What could cause permanent 4WD?

[[Dave B]]

I'm currently trying to diagnose some wierd noises coming from the rear of the car and my investigations lead me to believe something is locking the car in 4WD (as the rear wheels are dragging when on tight lock). Anyone any ideas what could cause this? My thoughts are:

- Siezed haldex clutch plates (is this realistic on 75k miles? could it be caused by low haldex oil or water ingress?)
- Something electrical causing the pre-charge pump to stay primed?
- Something mechanical causing the pre-charge pump to stay primed?

Haldex fluid is getting changed tomorrow to rule that out.

Cheers,
Dave.

 

[sportstractor]

The haldex should disengage on full lock, there's loads of threads on the matter but it never seems to be one fix, different things works for different people, but a fluid change is a good start

 

[<tuffty/>]

Dragging at low speeds is symptomatic of sticking haldex clutch plates....

Intersting read...
The Audi TT Forum :: View topic - Rear Diff Haldex Dissection After Diff Was Destroyed

<tuffty/>

 

[[Dave B]]

The haldex should disengage on full lock, there's loads of threads on the matter but it never seems to be one fix, different things works for different people, but a fluid change is a good start

This is what i've been finding on all the other forums. I'm not getting any errors in vagcom so no obvious joy there and up until last week when this noise started (and I stopped driving it) I was still getting 4WD when I nailed it. Your guess is as good as mine

Dragging at low speeds is symptomatic of sticking haldex clutch plates....

Intersting read...
The Audi TT Forum :: View topic - Rear Diff Haldex Dissection After Diff Was Destroyed

<tuffty/>

Yeah, I've read this thread already, cheered me up no end! If they have siezed and the fluid change doesn't sort it then I'm gonna feel pretty unlucky. The cars only done 75k and its had the required oil and filter changes either on or before schedule!

 

[S3featesV9]

Fluid change and earth strap solved my dragging issue.

 

[[Dave B]]

Fluid change and earth strap solved my dragging issue.

This is what I need to hear!!

Following more t'internet research I've had Vagcom on it again. I used the AWD module to run an output test on the pre-charge pump. This cycles the system through various states. The first state is 'Pre-charge pump OFF', at this stage I heard a very, very faint click from the rear underside of the car (not the 'clonk' I was expecting from reading horror stories on the net). On all other stages of the process I didn't hear a single noise from anywhere. I'm not sure if this is good news or bad news, but I'm airing on the positive side due to the pump not clonking....

I also used the measuring block 002 and noted the voltage of 12.20....I've no idea whether this is good or not either....

 

[murran]

if you unplug the haldex under the car you should have no drive to the rear axle at all. is it still dragging when turning corners? jack up one rear wheel and turn it. does it turn? it should. if it does, does the prop turn as you turn the wheel? it shouldnt.

plug the haldex back in and clear any fault codes. repeat the above? any different?

 

[doublem_1]

How can you tell if the Haldex is dragging?

 

[[Dave B]]

Progress! Prior to changing the oil I took the car out for spin to warm it up to the required temp, whilst out I removed fuse 31 to see if that would disengage the unit (hadn't tried this before as I didn't get any errors is VCDS) and low and behold no more dragging. Got underneath and discovered this:

I hope i'm right in saying that this is a snapped earth strap for the haldex/diff and is likely to be the cause of the problem (i'm surprised how clean the break is TBH). Just spoke to TPS and they're £10 each new but they won't have any until Wednesday so it looks like its 2WD for me until then.

Thanks for the input gents, much appreciated.

 

[S3featesV9]

Halfords mate, £5!!!

 

[murran]

careful you dont snap the stud off the body of the car when removing it.

 

[S3featesV9]

This cable was pretty much the exact size You need too. Other ones they stock are either far too long or short.

I didn't have any issue undoing the bolt and nut, Came off with ease.

 

[Westy]

The dreaded haldex earth strap strikes again.

 

[XFi]

careful you dont snap the stud off the body of the car when removing it.

I had this, "Can't believe this is actually budging... snap!"

 

[G60DAN]

Had the same thing with the earth strap but had to put a bolt in the boot floor as it snapped off .