Unsolvable ABS problem :(

[soku11]

Hi guys.
Patient:
Audi A4 B5 2.8 1999, tiptronic quattro

Current symptoms (no PC erros regarding ABS):
1. While braking from higher speeds (long braking distance) at about 20-40 km/h the ABS starts to "work". By work I mean a weird brake pedal feeling (scraping?). Then it starts to be softer and I push a little harder. In the end the ABS (?) activates and brakes the car by some kind of automatic brake punch. Probably it tries to compensate wheel speeds?
2. After letting go of a brake pedal on a stop with drive gear on, the ABS starts to buzz for a second and lights, etc dimms. Nothing else happens. The car rolls normally, no brake intervention.

Changed parts (by 4 or 5 mechanics):
1. 3 (!) ABS pumps changed - no improvement
2. ESP pump changed (the one underneath the ABS pump) - no improvement
3. ABS driver changed (under the rear seat) - no improvement
4. All 4 ABS sensors changed (the rear ones are different - they use some kind of screws) - no improvement

The last mechanic says that while braking the wheels speed read by the PC is not entirely correct. The speeds are not smooth. Unfortunately the car can only read wheel speeds under ~24km/h.

Does anyone know what to do? I'm hopeless. Should I buy a Yaris instead? :( Thanks for any help!

[MetalMan]

Sounds like the last mechanic at least is working on some diagnostic stuff instead of continually throwing parts at it.

Watching reported wheel speeds is the key, and in my experience this is how to properly diagnose the issue you have described.

If the car is driving straight on dry ground, all wheel speed sensors should report the same speed. If any corner's wheel speed reads differently from the rest, then the problem lays with that corner. Possible problems could be:
1) damaged sensor
2) sensor too far away from tone ring
3) damaged tone ring

(on my old '98 A4, I had ALL of these problems, also including a bad ABS module)

4) Less likely, but still possible, is damaged wiring between ABS module and speed sensor.

Since you say all sensor are new, you can probably rule out 1).
Hopefully the mechanic(s) is (are) competent, and know(s) how to properly install ABS sensors. If so, can rule out 2).

Again if a mechanic is competent, he should recognize a damaged tone ring. That would rule out 3).

Wiring is a whole different animal. You need a really competent mechanic to properly diagnose this.

[soku11]

1) The last mechanic changed all of the sensors. He claimed he had a car with similar problem.
2) 3 of the mechanics checked the sensor positioning. One of them even corrected front right one.
3) The last mechanic says that all of the abs rings are fine. His substantiation is also that the wheel speed is wrong only while braking.

The machanic is unfortunately on vacations for one week. He'll try to figure it out afterwards. Yesterday he checked ground and power cables - they seems ok. He thinks it's not related to the whole ABS system but other traction control systems. I don't know what to think - nobody can help my car :(

[seanf86]

Sounds like one of the rear tone rings are cracked, just dealt with this on a family members a4, I had to replace both rear axles, the rings crack and it's hard to notice sometimes

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[soku11]

Do You have any idea on how to diagnose a damaged ring? Is it possible that one cracked ring can cause weird wheel speeds on all wheels?
I spent a lot of money on it (I've also done the suspension and replaced brake disc, brake pads, etc) and I don't want to spent any more that is not needed (in my car the cost of a single axle i huge - the ring is in the bearing I think) :(

[seanf86]

Remove the end of the axle from the spindle and look close. This was the one that was causing me grief

[soku11]

One of the mechanics said that I have an ABS ring built-in the bearing in the rear wheels and it's hard to deassemble. Is it the one that you posted?

[seanf86]

One of the mechanics said that I have an ABS ring built-in the bearing in the rear wheels and it's hard to deassemble. Is it the one that you posted?

no, if your car is quattro there is no abs ring built into the wheel bearing, each corner of the car has a ring on the end of the axle.

[soku11]

I'm leaving the car today for the next approach. I'll talk about looking at the abs rings again.

[mfdk]

Hi guys.
Patient:
Audi A4 B5 2.8 1999, tiptronic quattro

Current symptoms (no PC erros regarding ABS):
1. While braking from higher speeds (long braking distance) at about 20-40 km/h the ABS starts to "work". By work I mean a weird brake pedal feeling (scraping?). Then it starts to be softer and I push a little harder. In the end the ABS (?) activates and brakes the car by some kind of automatic brake punch. Probably it tries to compensate wheel speeds?
2. After letting go of a brake pedal on a stop with drive gear on, the ABS starts to buzz for a second and lights, etc dimms. Nothing else happens. The car rolls normally, no brake intervention.

