Really odd Brake Problem?????
11-30-2001, 11:27 PM
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Really odd Brake Problem?????
87 928 s4
If any one can figure this one out, it sure would be appreciated.
The problem started with a on and off sort of a crunching of the brakes only when coming to a slow stop. The pedal also drops a little when the crunching/slip grabbing happens. It is worse when i would wash the wheels and get the rotors wet. Okay, so it did this for a couple of weeks on and off. Brought it to my mechanic, GULF performance and they could not find the problem. It did not happen while they had it. Then I started to get the same problem and the ABS warning light would come on. When the ABS disables itself, the problem goes away. Okay, so I pull the fuse on the ABS and take it back to GULF. They start doing some diagnostics and test all the sensors, ABS pump, wiring etc. and everything appears to check out okay. So they advise me to change the control unit. I procure another one from a running driving working ABS car and plug it in. Well now I no longer get ABS warnig lite and subsequent shutdown of ABS system, but I still have the crunching/slippage at low speed easy stops. I can go 100 miles an hour and jam on the brakes and they are completely normal. But if I slowly cruise up to a stop sign, right at the point where the car is going to stop I get CRUUUUNCHHHH. Help!!!!!!!
I could just get rid of the ABS but I really want to keep my car perfect.
Thanks in advance,
RichinFlorida
White 87 s4 with 86,000 miles.
If any one can figure this one out, it sure would be appreciated.
The problem started with a on and off sort of a crunching of the brakes only when coming to a slow stop. The pedal also drops a little when the crunching/slip grabbing happens. It is worse when i would wash the wheels and get the rotors wet. Okay, so it did this for a couple of weeks on and off. Brought it to my mechanic, GULF performance and they could not find the problem. It did not happen while they had it. Then I started to get the same problem and the ABS warning light would come on. When the ABS disables itself, the problem goes away. Okay, so I pull the fuse on the ABS and take it back to GULF. They start doing some diagnostics and test all the sensors, ABS pump, wiring etc. and everything appears to check out okay. So they advise me to change the control unit. I procure another one from a running driving working ABS car and plug it in. Well now I no longer get ABS warnig lite and subsequent shutdown of ABS system, but I still have the crunching/slippage at low speed easy stops. I can go 100 miles an hour and jam on the brakes and they are completely normal. But if I slowly cruise up to a stop sign, right at the point where the car is going to stop I get CRUUUUNCHHHH. Help!!!!!!!
I could just get rid of the ABS but I really want to keep my car perfect.
Thanks in advance,
RichinFlorida
White 87 s4 with 86,000 miles.
12-01-2001, 12:47 AM
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Gulf told me the rotors and pads were fine, I do not think they checked the wheel bearings. But wouldn't the problem persist with the ABS in or out of system? The problem vanishes when the fuse is pulled on ABS.
Rich
Rich
12-01-2001, 02:15 AM
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Rich,
I would absolutely first start with Snowy's answer because that's the usual cause of that kinda noise. (Always start with the simple and cheap) I would also pull the pads and look at them (HIGHLY unlikely) but they may have picked up a rock/debris.
However, there IS a way to tie the noise to the ABS. The first question I would have would be - where is the noise coming from? Is it all four wheels, just fronts, just rears? Also, did your mechanic use the factory tester?
The ABS hydraulic unit is basically a couple of electronically variable valves that control the amount of pressure between the master cylinder and the calipers. If one/both of those valves is "hanging up" due to internal stickiness, or bad input values from one of the wheel sensors, then you MAY achieve the symptomology you describe.
If you are confident of the testing that was done on the ABS system, and the rest of the mechanical aspects of the brakes and bearings check out OK, I would first try doing a very thorough flush and fill of the braking system - if something is just marginally stuck, this may free it.
If the noise is isolated to just the front or rear wheels, I would check the biasing valve (cylinder that looks like half a roll of nickels) attached to the outside of the ABS unit.
HTH,
Greg
I would absolutely first start with Snowy's answer because that's the usual cause of that kinda noise. (Always start with the simple and cheap) I would also pull the pads and look at them (HIGHLY unlikely) but they may have picked up a rock/debris.
However, there IS a way to tie the noise to the ABS. The first question I would have would be - where is the noise coming from? Is it all four wheels, just fronts, just rears? Also, did your mechanic use the factory tester?
The ABS hydraulic unit is basically a couple of electronically variable valves that control the amount of pressure between the master cylinder and the calipers. If one/both of those valves is "hanging up" due to internal stickiness, or bad input values from one of the wheel sensors, then you MAY achieve the symptomology you describe.
If you are confident of the testing that was done on the ABS system, and the rest of the mechanical aspects of the brakes and bearings check out OK, I would first try doing a very thorough flush and fill of the braking system - if something is just marginally stuck, this may free it.
If the noise is isolated to just the front or rear wheels, I would check the biasing valve (cylinder that looks like half a roll of nickels) attached to the outside of the ABS unit.
HTH,
Greg
12-01-2001, 12:26 PM
#4
An intermitant sticking caliper perhaps?
that may throw the ABS off and would have
more of a groaning sound.
Look for a discolored rotor after it happens
a blackish hue rather than shiny silver.
that may throw the ABS off and would have
more of a groaning sound.
Look for a discolored rotor after it happens
a blackish hue rather than shiny silver.