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Picture A is the oil pan drain plug. I believe the front diff only has a fill plug and you need to suck out the old fluid from there and refill till it overflows. It doesn't take much fluid. I am not sure if that is the fill plug but others who have done it know.
Not sure where this is going or the why, but back up a step. First figure out if you have a separate fill front diff, or a combined fluid unit with the center diff. See the ZF documentation or old Audi/Bentley (Publishers) manual/CD, If this is not familiar, candidly think about should you even be attempting this? The fill plug for a front diff is likely up by the CV joint area on USA passenger side of diff, and needs at least various shields removed. My memory of my W12 was basically, forget about getting at it in car. As always, always know your fill access before draining anything, and remove any fill plug first in case anything goes wrong with that step.
If it is a shared system, then I would approach it from the mid/Tosen diff and any gear cooler. What I in fact did on my D3 W12. It has a shared fluid system if you find a key large hollow tube area cast into the side of the transmission case running at an angle. Look for pics on web or in ZF docmunentation. Pretty obvious when you see it from under car. It was never clear to me which specific cars/motors did or didn't have that set up. IIRC one very knowledgable guy on forum swore he didn't on his 4.2, but then looked again and found he in fact did.
Last edited by MP4.2+6.0; Jan 20, 2025 at 06:22 PM.
In the nicest possible way, it sounds like you should leave this to the professionals. Not being able to tell the engine oil drain plug from any of the transmission drain plugs should be giving you serious pause as to weather or not you should be going anywhere near DIYing this job.
FWIW, the V10 and W12 motors are the ones with the common transfer case and front diff oil, with axle oil cooler. All the others have separate gearbox, transfer case, and front diff oil with only an ATF cooler.
Deep dive here in SSP 385 on the different variants of the ZF 6HP transmissions Audi used.
Was changing Front Right Bearing and preemptively changing inner cv boot... so this was a good time to do front differential fluid as well.
You need to remove the heat shield above the inner right CV joint. This is done by remove three 6mm hex bolts with a long ball tip 6mm hex (from harbor freight for 14$).
There is no drain bolt, only a fill bolt with an 8mm hex key that is on pretty tight. the fill bolt had a green gasket inset into it. Next you must snake a suction tube down the wall under the fill port till you get down. I got a little less than 1L out, tilting the car towards me. Old fluid was dark, but not particulate filled. Then I put in about 1L of new fluid. Tightened the bolt somewhere between 20-30 nM.. I didn't want to over tighten to 30nM as the next time it comes off, the hex may strip. I used G052145S2, same stuff rear uses, as there is no connection between front diff and tranny or center diff for the 2009 A8L 4.2L
my axle was off already so pics show a pretty good angle, but access is possible with heat shield off and axle on. I didn't really notice any difference in smoothness driving forward with the wheels turned extremely right or left, but I never really had any issues or sounds before, now at 230k miles, so you maybe it could last the "lifetime" of the care and be fine, but it was still reassuring to replace the fluid.
Last edited by Sci-fi_Wasabi; Jun 1, 2025 at 03:04 PM.
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Front differential fluid change D3 A8
