It pays to find a good mechanic!
 

I thought this might be of interest to some.
Warning: I write long stories!
I've also posted this on another popular Audi forum, so no need to read it twice!



Since buying my S8 back in 2013, the shifting in the transmission was never quite right. It would do a rough thump into 4th gear on every fresh start (hot or cold) and displayed some other intermittent shifting roughness on heavy throttle. Over time the 4th gear thump got a little bit worse to the point that I adjusted my driving habits around it and the 1st to 2nd WOT shift was also affected with a prolonged slipping change with a thump - a bit better in tip.

The transmission had been replaced by the PO at 135,000km (84,000 miles) by an Audi specialist shop I don't particularly like or trust. There are reports in the PO's records for complaints about rough shifting of the replacement transmission which were not properly addressed with the shop stating “Car test driven, transmission shifted smoothly”. Another service report has a warranty fix stating “Transmission removed, reverse solenoid replaced, transmission refitted” - What? That's not a transmission removal job!

Anyway, when I bought it it had 186,000km (115,000 miles) on the odometer, putting around 50,000km (35,000 miles) on the transmission. Although it wasn't quite right, It initially wasn't too bad and after that first thump into 4th the transmission would operate quite smoothly except for rough transitions from off-throttle to on-throttle (eg. at an intersection). I had it serviced with Nulon Synthetic MV Transmission Fluid soon after buying to eliminate bad fluid as the cause and to rejuvenate seals and to generally clean out the transmission.
It essentially behaved as before, but I was more satisfied.


Mid 2016 and we're at 210,000km (130,000 miles). The thump into 4th is heavier and the transmission is giving it's trouble with 1-2 shifts to on WOT... JFrahm (AW member) draws my attention to the TransGo replacement pressure valve as a possible solution to low pressure or pressure fluctuations caused by a worn original valve. The original valve is a metal to metal fit with no seals, which once worn sufficiently will allow pressure leaks and may not operate smoothly. The TransGo Valve fixes this issue by changing the design slightly to incorporate o-ring seals and has larger chambers to handle more fluid. So I have the TransGo Valve fitted and fluid changed once again.
To begin with there seemed to be a great improvement. Smoother shifting and reduced 4th gear thump.

I took a run up to Echuca on the Murray River on the Victoria / New South Wales border (about 3 1/2 hours drive) and was able to test heavy throttle overtaking manoeuvres. It was very hesitant and not all that smooth in down shifting and upshifting during those runs. So I gave it the hard test - standing start WOT. Big mistake! It tried to change into 2nd at 5,000rpm, changed its mind half way through the change and went back into 1st then another long slipping/flaring change into 2nd at 6,500rpm. After which all the poor behaviors prior to the TransGo Valve fitment returned. It was not happy!
After cooling down, it became smooth again, but any push would make it rough once more. finally on a climb up a steep hill nearby a couple of days later it fell into limp mode. A shut down and restart would reset it, but I was left with having to drive it gently - any push and I'd end up back in limp mode.

This wasn't good!

On the off chance that we might find something blaringly obvious, my mechanic friend and I pulled the transmission pan once again on his hoist. We had nothing to lose, but as I expected there wasn't much to see. It was worth it for the exercise despite the fact we didn't achieve much! A test drive proved it was still faulty.

I phoned the Melbourne authorised ZF repairer and described my problem. “Transmission's stuffed mate. We can fit a rebuilt change-over transmission for AU$7000 +GST (10%)”!

I had a chat with my German mechanic who had fitted the valve and we both thought that the gear selector clutches couldn't have worn out with only 78,000km (48,000 miles) on the transmission and that the problem must be in the Valve Body. He suggested another transmission shop, who were friendly and knew the ZF transmissions well.
I had a good chat with the owner of the shop who was very amenable and more than willing to perform proper diagnosis. In the process of testing they found the following broken pipe:

http://cimg3.ibsrv.net/gimg/www.audi...0caaa54c0a.jpg

Pipe fractured clean across the pressed flange. Looks like a fatigue crack, but could even be a manufacturing fault - history of rebuilt transmission is unknown
http://cimg5.ibsrv.net/gimg/www.audi...1bd5c008cb.jpg

Location of pipe within transmission above Valve Body (Valve Body removed)
http://cimg7.ibsrv.net/gimg/www.audi...59893b53f6.jpg




One $22.40 pipe later (plus $60 airfreight from Germany!) and the transmission now shifts perfectly and is very smooth in its operation. Thumps, bumps and flares are all gone. I finally realise how well this transmission is supposed to shift!

Total cost of repair AU$650!

.

Thats a useful one to know about +++ Glad you can properly enjoy it now :)

Great story... Transmissions scare me!

Thanks for an informative, and entertaining, write up. Great result +++

Glas to hear it was sorted. +++ Do you know whether the replacement transmission at 84k was a new or reconditioned box?

The Previous Owner assured me it was a reconditioned box.
The receipt I have from 2011 certainly seems to bare that out, if a little on the cheap end. It states that a reconditioned unit was fitted at a total cost of AU$4610 with the reconditioned transmission costing AU$3620.
Considering ZF's distributer wanted $7000+...
However with labour at $120 per hour in any decent tranny shop, $3620 buys an awful lot of time and many parts in the transmission would be perfectly serviceable. If they reused the Valve body for instance, there's a $2500 saving on the cost of a new one.