Brakes releasing momentarily before stopping.

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  #1  
Old 03-13-10, 03:08 PM
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Brakes releasing momentarily before stopping.

This is a well maintained 1993 Olds Silhouette, 3.8L engine, with 232000 miles on it. Last year (about 10k miles ago) the brakes were worked on (new front pads, new rotors, resurfaced rear drums).

When coming to a stop, the braking works fine (slows the car in a normal fashion, no noises, no pulling); however, once the car is moving very slowly... just about to stop... the brakes release, making a graveling type of noise, allowing the car to slide forward a few feet and then the brakes catch again to stop the car.

Scared me to have the car release and slide like that so I took it to a local repair shop (as opposed to driving it to my normal mechanic who is near my office not my home).

The shop folks called me in after about 45 minutes to say they think the problem is with the Master Cylinder, but it could be because of burn marks on the rotors. They also said they had never heard of this type of brake problem so they weren't 100% sure. They tried to show me that the Master Cylinder was leaking onto the booster, but I just don't have the eye for that kind of thing (I didn't 'see' the burn marks either...). He said the leaking was letting air into the system and that caused my problem. I said I thought air in the system would cause the brakes to be spongy or the pedal would sink to the floor. He said something about the engine vacuum system and leaking only under pressure... In case it makes a difference, I noticed the repair ticket just said to check for Brake Fade.

So 4 hours, and $463 later, I picked up my car. They said they couldn't test drive it thoroughly because of the rain and the ABS system kicking in. When I drove it out of the parking lot, I could tell it still had the same problem as before they replaced the Master Cylinder.

I have a couple of questions that I would appreciate feedback on:

1. I suspect the brakes releasing with the graveling type of noise was the ABS and my real problem is with the ABS kicking in to release the brakes when it should not. (it was dry when I first experienced the problem... raining when the repair guys were finishing). Maybe I should not 'self-diagnosis', sorry about that... what do you think the problem might be?

2. Having spent $463 on what was clearly not the problem is an issue for me. It's just way too much money for my current circumstances. And to now go back to pay for who-knows what they will suggest next, well, that kind of rubs me the wrong way. Should I go back or should I take it somewhere else where they sound more confident about understanding my braking problem?

Thank you, in advance, for your insight and suggestions.
 
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  #2  
Old 03-13-10, 04:22 PM
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Go back to that mechanic and say you want your money back because that did not fix the problem! Perhaps call the state attorney general.

I'm thinking ABS...

The way this works is if the wheels are about to lock up, or one wheel is about to lock up, the ABS system "takes control" of the brakes!

When it takes control, it quickly "pulses" the pressure in the brakes (perhaps the "graveling" noise) allowing the wheels to continue rotating yet still have braking. You might feel a pulsing on the brake pedal.

This is as if you were quickly pressing and releasing the brakes many times per second which would of course be impossible to do.

The good thing about this is there should be an electrical connector on the ABS. You should be able to disconnect that for testing. And see if the problem then goes away. If yes, then it is the ABS system.

BUT!!! Is the ABS system functioning normally? Or is this a malfunction with the ABS system? That would be the next thing.

There can also be problems with the power brake booster which would be vacuum powered. With the ABS disconnected, test drive the vehicle and see if the brakes seem to work normally - like other vehicles with power brakes. And be sure there is no weirdness with the brake pedal staying down by itself or anything else weird. I suppose there could be a problem with the brake booster which is applying too much force on the brakes, then causing a properly operating ABS system to activate.

Then another test with the ABS disconnected. Have someone watch while you drive and stop the vehicle. See if one wheel is locking up. Look from one side. Then test again while looking from the other side. If a wheel is locking up, then that would activate a normally operating ABS system to take control and keep that wheel moving.

Then if the brake system is working OK other than the ABS, and when this problem occurs, you are on a good surface as opposed to a dirt / mud road where a wheel could easily lock up, next I would check the tires. Is there a bald tire not getting good traction? If yes, perhaps that tire is locking up and skidding? (Causing the ABS to then activate).

If all the above checks out, then I would check the ABS wheel / speed sensors and wires. These sensors tell the ABS a wheel is rotating. If there was a poor connection or a weak signal, perhaps the ABS would think a wheel was about to lock up when in fact it was not about to lock up.

I think the only way to properly test the wheel speed sensors for proper operation would be to use an oscilloscope. And then you would need the factory service manual set to tell you what readings / voltages you should get.

Note: The following are not for your specific vehicle...

Here is what a wheel speed sensor might look like...
http://www.instructables.com/image/F...eed-sensor.jpg

And here is what the speed signal might look like on an oscilloscope...
http://images.picoauto.com/abs-fig1.png

More on ABS systems...
Real-Life Diagnostics - Diagnosing ABS

Anti-lock braking system - Wikipedia, the free encyclopedia
 
  #3  
Old 03-22-10, 05:25 PM
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Location: Northeastern PA.
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Real simple......

Go to the fuse box under the hood. Remove the fuse marked ABS. Drive the car...If the slip and chatter are now rectified (Ignore the ABS lamp For the moment)...Look at the front axles for a chipped or missing tooth on the tone ring. ( If it has an integral Hub Bearing, the tone ring is sealed and you wont be able to see it).

Anytime the ABS light is illuminated, ABS function is disabled, but Service brakes still function normally. If your problem is solved by deactivating the ABS system, then 100% , you can blame the ABS system for your problem.
 
  #4  
Old 03-22-10, 08:03 PM
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resolved as...

The service manager was very receptive to my explanation of my concerns. He said that if I was concerned about having to pay for repairs that did not fix the problem... don't be. They will not charge me for work that did not fix the problem.

So I left the car with them again. They retrieved the error codes and ran some ABS diagnostics to determine that there was a problem with the front right wheel ABS sensor. He ran other tests to confirm that it was just the sensor and not any wiring. The sensor component was part of the hub assembly so he had to replace the assembly. The costs came to just a little over what I had originally paid. He gave me full credit for the prior work even though it included some kind of fluid flush (I think that's what he said). Whatever, he did give me full credit for the entire amount originally paid and it cost me only another $40 (including the cost for an oil change that I requested).

Thank you all for your insight and suggestions.
 
  #5  
Old 03-22-10, 08:32 PM
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Good mechanic!

Tell all your friends they are good guys!
 
  #6  
Old 03-23-10, 07:34 PM
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Location: Pittsburgh, PA
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Sounds pretty competent BUT.... any time I have had a problem as you describe and it was a wheel sensor you DID NOT need a hub. Unfortunately that is what they will do in most cases.

Here is how I fixed my son's ZR2 truck.. remove the wheel and unplug the wheel sensor. With a volt meter you need at least 200mv when you turn the hub, if not remove and clean the sensor. If that didn't work, and here is where they change your hub when not needed.... remove the wheel sensor, pop up the rivet that holds the stainless spacer that is under the wheel sensor, stuff a piece of a paper towel in the sensor hole and flat file the rust off of the hub assembly. The rust UNDER THE STAINLESS SPACER creats a larger gap and drops the wheel sensor voltage due to increased gap. In my case the difference between flat filed and not filed was 120mv to 850 mv! Reinstall the spacer/rivet/sensor..problem solved and NO PARTS NEEDED! You cannot see the rust under the spacer without removing it.

BTW.. NEVER put it back together without the stainless spacer, YOU WILL destroy your reluctor wheel in the hub.. YES I did it once!
 
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