Avant20v

Do you guys know if the rear diff is the same on the A6 as our S6? I ran in to a low milage replacement that I was going to pick up as a stand by just in case.

UrS4boy

What you need to check is what the build tag says. It needs to say "CKL" as shown in this diagram. The North American version of the UrS6, has a CKL rear diff with 37 teeth on the crown and 9 on the pinion gear yields a 4.11 to 1 ratio. The UrS6 MANUAL transmission has a 4.11 to 1 ratio for the front diff and therefore the rear diff has to be the same.

The build tag is in the trunk, as shown here:

UrS4boy

Note: If the A6 is an autobox, then the rear diff is very likely not compatible with the UrS6 transmission (4:11 vs 3.89:1)

Avant20v

Thanks. When you say auto box is that an automatic transmission. On a related note do you think that it could be my diff when I accelerate up hill it starts to make a rattling or alomst a small thumping noise? It seems to only happen under acceleration and uphill, I just changed the fluid in the rear and what I could read it takes 2 quarts.

UrS4boy

More likely to be the center driveshaft bearing. Have a read on this:

Drive Shaft and Centre Bearing and DIYs