I performed various long term maintenance this weekend on the W12. More to come, but for starters the rear differential fluid change. This service should apply to any of the D3's by motor. Not much more difficult than an oil change.

Vehicle is approaching the 95K mile service point, so I thought I would take care of some of the very long term service items I tend to do on Audi's I own for 10-15 years+--out to 200K miles+ by me or family successors. I try to roll through the various "lifetime fluids" and other specialty items somewhere in the 5-7 year timeframe and by 100K miles.

Since I just have a home environment, I put my car on ramps on a sloped driveway to get it back to level for any of the drivetrain fluids where level matters, and so I can get under easily and safely--no jackstands, jacking, etc. I have "field verified" it before with a level (easier on the A6 where the small level magnet actually sticks to the roof). Net, plenty close enough. Car was also in lift mode. Picture on ramps follows in fuel filter change thread (5 pic limit in threads?)

Fluid change materials in first shot:

• Basically it takes a 10mm allen socket to get the drain and fill plugs out. The fill plug is very high up at the extreme rear of the differential on the (U.S.) passenger side. See the picture where the socket drive extension points toward it. It was actually quite hard to find, lurking in the shadows and farther back than I remember on other Audis; I have done the rear diff service on prior Audi's several times. That picture was taken almost from the side of the fill port and from above the exhaust resonator pipe--well above where you can see by line of sight. I ALWAYS open the fill port first. If I can't, the job is over and I get other tools, new plugs or whatever is needed. Sometimes like in the tranny area I just can't get to the fill plug even if I see the drain one, like the front diff on my C5 4.2 for example. It's a proverbial nightmare if you open the drain plug first and then find you are screwed on access or something on the fill side, or something strips out or jams. On the drain plug it was quite stiff actually, even as a California car, and it took just the right combination of socket extensions to get in there and have the wrench underneath for leverage. It finally took a sharp impact to break it free, with about a 10-12" long socket drive.
  
• The drain plug is obvious. Right there underneath. Took less force to get this open.
  
• Notice in the parts picture I bought the drain plugs too. From experience on the front diffs, those often get burred up if they use smaller allen sizes. Having done these, the old ones were still fine, one quite clean, the fill plug somewhat burred from the force I had to apply. Part # N-100-371-06. If I did it again, I should have only bought one; usually the higher up side accessed ones are the only that give real problems, and the 10mm head size is robust enough for reuses.
  
• The older diffs like mine (black in color on the whole body) take 1.5L of gear oil. Thus I had two bottles on hand; using a cheap plastic hand pump with the simple clear hose and an "L" barbed fitting to hook into the fill port better, I went through most of both before I took the time to pour the leftovers from one bottle to the other. Audi part number is G 052 145 S2. Available from dealers, ECS and others. Basically a 75W-90 GL-5 gear oil, synthetic now for many years on Audis. Mobil1 and some others are available as GL5 too; need to read labels. Given cost of diff, I just use the Audi spec fluids here. For those buying or following Audi gear oils, this is basically the "regular" stuff that has been around for a long time. My 2000 A6 4.2 used the same. It is NOT the quite expensive oil with the friction modifier additive used in the Torsen differential further forward (or in the W12 or any other where the front and center diffs are inter tied with common fluids, apparently all the forward diff units; still TBD in a future service).
  
• Newer differentials are silver in color and supposedly take 0.9L. They apparently arrived in at least the W12's by sometime in 2007.

Diagnostics/what I found:

• First I opened the fill port per above, a very small amount of fluid ran out, no more than a few teaspoons; so it was still "full." Similarly, I have never seen signs of fluid leaks from seals or otherwise on the case; just usual road grime.
• I got a shot of the fluid draining. I was actually surprised at the still apparent golden color. Picture color is basically true to what I saw visually (ignoring the tiny amount of purple tinge I see in the uploads on screen). Prior Audi's I have done by this mileage have come out deeper brown and more muddy, including my C5 4.2. The fluid also did not smell burnt or rancid as I often find with used gear oils. My initial reaction was I didn't need to bother with the drain after all. BUT, when I went to pour the oil from the pan into the recycling container, I definitely saw the often found look of a reasonable amount of super fine metal particles in it. Less than I have seen with other diffs and no obvious shavings or similar sediment, but still present visually, let alone in microscopic form. Given there is a TSB that says the earlier black diffs used in W12's and early S8's are prone to developing noise and the fix is a forklift upgrade to the newer silver colored one, I was motivated toward the preventative. Diffs are big bucks from Audi, and eBay or junkyard ones ones erratic in availability and often high mileage. Mishar has found a source before for the black ones who is perhaps selling the superseded Audi diff replacement stock off.

Net, it doesn't look like a must do out to 150K miles +, but w/ the fine metal observed I didn't feel I had wasted my time or (small amount of parts) money either.


Final shot of the diff cleaned up so you get a clear sense of what's back there seen from directly below it.

 

Nice write up. I did all of them few weeks ago. Now I am waiting to see what are you going to do with the middle one. I couldn't figure out how to drain (or suck) it so I just added some Red Line Limited Slip Differential additive and some oil, all together close to 0.1 liter. That much it accepted before it was full. So not enough oil and absolutely no leak. Like that from factory??? Not to mention that plug position almost covered by transmission support. Good thing is that cleared all LSD chattering, widely known as "Quattro working its job".

 

That oil is nice and golden, I think I did mine at around 175,000 and it was as dark as cola.
Mine is the one that takes only 0.9l so that may age quicker.

