dvs_dave

Right, no 100% lock, just a figure of speech. TORSEN is a torque biasing differential and it relies on friction in the gear sets to achieve that. Putting an incorrect amount of friction modifier in messes up the torque biasing ratio (TBR). D3 Quattro has simulated diff locks (auto brake application) front and rear to avoid the situation of a wheel with near zero traction causing the whole system to spin away its power. This system works well, but if you've inadvertently jacked up the TBR through imprecise addition of friction modifier, it's not going to work. This is because the TORSEN diff won't be able to push enough torque to the other axle as the TBR has been reduced. So you'll just sit there spinning and wondering why your AWD car doesn't appear to be all that AWD.

Torsen diffs are very effective and work very well in the right application, such as in a road car where a true mechanical full time AWD system is needed, rather than the more usual electronically controlled clutch based systems. But mess with its TBR and they become pretty ineffective.

Ron a8

good evening,

came across this post and having the same issue. Just wondering if any fix has been discussed?

dvs_dave

There is nothing to fix. It’s a normal function of the quattro system in the D3 related to how the TORSEN center differential works.

If you don’t like it, only apply the lightest of gas when turning full lock, or whilst coasting only. Hey presto, no “grumble”!

Ron a8

Good evening,
How much did you add? Have you had any other issues since you put it in? I remember reading where if you put to much it can destroy your differential. I welcome your thoughts and opinion on this.

dvs_dave

I give up.

Rob09A8

Ron did you resolve the thumping under the back seat at 300,000 miles or is that kms when you posted about it in 2021?
Is the diff central hanger bearing support still intact?
How about the other wear items, like rubber support mounts for the drive train?
Audi are very clear about not adding friction modifier to a Torsen unless it is rebuilt and starting with completely clean metal gears without the friction modifier on the surface.

jeasley4

Had the same problem in my 2021 Q8. Replaced with different fluid and didn't fix it. Replaced center diff and all is good. At least until another 30-40K miles.