. I'll fix it, but I'm staying with standard brakes if leaving the fuse out won't cause other issues--at least then I know 100% of the time the brakes will ENGAGE... Figure, I'll put the fuse back in prior to inspections so I pass without the idiot lights...

Will I be any worse leaving the fuse out?

[LiftedAWDAstro]

Leave the fuse out and the system reverts to standard brakes.

[stefko]

I have a 2000 Astro AWD Van with ABS that appears to do the same thing. At the end of a high speed stop the pedal fades and a noise is heard. I have to apply more pressure to the pedal to get the van to stop. I also felt this when stopping in reverse. At the end of the stop I fell the steering wheel pull slightly to the right. According to the Chevy service manual that I purchased for $150, the ABS system should be disabled under 8 MPH. I sense from other posts that this may not be the case. I will be pulling the fuse in the morning and giving it a test.

Is it just a cleaning of the front speed sensor that is needed. I heard from a mechanic that I just met that they change the sensors quite a bit. does anyone know of a voltage check that can be done to see if the sensor is bad?

thanks for the posts guys! Maybe the wife will be more loving when this problem is fixed.

I just finished replacing all of the rusted brake lines, master cylinder, leaking rear wheel cylinders and rear shoes. What a job!!!!!!!!!!!!!

[LiftedAWDAstro]

On the AWD's, the front speed sensors are located in the center of the front hub bearings. You can remove them to clean them but seeing as the hub bearings are sealed, the sensor should be clean. If they are dirty, the bearings are probably starting to fail. To pull the sensors may require you to pull the hubs.