ABS problems? Need some advice...

[AnniinaW]

Hi Guys,

I need a bit of advice. I have a 1998 Audi A4 (B5) 2.5 TDi Quattro and I have some kind of problem when I am braking.

Issues with the car:
The brakes are giving feedback at slow speeds but seem to work fine at higher speeds. While breaking the peddle kicks back slightly with a noise that is like a spring moving (I suspect this is the same feeling as when the ABS normally kicks in) but the road is dry and there are no problems with traction.

What has been done so far:
The front pads have been changed 2,5 months ago. Another garage had a look of the breaks and said they looked mechanically ok, suspected ABS-issues. Then the car was taken to an auto-electrician. They also suspected ABS and said that the error is in the front right wheel. They said that the front left wheel picks up the speed slower than the others. They weren’t sure if the issue is the ABS unit or the Quattro-system. During the test drive at the garage the front left tyre blew out (while at low speed) and I have now replaced all the tyres (not sure if this was related or not or caused by the fault).
Nobody sofar has been able to give a reason for why this is happening. The garage said that I should change the ABS brain to see if that fixes the issue. I was quoted 700€ for that job which I am unwilling to pay on the off chance that it 'might' fix the problem.

Any ideas or a more definitive diagnosis?

Thanks
AW

Update: There are no warning lights for the ABS or anything else and the check comes back saying OK before I start the car.

 

[jbh]

you're quattro system is purely mechanical so the fault cant be there.

has the sensor been changed?
before spending lots of cash doing the abs ecu I would fit a genuine abs sensor and test. if still the same change the CV joint as this has the pick up ring built in for the abs sensor.

 

[aragorn]

The fault is almost certainly one of the reluctor rings on the CV joints. Its also almost always the rears that do it.

Unfortunately to find it, you'll need to remove each driveshaft from the hub, and inspect the ring. One of them will be cracked, and the crack creates a disturbance in the magnetic signal at low speeds, which makes the ABS unit think the wheel has locked up.

Audi will tell you you need to replace the CV joints themselves, but replacement rings are available if you search online.

 

[AnniinaW]

Thanks for the replies! I was hoping it wasn't the ECU and I'm not sure why the garage suggested I change it. They never mentioned the rings at all. I will check out the rings and test. Thanks again

 

[Arrkon]

As Kev has said, the rings cracking is a very common affair, and considering the cost its well worth checking first. as this pic shows.

If memory serves, part number is 8D0 614 149 for the rear (on the back of the disc) but cant remember the front, it does however fit around the CV joint body. Both are shrink fit, so need to get the ring glowing hot to fit them properly (and i also left the new discs in the freezer overnight to help).

 

[aragorn]

Audi dont sell the CV joint ones, and as its a quattro they're all on the CV joints

 

[PAULF]

You can see which one has gone by hooking up VCDS and running the VAGscope (?) on the wheel speed sensors.
If you drive below 20 (KMH?) you can see the sensor speeds. Each sensor is a different colour on the graph.
They should all be the same. The cracked one will spike, the colour will tell you which one it is.
The pic above is a rear. I had one rear on one car and both on the other cracked.
http://www.audi-sport.net/vb/a4-s4-...replace-front-rear-suspension-parts-pics.html
It's a bit of a ramble about suspension generally, but I did manage to weld a ring up and refit it. As was said previously, you need to heat it to at least dull red to fit it - and you need to be quick or it will cool and stick on!

 

[spartacus 68]

Sometimes it's not obvious there's a crack in the reluctor ring compared to Arkkon's picture. This was the hairline crack on mine. I released the splined bolts at the driveshaft on the diff to remove.

From memory it was rear offside. Bingo!

 

[AnniinaW]

Took the car back to the garage as I am not capable of doing this work on my own. They claim they tested each sensor and ring and can't find anything wrong with them. Any other ideas or should I get a second opinion?

 

[jbh]

Took the car back to the garage as I am not capable of doing this work on my own. They claim they tested each sensor and ring and can't find anything wrong with them. Any other ideas or should I get a second opinion?

its always worth getting a second opinion

 

[aragorn]

They're idiots then.

You need to physically remove, and inspect the shafts.

It might be worth printing out that photo of the cracked ring above and taking them it so they can see what they're actually looking for...

Rear offside seems common, not sure why, but i'd be starting there, then the other rear, then the fronts if you've not found anything.