A4 Tdi Quattro, Rear locking up after losing grip... Help!

[SilverSaloon]

Merry Christmas everyone, I havent had the quattro long and bought it for the extra traction at this time of year, Like many i couldnt resist lighting up all four wheels on a snowy car park near me, After spinning the wheels and letting off the throttle, The rear drivetrain seems to lock up as if the handbrake is on for a couple of seconds then free itself, if i press the throttle when its tight (after the initial spin) it will stay tight until i come to almost a stop then it will be free, Now i know it will either be the abs or the rear diff, How can i solve this problem> I also get the abs feeling at the pedal at very low speeds and the abs light comes on sometimes and goes off on its own, but i cant help feeling its the diff as it only happens after a full on 4wd spin and its definitely the rear, (i cant tell if its one sided or both, I dont really want to keep doing it in case something happens. Look forward to your replys people. Many thanks. Nicky.

P's The abs light does not come on when this problem happens.

 

[murran]

does it have traction/esp? find the abs fuse, pull it out and render the interfering nannying invention inop. these systems are only fitted to cars because 90% of people have no idea how to drive!!

 

[aragorn]

not entirely.

Audi fitted EDL to quattros to get round the fact that the car has two open diffs and a centre which will only lock up when its detecting a torque differential, which it often wont do on ice.

I suspect your locking the wheels under engine braking, much like you would do on a motorbike if you downshifted too hard. Doesnt usually happen on cars due to the much larger tyres etc, however on ice or similar its entirely possible.

 

[SilverSaloon]

does it have traction/esp? find the abs fuse, pull it out and render the interfering nannying invention inop. these systems are only fitted to cars because 90% of people have no idea how to drive!!

Yes that thought crossed my mind an hour or two ago, can then isolate the problem. Thankyou.

 

[SilverSaloon]

not entirely.

Audi fitted EDL to quattros to get round the fact that the car has two open diffs and a centre which will only lock up when its detecting a torque differential, which it often wont do on ice.

I suspect your locking the wheels under engine braking, much like you would do on a motorbike if you downshifted too hard. Doesnt usually happen on cars due to the much larger tyres etc, however on ice or similar its entirely possible.

Hi, Thanks for the reply, Its definitely not engine braking, when it does it i can floor the throttle and its as if the rear brakes are on, then it will disengage on its own after a second or so, I assume it doesnt have t/c i dont think as theres no switch to disable it. cheers.

 

[NineNails]

Could be ABS, I had similar when I had a rear sensor intermittently faulty. It was as if the car would just put on the brakes by itself. I would have said the ABS kicking in may be normal for the conditions you are driving in BUT the ABS warning is telling you that you have a problem and you make it sound like it's happening at random so... Vag-Com is the way to go, a scan will probably tell you what's going on. I think it's very rare to get a diff fail on these cars

 

[Broken Byzan]

EDL detecting the slip and then locking the rear brakes to push the power forward to get traction would be my guess

 

[aragorn]

EDL only works across the axle, and each axle is independent.

IE it wouldnt lock both rears if it detected slip, the most it could do is lock one front and one rear, but even then, as soon as the wheel locked the system would disengage it., it wouldnt stay locked for a few seconds like the OP describes.

Given it stays locked upon reapplying power, i'm going to guess its either some weird ABS issue (even though ABS only activates when you brake...) or more likely something mechanical, like binding handbrake cables or a sticky rear caliper causing enough drag to lock the wheels until the Torsen locks up and does its thing. On normal roads this wouldnt even be noticable, because the front wheels would simply drag the car along, but on low traction surface the torsen wont lock up until its actually under load, and the torque output is so low on a very slippy surface that it may just do exactly what you describe until you build up a little speed and the loads on the diff increase.

 

[SilverSaloon]

Thanks for the replys people, I havent found the exact problem but it was the abs, i since noticed when it was giving this fault the brake pedal would become solid, I have for now removed the abs fuse, Its stopped doing it after loss of grip, possibly one of the problems described below, I Just have the bleeping at startup now and unusable obc, Small price to pay for a trouble free winter, Thanks again everyone. Off to "panic buy" lots of diesel!!

 

[aragorn]

You need to fix it properly.

Driving around with the ABS disabled screws up the brake bias, meaning the rears are likely to lock up under heavy braking (and ofcourse in snow/ice they'll potentially be able to lock up even under light braking due to the much reduced levels of grip).

The fact you say the ABS kicks in at slow speeds is the key, it will be caused by a crudded up or possibly even broken reluctor, an incorrect sensor airgap, or flakey sensor.

Given you said it sometimes braught the ABS light on, there should be a fault logged as to which wheel its getting upset with.

 

[SilverSaloon]

You need to fix it properly.

Driving around with the ABS disabled screws up the brake bias, meaning the rears are likely to lock up under heavy braking (and ofcourse in snow/ice they'll potentially be able to lock up even under light braking due to the much reduced levels of grip).

The fact you say the ABS kicks in at slow speeds is the key, it will be caused by a crudded up or possibly even broken reluctor, an incorrect sensor airgap, or flakey sensor.

Given you said it sometimes braught the ABS light on, there should be a fault logged as to which wheel its getting upset with.

Yes, I have a vagcom program and only recently replaced a damaged rear abs sensor and may have disturbed the front one during cv boot change, I will give it a scan and report back, it may be the gapping on the sensor i replaced, i reset the fault after i replaced the sensor, passed the mot then the abs light came back on after approx 200 miles.. But goes off and on when it pleases, then i had the rears locking after a wheelspin.. Also i dont want to start a new thread for this but it feels like under engine braking there is a lot more drag/resistance than my other 2wd tdi's i assume this is the 4wd system, anybody else feel this? thnks.

 

[Audi2ptzero]

EDL only works across the axle, and each axle is independent.

IE it wouldnt lock both rears if it detected slip, the most it could do is lock one front and one rear, but even then, as soon as the wheel locked the system would disengage it., it wouldnt stay locked for a few seconds like the OP describes.

Given it stays locked upon reapplying power, i'm going to guess its either some weird ABS issue (even though ABS only activates when you brake...) or more likely something mechanical, like binding handbrake cables or a sticky rear caliper causing enough drag to lock the wheels until the Torsen locks up and does its thing. On normal roads this wouldnt even be noticable, because the front wheels would simply drag the car along, but on low traction surface the torsen wont lock up until its actually under load, and the torque output is so low on a very slippy surface that it may just do exactly what you describe until you build up a little speed and the loads on the diff increase.

Actually it can lock up both rear tires at the same time if both of them are spinning faster then the front tire. I actually snapped the rear output shaft during a launch because the control unit saw that the 2 rear tires were spinning faster then the fronts because of a bad front left abs sensor. Not a good thing when launching a car that is making over 500awhp. I ended up just unplugging my ABS completely since the car is only used at the drag strip, never had the problem again after that.