B5 S4 Disable ABS

[HibiscusS4]

Right I have an ABS fault that is not quite faulty enough to raise a fault code and tell me what to fix. Can I disable ABS in VCDS either Lite of full?

I have a judder through the pedal under certain braking conditions, I have diagnosed this myself to the ABS system as when I pull the fuse the judder goes away but the dash lights up. I know for an MoT I cannot have any warning lights on but on a pre 2001 car I am allowed to remove ABS. I am sure my MoT place would over look the sensors and other gunf still being present as long as there were no warning lights on.

I tried search and found lots of posts for disabling ESP and keeping ABS. I dont mind loosing the ESP as its only for the purpose of the test after which I will revert back. I need the car on the road to try to find the fault.

 

[adamss24]

I would check the rear ABS reluctor rings on the driveshafts/rear cv joints. They snap causing the ABS to work as it seems the car is locking a wheel- not throwing a code every time. Start with the driver side first as it's the one that snaps the most although i have seen it on the passenger side as well !

 

[PAULF]

Assuming this is pulsing/buzzing at low speeds, as was said above.

I have a couple of pics here How to replace front and rear suspension parts (with pics) of a rear cv joint with a broken ring.

To find out if this is the case, hook up your VCDS, go to 03 ABS, Measuring Blocks and select the wheel speed sensors. Log the results, and drive off, keeping below 20 (I think) kph - if you go above this speed, the safety feature will kick you out. Play back the graph and you will see a spike on the duff wheel.

If you can't do this with VCDS Lite, get somebody else to drive whilst you watch the readings for a jump in values on one wheel - such as going from 8 mph to 0 and back.

You can get new rings, but they are not cheap for the rear - and you need to heat them to dull red, then pop them over the cv joint (the correct way around) and they shrink fit on.

You shouldn't fail your MOT with this fault but if you pull fuses and get a light, you will!

 

[HibiscusS4]

Thanks, I have seen measuring blocks and I can log it in VCDS lite. I did not know about the safety feature so thats a good tip. I have looked at the front and the reluctor rings are buried right inside the hub not easy to see is it easier at the back or should I try the VCDS diagnostic first before jacking it up and taking wheels off?

Will it really not fail an MoT if they have pedal judder on the brake test? I assume they cant use the rollers as its a quattro so have to use the G force metre that goes on the floor while driving? (they dont have a 4 wheel roller at my place).

Assuming this is pulsing/buzzing at low speeds, as was said above.

I have a couple of pics here How to replace front and rear suspension parts (with pics) of a rear cv joint with a broken ring.

To find out if this is the case, hook up your VCDS, go to 03 ABS, Measuring Blocks and select the wheel speed sensors. Log the results, and drive off, keeping below 20 (I think) kph - if you go above this speed, the safety feature will kick you out. Play back the graph and you will see a spike on the duff wheel.

If you can't do this with VCDS Lite, get somebody else to drive whilst you watch the readings for a jump in values on one wheel - such as going from 8 mph to 0 and back.

You can get new rings, but they are not cheap for the rear - and you need to heat them to dull red, then pop them over the cv joint (the correct way around) and they shrink fit on.

You shouldn't fail your MOT with this fault but if you pull fuses and get a light, you will!

 

[HibiscusS4]

Well measuring blocks was inconclusive. You can only go 20KPH before getting disconnected from the controller. With VCDS connected the ABS light stays on and the fault goes away, when you get kicked out the fault instantly comes back. Logs show never more than 1kph difference between wheels and thats probably just caused by the sample rate of the dongle.

Going to run it down the the garage tomorrow and ask about an MoT, it runs out Tuesday and I have only just got the cam belt all put back on it. If they fail it I will at least know it is ok on everything else. If they use the decelerometer and its ok I might be able to pursuade them the jerkyness is not servere (its only servere jerking brakes thats a fail point). They dont jerk at all above 20mph.

 

[PAULF]

Did you log the results and play it back as a graph?

You should see all 4 wheels on the graph running within 1 kph. If a reluctor is damaged, you will see a spike every now and then on one wheel. The low speed doesn't matter, because as you go faster, the effect of the broken ring is lessened.

The fault goes away with VCDS hooked up because the ABS is switched off as a safety feature - so you don't accidentally switch your brakes off whilst driving. That is also why you get kicked out above 20kph.

You may not be seeing anything strange because your rings are ok!

Other people on here have had a problem with the sensors being slightly out of adjustment, or contaminated with rust dust.

One of the testers on here may be able to explain further, but both my cars had cracked reluctors at some point, and I had no problems with the test.

 

[HibiscusS4]

I recorded the log to a CSV then looked at that, if I turn it into a graph I assume it will still be base on this data? I am taking it for the test after work today so I will see. Pass or fail I will get all the wheels off one by one, remove the sensors and inspect the rings/clean the sensors to see if it goes away.

If you brake above 20mph it does not do it even if you hold the brakes down until under 20mph. If you brake between 10mph and 20mph it happens, a judder through the pedal. under 10 the car either stops before 1 revolution of the wheel or the effort is too light to make it happen.

 

[PAULF]

Yes, it is the same data. It's just so much easier to see all four wheels going at the same speed on the moving graph, so any difference stands out a mile.

You need to remove the driveshaft bolts and pull out the driveshafts to get to the ring.

When you tighten up the bolt, it is ***** tight, so check the torque settings (I can't remember but it's something like 70 lb./ft then a further 180 degrees)

It is a REALLY annoying fault though.

I found on one of mine that after a couple of hours on the duel carriageway at 70 MPH, the light came on and I had a fault code logged. There was no other codes appeared at any other time.

 

[HibiscusS4]

Well it failed due to the front brakes binding. Probably in part due to the car not moving for close to a month with the cam belt being done (funny noises and being a year over due ment it didnt get used until I had time to fix it).

Got the wheels off and the brakes do rub and after they are pressed its almost impossible to turn either side without the wheel on and then its easier but they dont spin until you tap the caliper to loosen them, then all is fine.

Problem is I cant see it being the cause of the judder, maybe it is.

 

[PAULF]

Binding brakes can cause the disc to warp, and then you get judder through the pedal. I have gone through this and had to get new discs.

If it is your ABS, you should hear a 'buzzing' when it kicks in.

 

[HibiscusS4]

Gutted, just spent what feels like a life time rebuilding the 2 front calipers, binding gone but judder still there. I havent checked the disc run out but I dont think its warped discs because it does not judder above 20mph, the brakes are perfect and it does not judder with the ABS fuse removed.

Got to choose now if I take the car again for an MoT or try to fix the judder. Everything I have read says you have to remove the whole drive shaft at the front to inspet the reluctor ring and that the sensor will break if removed!