hunterp27

Alright teammates,

I think I am zeroing in on the culprit of the binding feeling of the front end of my 2007 S8 w 72K. I have read as much as I can find on this topic and have chased just about every potential lead that I have researched. I think I have narrowed this grinding, rubbing, moaning, groaning, binding, slightly shuddery sound/feel to the front differential. And, here is why. After eliminating suspension component wear (uppers, lowers, tie-rods, sway bars, struts, steering rack, power steering fluid and so on), I think I have dirty fluid in the front differential. The symptoms are almost exclusively when I am at very low speed while backing mostly (but moving forward too) and have the wheel turned somewhere between half to full lock. Yesterday when I was backing out of the driveway I slipped the transmission into neutral while all conditions and symptoms were present and the noise and feeling disappeared instantly. So, in other words when no "drive force" was being applied thru the quattro differential, the symptoms disappeared. That can only mean one thing right? The fluid is either low, dirty or both??? If this sounds right to the experts here, I think I'm gonna have the diff fluid drained and replaced. From what I have read here, the front is the only one that a weekend warrior cannot really do without a lift and all the right tools and so forth. Again, does this sound about right? If I reach consensus here, then its time to hand the keys over to a trusted Indy here in the North Dallas area and get this flaw fixed. Its driving me nuts and sucking the fun outta driving her. I anxiously await the experts to weigh in and give me the confidence that this is my culprit.

MP4.2+6.0

Your diagnosis is plausible. I have never had an issue with the front diff on any Audi, but it still could be. Might well be the center/Torsen instead. Quattro can stutter a bit and make some tire noise like a 4WD type vehicle (since it is always engaged at all wheels); I haven't noticed that much though since my 1996 C4 A6, and my W12 doesn't have any apparent noise or groan like you describe. If you suspect low fluid, obviously you want to inspect for leaks, probably oat likely (albeit uncommon) on the front side seals where the CV axles bolt up or on the rear center diff seal (had this one on my C4 under warranty).

Given you have an S8, actually you are somewhat more "in luck" since there is a trick by which these can be drained. Not documented by Audi/Bentley or ZF. First thing you need to check though is if you have a shared fluid system between the front and center diffs. My W12 does, and I think the S8 shares the same set up. You are looking for two things: an auxiliary gear oil cooler, and some external fluid lines. The gear oil cooler is under the tranny far forward on the (U.S.) passenger side. If you drop the motor belly pan, it is toward the back. Don't confuse it with the tranny fluid cooler, which is toward the driver's side and basically right beside it. You are also looking for a couple of fluid lines with some silver insulating material high up on the passenger side of the tranny in the area where the pan is. You can just barely catch sight of a little of the insulating material if you shine a flashlight up in that area.

Assuming you have this set up, basically what you do is remove the gear oil cooler and drain both the front and center diffs from there--it is a shared fluid system via those lines you see along the side and via the gear oil cooler underneath. Before doing so, you open both the front and center diff fill ports. Neither is that easy to get at, especially the center one. I did mine with the exhaust dropped because I also changed the tranny fluid and filter, and on a W12 or S8 you need to drop the exhaust to do that. You can get to the fill ports though with the exhaust in place; I know since I rechecked the fluid level once I ran it. Also, the center fill plug is hard to get out, and extremely hard to get back in--the allen head hole in the plug is off angle from any wrench you can get in there. What I did was drill a hole in the cross member that is in the way. See the second link below.

Not all the fluid will come out by volume, but you drain what you can. You can use some very LOW pressure air to try to coax a little more out. Afterward you bolt the cooler back in place. You have to fill from both diffs until you get it to spill back out, then recheck once you have it running.

As far as parts, you should replace the two round seals for the diff cooler when you drop it. The Torsen diff takes the expensive gear oil with the magic additive (ZF calls it Sturaco); because of the shared fluid system, you use it in both, unlike the 4.2. You need probably 2L; dealers sell it, including audiusaparts.com where I get my stuff typically. You may want to replace the fill plugs too; I've also posted on that--the front diff plug listing is wrong in the Audi EKTA parts system last I ordered it; they will send you one just like the small center diff. plug unless you get the very similar correct part # for the front one. IIRC the D2 listing for the front diff fill plug is correct for the D3 too.

A prior post of mine on this subject: W12 tranny service at 100K; general appearance - AudiWorld Forums

How to modify the rear support to get at the center diff drain port: W12 tranny service at 100K; general appearance - AudiWorld Forums

(both are specific sub posts under a main post I did about the whole tranny fluid change subject on W12's)

mishar

I am almost positive that is middle differential, but in S8 case (shared oil) it doesn't make much difference.

It may worth trying .

I used this and it worked for quite some time but recently sound is back. It looks like it wears out. Adding it to the middle differential is all I could do since it can't be drained on 4.2.

Drilling that extra access hole saves a lot of time and even more nerves.