a65l

So my 2010 4S started making noise. Unpleseant noise. From the rear of the car. Shop diagnosed it as the rear diff, specifically input pinion side. Since rebuilt/new units are unobtanium right now with no ETA, I bought a used unit off eBay. My research says that there is no difference in ratios between V6 cars and V8 cars. Same final drive ratio, to be clear. So, I bought a used diff from a 2011 V6 with fairly low miles and had it installed. The car is nice and quiet now, and I don't feel any jerking/grabbing that I would associate with having different final drive ratios on the front and rear.

But.

PDK has been acting strangely since I picked the car up., Almost like I have turbo lag... upshifts in auto mode occur, and you can feel it, and then a split second or so later I feel a very firm engagement of the clutch. But unless you're fairly hard on the throttle, the clutch engagement seems to take longer than normal. Kickdown is where it really shows up, the transmission downshifts, and engine rpm increases, but it feels like the clutch isn't engaged at all, and then when it does it's very noticable and firm. The lag seems to vary, but it's always there. I note it going straight, around corners, it occurs wether I let the transmission kickdown automatically or select a lower gear myself. I also note that just selecting a lower gear, it seems to engage, I.E. engine RPM increases, but when I get on the gas there's no drive for a second or two untill the clutch engages.

I ran through the PDK/Hyd/Clutch calibration on the PIWIS the other night, and drove it today, no change. It's not throwing any error codes. Low speed engagement and to reverse from a stop seems absolutely normal. There is no indication that tthe clutches are slipping after engagement, while in manual mode I ran thru 5tth,6th and 7thg gears and none of them showed any sign of slippage. This issue occurs through all gears, not odd or even. Kickdown to 3rd does the same as a kickdown to 4th or 5th. Or even 6th.

So, my quesitons for the collective are obvious, I guess.. is there a difference in final drive ratios, and would that be causing the PDK issues? Or do I have an issue with the PDK? I'll restate, that it was not doing this on the way to the shop. I called teh shop and they verified that they did nothing with/to the transmission.. drop driveshaft and swap out diff. PDK has been serviced in the last 3 months by same shop. Little puzzled here... and a little annoyed.

justabout

I can’t confirm the ratios but this is too much of a coincidence. I would search for a V8 diff. Don’t drive it!

a65l

Well ratio issue is confirmed. I sourced another used diff, this time exact part number not "crosses over according to the ebay seller". Picked up car from shop today and all is well. 4 new tires and an alignment... so time to have some fun.