Changed parts (by 4 or 5 mechanics):
1. 3 (!) ABS pumps changed - no improvement
2. ESP pump changed (the one underneath the ABS pump) - no improvement
3. ABS driver changed (under the rear seat) - no improvement
4. All 4 ABS sensors changed (the rear ones are different - they use some kind of screws) - no improvement

The last mechanic says that while braking the wheels speed read by the PC is not entirely correct. The speeds are not smooth. Unfortunately the car can only read wheel speeds under ~24km/h.

Does anyone know what to do? I'm hopeless. Should I buy a Yaris instead? :( Thanks for any help!

Dude,im not sure if this helps you at all but i have the same issue more of less, with me thought it started with ABS module going bad. I would get brake light flashing whilst traction and abs light lit solid, after getting the ABS module replaced the problem went away but i still get the brake pulsating intermittently at speeds your describing, i might be wrong but to me the problem seem more persistent at warmer weather and definitly none existon when its cold outside or raining. Having said that another issue thats buggling me that may very well be related to Brake Pulsating; if the car has been parked outside in a rainy night or high humidity night for more then 5-6hrs as it starts moving i hear a loud squeeky noise coming from the front drive train, sounds as if something is rubbing against the Rotors, it only last for couple meters and then its gone.

As far Brake pulsating i had it diagnosed and they told me that both front wheel speed sensors are showing ubnormal reading, they told me to replace them and then see what happens, i didnt get the chance to do that yet, i have though inspected and cleaning them up.

Let me know if you do find the root cause to this issue...

Thanks

[soku11]

In my car there's no sign of a problem on the dashboard - no ABS/ESP blinking, etc. Even wheel speed sensors seems to work correcty - while driving all of them show the correct speed. The problem is only with braking.
The problem occurs independently of the weather and temperature.

The mechanic said that he will check the abs rings again and do other diagnosis of other ABS related mechanisms. I'll post the news and hopefully the solution :(

[crazyquik22023]

1) The last mechanic changed all of the sensors. He claimed he had a car with similar problem.
2) 3 of the mechanics checked the sensor positioning. One of them even corrected front right one.
3) The last mechanic says that all of the abs rings are fine. His substantiation is also that the wheel speed is wrong only while braking.

The machanic is unfortunately on vacations for one week. He'll try to figure it out afterwards. Yesterday he checked ground and power cables - they seems ok. He thinks it's not related to the whole ABS system but other traction control systems. I don't know what to think - nobody can help my car :(

How does one adjust the positioning of a wheel speed sensor on a b5?

[soku11]

No clue, just passing the information the mechanic gave me :(

[walky_talky20]

I'm going to second the cracked tone ring.

Really it comes down to signal quality. So if there are no codes or obvious faults, you can crank the diagnostics up to "hard mode". Get a 4-channel oscilloscope and capture all 4 wheel speed signals at once. There will probably be some that look good, and one or 2 wheels that have an obvious problem with the quality of the waveform. You get, like, 1000 bonus points if you post your captured wheel speed sensor waveforms in this thread, BTW. ;)

[soku11]

I got a phone call yesterday. Two of the back abs rings looks a little bit damaged. One of the has only a scratch, so firstly the mechanic will change the other one... I hope it'll finally solve the problem.

[mfdk]

I got a phone call yesterday. Two of the back abs rings looks a little bit damaged. One of the has only a scratch, so firstly the mechanic will change the other one... I hope it'll finally solve the problem.

Keep us updated please,

Thanks


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[soku11]

We've got a winner ;) It was a damaged back left abs ring. The machanic changed it and the problem is gone. No magic braking, no weird brake pedal feeling. The only thing left is that sometimes after releasing the brake pedal from the start I hear a buzz. Don't know why, but maybe it must be like this.

Thanks for You guys for helping me fix my precious car :D

[mfdk]

We've got a winner ;) It was a damaged back left abs ring. The machanic changed it and the problem is gone. No magic braking, no weird brake pedal feeling. The only thing left is that sometimes after releasing the brake pedal from the start I hear a buzz. Don't know why, but maybe it must be like this.

Thanks for You guys for helping me fix my precious car :D

Im still experiencing this issue, when i check the measuring Block with VCD to me all the reading seem the same from all 4 abs sensors.

Im assuming when the Tone ring/ABS ring is bad it would still show some abnormal reading on that particular abs sensor...did they just physically inspect the tone ring or was vcd showing abnormal reading too..!

Thanks



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[soku11]

The mechanic said that the reading was "weird" while braking, but not that one wheel had 0 speed. He inspected all of the wheel abs rings and physically diagnosed that one of them is "burned".