 

Not even remotely covered in Bentley but just put together from snippets of info from ZF about the coolers and endlessly trying to tie down which gear oils mine use. For reasons that may become clearer below, this may only work on W12's and S8's if a gear oil cooler is used only by them.

Remember it's unclear (at least to me) when the fluids become the same and are shared in common between the front and center diffs. I think mine qualifies for that sharing. Getting on the ZF site and looking at the service/parts manual for the tranny I get to that same conclusion too. Check this out; best reference I have found that is quite specific to our trannies and the fluids: http://www.zf.com/na/content/media/u...61_Catalog.pdf Worth holding onto a pdf copy BTW; I grabbed one. Notice also they document how it can be one or the other--shared or not--but they ever explain the basic set up/vehicle spec of how that comes to be.

Now, look at the parts diagram from Audi. Here's one I pulled off the Jim Ellis site (the AudiUSA ones we both frequent often seem to fall down on some of these deeper ones, and don't have part #'s): http://www.jimellisaudiparts.com/sho...=0&modelYear=0

Notice cooler #6 in the diagram. My car definitely has that, and I'm now almost sure it is the gear oil cooler, and in turn among the key reasons the fluids end up in common circulation between the two diffs. Go back to the ZF doc't and read how it is the lowest part of the system in terms of where the shared/otherwise unaccounted for total gear fluid capacity ends up. But, not sure if 4.2's have that particular cooler. Notice there is also cooler #3 in the diagram; I think that one is for the tranny fluid and the 4.2's may well have that one. Mine has both. You would know from your own 4.2 what they have. Both coolers are oil to water types given the obvious water lines, even though--especially the tranny fluid cooler--has what seem like a lot of fin type construction to it.

Separately in the same cooler area, by chance I found previously to really drain the cooling system, if I simply pull one of those lines back at the coolers I can get a bunch more out than just by the usual lower front hose type method. Still would have coolant back there even w/ opening the buried 4.2 block drain like on my A6.

So, put it all together. First, drain coolant. On my list anyway, including pulling rear cooler hoses to really empty it out. Next and the key, DROP THE TRANNY GEAR OIL COOLER TO DRAIN IT ALL OF THE APPARENTLY SHARED GEAR OIL AT ONCE. Oh and if I have my coolers by any chance mixed up and I happen to hit tranny fluid, I'm going for that anyway, plus the cooler seals involved are also the same at the parts # [I bought a set]. ... And... on at least the W12 with the mega exhaust, it is definitely in the way of a likely successful tranny pan drop to get at the trans filter, and I don't want to drop all the front pipes. That's work even on my C5 4.2 when I did only one side, let alone the W12 stuff. BUT, it looks like if those coolers are out of there and a related support, the tranny pan probably can just come forward and then out. Bonus prize for the same work.

My last step on this is successfully opening the front and center diff fill ports. So far I have gotten at the forward diff one as a test. The rear one is hard on mine--off angle and tight space. But I think I now likely have the right ball type allen head tool to get at it off angle. Hopefully not too tightly in there where the fill plug starts to strip. As with the rear diff notes, getting those fill ports open first are clear prerequisites to proceeding any further. Unless I can get both, I won't proceed with either "end." In a shared fluid set up, I might be able to pull it off by filling from one end only and then rechecking after a brief cycle and measuring all amounts used, but I don't want to take any chances here with important fluids.

Anyway, thought I would post the draft plan either for any reactions or if any of the heavy service guys among us have gone anywhere near here. As I found with the pull the whole manifold trick on the W12 to do the spark plugs, some of this is totally undocumented by Bentley, Audi, ZF or whomever, yet seem like may be quite practical work arounds. Here it takes draining and replacing several major fluids, but that is EXACTLY my goal here anyway.

 

First, thanks for that ZF PDF. My PDF library is more complete with it.

I believe you are right about shared oil. If there are oil pipes on the right side that's it. Mine doesn't have them and oils are separate. That makes oil change cheaper if you are using that Audi special oil. Somebody here helped me to dig deeper into additives (thanks and sorry I forgot your name), so I came with Red Line LSD additive and their gear oil. I can recommend that additive. Stopped chattering completely and makes oil change much cheaper, especially for W12 and S8. Won't hurt front differential at all.

Now to the middle one. I believe that you can't drain it even with shared oil. I tried every small hose in my garage in order to vacuum it out but no luck. And about the "art of engineering", is not that difficult to take out that plug with boll tipped Alan key. It sets deep enough. Problem is putting it back. Angle, slippery hands and confined space made me remember those engineers mothers many times. If only they pushed that transmission support 10 mm to the back ...

Just a little tease

 

I remember those from last season and wondered where they went.

"Special" fluid. Some kind of friction modifier. Read the ZF material for a while and I think you will conclude it actually doesn't need to be used a second time (like a refill) if already in there. They sem to say that in kind of a roundabout way. But, if not there to start, different story. I just bought the bigger bucks stuff since the fluid cost was pretty minor in all this. The tranny filter and fluid is where most of the $ are. Still small change compared to the last $4K+ I spent on the bound to fail 5HP24A A6 ZF rebuild and R&R.

 

Here they are. My winter wheels too. Mini is not mine. Friend borrowed my truck so his Mini is just a guest.

 

Front and rear are easy enough, the center is a PITA. I bent some thin copper water maker line tubing to be able to get in there and vacuum out the old and pump in the new.

All of my fluid came out looking gorgeous, and thus really didn't need to be done, so go figure on that....

 

Going to flush the rear diff on my silver 05 A8. Making a loud humming noise when up to speed, going to flush the diff as well as change whatever wheel bearings are bad.

 

I would like to see that. Tried everything but no luck. May be I am getting too old for this hobby?