Transmission Vibration?

[SR71]

I posted about this before but bar being pointed in the direction of a problem with the transmission, I'm at a loss for sources of the problem.

My car has developed a vibration that appears to be boost/load/speed related.

In 4/5/6th gears, above about 60mph, whenever I get the turbo into boost, I get an a vibration of increasing frequency as the car accelerates. If I back off, it goes away.

If I accelerate very slowly, without getting the turbo into boost, I can avoid the vibration.

It would thus appear to be torque related.

In the first 3 gears, I can accelerate at WOT and completely avoid the vibration (except perhaps until very high rpms).

Audi have looked at the car and want to change the gearbox and rear diff.

They say £5K for the gearbox and £2.5K for the rear diff.

I think not.

I am wondering whether the flywheel (the car is a 1994 ABY) might be in need of an overhaul? How do flywheels fail and/or what are the symptoms of such a failure? The vibration is only apparent when there would appear to be a significant torque being generated.

The car is driveable but it is an annoying vibration that has developed gradually over the last few months and is now present as soon as I step on the thottle when cruising. As most of my time in the car is M-way driving, it needs to be fixed.

Any sources of advice much appreciated and any garages in South Wales who might be a better diagnostic bet than Audi also would be helpful.

Cheers guys.

[SR71]

I have now discovered the source of the vibration...complete fluke tho.

Its the rear diff.

I got a puncture in one of my rears and switched to the spare whose rolling circumference is far smaller than the original tyre as it is virtually bald.

Once fitted and back on the road, all my vibrations vanished and the car runs smooth as a dream again.

That being the case, can any gurus comment on what the significance of this observation is regards the health of my diff?

Does it need over-hauling? Re-building? Changing?

Much obliged as I've never seen anything other than the bottom of mine.

Cheers.

[caterpillar]

IF you have alittle grease in your outer part of propshaft(it is too little no) i think the vibbs are gone. it is some tricki behind the rubber.use mobil xhp2222.

[newsh]

I think the cat says grease the outer cv joints, under the rubber boots?

[SR71]

Thanks for the pointers guys....

That said, I'm confused as to how a lack of grease on the CV joints would manifest these symptoms?

Surely, they'd be there all the time rather than a function of a changed rolling circumference?



I've got a few more things to try like new exhaust hangars, changing the rear axle mounts...

[audiman]

If all you did was change the tyre, then either a) it was the tyre or b) the diff. Right?

[SR71]

Yeah I agree Tom...I didn't think I was being a dummy to arrive at the same conclusion.

But not being able to (easily) get under the car, and having never seen the inside of a diff, I was hoping a guru would confirm my diagnosis.

Having rotated all the wheels around in various combinations, I can't seem to get it to run without the vibes when using two tyres of the same circumference whereupon I suspect the diff.

But I've never heard of anyone with a similar failure and that suggests to me the failure may be something simpler but torque related...

But I'm not sure what that might be?

How would I know if its the open rear diff that is at fault or the Torsen diff?

Thanks for the help.

[audiman]

Thats a hard one, i guess it would be the diff. But i wouldnt like to be 100% certain on that one. Glad its not one of our customers!! Sounds like a right pig to diagnose.

[dan_b]

What tyres do you have, how old are they and what condition are they all in? Are all your alloys in good condition?

[SR71]

Tom & co.,

Don't laugh too hard. You must be the nearest Audi guru with a garage to me in Cardiff! Its about time I met some MRC personnel!

I'm tempted to drive around with the spare on all the time. The open diff can cope with that, but the Torsen?

As for the tyres Dan....well they were all changed at the same time about 10 months ago and still have ~4mm tread left on them. One of the tyres has been puncture repaired twice now so obviously has two plugs in it but I presume that having been balanced when it was re-fitted, this would not enter the equation as far as a consideration that is generating my symptoms.

The spare is virtually bald all over.

Remember the vibration only manifests itself above 65mph, in 4/5/6 gear, when the turbo is in boost...AND I've got tyres of the same circumference on the back!

Diagnose that!

At a recent service at Audi, I've been told the rear axle mounts are worn/loose ( a potential source of the problem?) so my only course of action would be to gradually work through all the mounts/bearings replacing them one by one before I replace a diff.

I can't believe its the diff myself.

The car has not been abused, only has 160K on it, I don't drop the clutch at 6000rpm, and I don't know of anyone who has had a S2 who has replaced a diff. Why would it go?

The problems manifest themselves over a period of time, First they were only present at very high rpms, but now they are present further down the rev range.

The car still performs like it should in terms of boost/performance etc but its annoying and shouldn't be there.

Comments and diagnoses gratefully received.

[SR71]

I finally managed to get right under the car (when at the garage for a MOT) and see what the problem was.

Sure enough one or two of my exhaust hangars have dropped and the exhaust was rubbing on the OSR drive shaft creating the vibration.

When generating torque, the free-play in the diff is taken up enough to cause the shaft to come into contact with the exhaust and rub.

Nevermind. A 3" system from Prospeed a la Bibby is going on in a couple of weeks and that little gremlin should be nicely ironed out.

So much for the Audi stealer saying my gearbox and diff were fubar and it'd be £5.5K to change out.

[volcano rs]

the dealers just change the parts until they cure the problem .sometimes they dont even cure the problem but still charge you for the parts they have replaced . only difference between them and a diy mechanic is they have parts they can try and we dont .