ADRIAN GRAHAM

I currently own a 2001 Audi S8. I've recently installed another transmission, exact TCM replacement, output rpm sensor, and selector module. I also did the proper A/T service w/ filter and EURO syn trans fluid. The car drove fine for the last few weeks but now when I engage D , it goes into limp mode w/ Incorrect gear ratio code. I've checked the input and output speeds of the transmission. They seem to only zero out after limp mode is engaged. Btw, when I pull off at a roll w/ limited revving, it shifts through the gears although irregularly. Could this be a wiring issue from the output rpm sensor? Could it be the A clutch slipping? (It engaged in drive, it's just not a strong hold as it feels in R) I know this topic is over discussed but any insight is appreciated

twentysevenlitres

I had an issue about 18 months ago that gave me an incorrect gear ratio code, but on light throttle it would function ok for a time.
Turned out to be a fractured pressure pipe from the valve body into the transmission. Au$49 part, au$500 expertise/labour!

Had the Transgo valve fitted too to prevent future pressure spikes.

ocles_inc

I hope the fix is quick and simple, but I think there is some data in support of your "maybe it's the clutch 'A' drum" hypothesis, and I don't know anything that would enable you to rule that out, as yet, sadly.

There's an important point I would like to make. At 100K the ZF 5HP-24A transmission is at risk of its clutch "A" drum failing due to a worn out pressure regulator in the lower front valve body housing. As I understand it, one hypothesis is that at high RPM the worn regulator gets stuck and allows a pressure spike through, and that blows out the snap ring atop the clutch "A" drum. Here's a picture of one of several that I own, like that:



I'm clear of the danger of overcapitalizing, as in spending so much on a rebuilt transmission that it's unjustifiable given the likely remaining life, and market value, of the vehicle. On the other side of the extreme, there's the lure of buying a used working transmission. The problem is that all of these ZF 5HP-24A transmissions are about the same age, which means that if the used replacement transmission hasn't failed yet, it might well imminently do so. That seems like a possible explanation for what might have happened here: you had a bad transmission, you replaced it with a good transmission, which then also became a bad transmission.

Whatever you do, if you do buy one more used transmission, I would urge you to remove the lower front valve body housing (easy to do), mail it to the ZF distributor in Connecticut, and pay them to renew the pressure regulator. I do that to every ZF 5HP-24A transmission I have installed in an A6 Quattro 4.2 V8 or A8 Quattro (I'm not in the S6 or RS6 or S8 bracket, sadly, much as I'm cheering you on). With this fix, these transmissions have been rock solid for me. I did this fix to my own daily driver 2000 A6 Quattro 4.2 V8 about 15,000 miles ago. I drive it very hard (for fun, but also to prove a point) and it's still running just fine. I even entered it in the Virginia City Hill climb time trials this summer, just to prove how solid these transmissions can be once the pressure regulator has been renewed. Roaring up a steep hill under near-full-power for several miles on end ... that was a nice real-world test of the transmission. So there's no reason to be wary of them, as long as you take that one essential precaution.

Good luck ...

~Tanya

jfrahm

Sounds like no rebuild or valve body rebuild, pressure valve fix. So yes perhaps the used transmission failed. IMO it's nuts to do any trans service without doing the pressure valve fix (or TransGo replacement) or determining if the pressure valve has already been addressed.