Brake pedal vibration/chatter

[Avant20v]

I am having an issue with my brake pedal chattering or vibrating. It feels similar to the ABS but there is no sound that goes along with the chatter. When I start the car and depress the brake it will chatter (while standing still). The problem will persist even while driving. In the past this has occurred if I had not driven the car for a while but it would go away by itself or if I held the pedal down for a few seconds. I have newer brakes (pads/rotors) on the car right now so I am sure that they are good. I have been running my steel wheels with snow tires on the car for the winter and have done so since October, so no change there. The brake light has become static ON since this has occurred. I think that the abs unit/module has gone out or is malfunctioning. I have not bled the brakes but I guess I could try that out. But I wanted to throw it out there just incase you guys have seen something similar or have personal experience with a problem such as this?

[scarman]

Originally Posted by Avant20v

I am having an issue with my brake pedal chattering or vibrating. It feels similar to the ABS but there is no sound that goes along with the chatter. When I start the car and depress the brake it will chatter (while standing still). The problem will persist even while driving. In the past this has occurred if I had not driven the car for a while but it would go away by itself or if I held the pedal down for a few seconds. I have newer brakes (pads/rotors) on the car right now so I am sure that they are good. I have been running my steel wheels with snow tires on the car for the winter and have done so since October, so no change there. The brake light has become static ON since this has occurred. I think that the abs unit/module has gone out or is malfunctioning. I have not bled the brakes but I guess I could try that out.

But I wanted to throw it out there just incase you guys have seen something similar or have personal experience with a problem such as this?

Yes, I have seen a few drivers dumb enought to drive the car with a brake warning light in their face.

My guess from here is that the pedal pulsing is pressure pulses from the pump and the warning light is a result of low pressure at the hydraulic brake boost cylinder. If it stays on continuously, it appears the pump cannot provide sufficient pressure to charge the system to operating pressure. This could be because of insufficient hydraulic fluid in the system, a leak, obstruction in the lines or filters. or a bad pump. This would seem to indicate a badly neglected hydraulic system. You can unplug the connector on the pressure sensor located on the boost servo to see if that is causing the light. This doesn't fix anything, but it does confirm the cause, since there are several reasons for the light. Plug the connector back on the pressure sensor.

[Avant20v]

Thanks for the reply. I have been driving my 928 since this issue has come up. So I took out the 6 today to see if I could gather anymore details and it turns out after a couple minutes the pulsing stopped and the light went off. But I did read your reply and I think there is more validity to the pump being the issue. I watch the hydraulic fluid like a hawk to be sure there are not issues to that system.

Hydraulic fluid in the system checked and good.
No leaks detected.
Obstruction in the lines or filters possible but I have recently replaced the rack and no obstructions were detected but it can be placed on the list.
Or a bad pump—I think it is this more than anything. To be clear the pump you are referencing is the abs module/pump? I think they used the same pump in the A6 and 100 but I will have to look it up. I'll have to track down a pump.

Next time the light comes on I will unplug it and see if that affect it.

I'll keep you guys posted.

[SchwarzS6]

This problem could also be due to a completely depleted hydraulic pressure reservoir (brake bomb). This would allow the pulses of the pump to be felt directly in the pedal rather than be smoothed out by the diaphragm in the bomb.

[scarman]

I'm with you. Of course, if this occurs every morning, you will need to find the cause. If it is a defective bomb check valve or diaphram, the pump should be able to charge the system and extinguish the light in ~30sec.

The reserve pressure that is stored in the bomb is the emergency reserve in the event the engine quits or belt fails while on the road.

[SchwarzS6]

I think that it may also act like a capacitor and filter out the high frequency pulses from the pump. If this is the cause I would not be surprised if he also felt some vibration at a high frequency in the power steering.

[scarman]

I am talking about the hydraulic pump for the steering and power brake systems. It is unique to UrS models; Isn't that great news. : ) You can look for part # 054 145 165 LX (ZF rebuilt pump) at San Diego Audi-genuineaudiparts.com or at online dealers that sell the ZF rebuilt:
< http://www.germanautoparts.com/Audi/S6/Steering/164/1> or you could get a rebuild kit (o-rings and seals) and do your own If indeed it is a bad pump.

At $500 for a pump or $300 for an accumulator, this is one problem that is not best solved by throwing parts at it. This one is a candidate for real honest troubleshooting, but without a pressure guage to put on the system, it can be difficult. This is why I am not ready to quit with the guess that the pump is faulty.

You have done some already when the steering system was repaired. The Jorgen rack rebuilds require a fluid flush and a Magnefine inline filter in the return line to the reservoir. If the system was flushed and is now bright and clean, that is a good start. Next, I would check the filter screen in the reservoir. It should be bright metal. If it is black, that is hose residue. The brake "J" hose that provides pump pressure to the bomb has a small screen at the bomb end banjo fitting and a restrictor in the center of the hose that could catch a larger piece of hose. If I am opening and cleaning my system, I would put one of those 3/8" Magnefine filters in the reservoir return. If the system and filters are clean and the fluid bright, and you see no improvement, things get harder. If the system uses fluid, you know there is a leak. Squeeze that "J" hose fat insulator to make sure it is not saturated with fluid from a very small leak that drips unnoticed from the bottom connection. Test the accumulator function by checking the # of brake depressions after shutdown before the pedal becomes hard. Check the servo return line to make sure it is not leaking fluid by without the brakes depressed, returning the pressurized fluid back to the reservoir. If it leaks, some hard brake applications may force enough fluid through to remove a small piece of hose junk from the check valve. You would rather not change the servo for sure.

If you can't find a leak or obstruction and the accumulator stores pressure for pump off braking, then the pump is the only part left.

[Avant20v]

Thanks guys. I will have more time this next week to run through your suggestions. Can I rebuild the pump I have if is indeed the problem?

[littleyellowbat]

I have been having a similar problem to the fellow that started this thread. The "Brake" light commonly stays on upon start up. It normally goes away after a few minutes but can come on during hard braking while driving. I tried pumping the brake pedal quickly after shutting down the car and it takes normally 38 or so pushes before the pedal gets hard. Does that mean that my brake accumulator is fine? I tried holding the steering to lock for 30 sec when the light was off and then pushed the brake pedal - the light immediately came on. I have not detected any fluid leaks. Unplugging the sensor wire that scarman spoke of does not turn the light out.

I have a rebuilt bomb for the car but I'm not sure that thats the problem. Would it be worth it to just flush the system of fluid and put new in? I think its been many years since it was replaced.

Thanks for the help.

[scarman]

The pedal pushes could have been too rapid for the system to fully repressurize from the accumulator so the 38 may not be accurate, but I think you can assume that the accumulator is working properly.

I don't know exactly when you unplugged the servo pressure sensor, so I'm not sure what your check indicates.

I think you can examine the fluid visually and determine if a fluid flush/change would be beneficial. Your issues seem to be a weak pump or some restrictions in the system from dirty fluid (which usually results from rubber hose deterioration).