2001 S8 gearbox limp mode only on cold start
 

I have a problem with my 2001 S8 auto transmission.
Only occasionally, and only on cold start-up, the gearbox goes into limp mode.

I will turn on the ignition, and the P will light on the dash as normal. Start the engine, and it goes immediately into limp mode and lights all the gear lights on the dash. Switch off the engine and immediately re-start, and all is ok again.
The gearbox works fine at all other times.
When in limp mode I only have 4th gear and reverse, and selecting either is harsher than normal.

I have checked & cleaned the plug for the range switch on the gearbox, and checked & cleaned the plug at the TCU.
This same problem occurred last winter, was fine all summer, now the weather is cold, its back.

Gearbox fluid & filter was changed about 20K ago. Car has approx 150K miles now.
I'm thinking it could be the gearbox switch, but I'm also suspicious of the battery. Anyone had a weak battery cause this?

The F125 (range) switch would definitely be the first suspect. Have you done a VCDS scan to see what codes you get?

Nope. As HP says, its most probably the F125 switch which is an electromechanical nightmare giving up the ghost on the inside.
Search this forum using F125 and you will find the various threads in the D2 transmission section including one where I disembled mine after its failure.

PS. why, as an S8 owner who appears to look after his car, is this your first post??? :) Any details/pictures of your car? +++

Yes, please tell us more including photos, put some specification details in your signature and let us know what area you're in.

I was also thinking F125 as that's the exact same behaviour mine exhibited, right up until you said it did the same last winter but was fine all summer. If it was the switch it should have got worse until it did it in warm weather too, and then even when the engine was warm. Not ruling it out, since its due a change at that age and mileage, but its odd for it to go away rather than get worse.

Where are you? There's probably someone relatively nearby with VCDS who can read the codes for you, then we'll have a much better idea of whats happening +++

Have updated my signature and location.

I can read the codes. I haven't bothered yet this winter, and I can't remember what code(s) I had before, but I think it did sound F125 related.

I'm suspicious of the battery because I did some work on the car last saturday (O/S front upper control arms with the lovely pinch bolt), and when I was done, the car only just started due to flat battery.

I too would expect a failing switch to just get worse until it didn't work at all. I added that up with the flat battery and thought maybe it can set a F125 related code due to low Voltage during cranking.

I've had the car a couple of years now, and been shopping at A8 parts a few times, just never got around to posting here.
Jobs I've done on it:-
Oil cooler water pipe (Bufkin pipe) - fitted a new oil cooler while I was there.
Both front outer CV joints
Both front strut top mounts
Front upper control arms
O/S front lower control arm (rear)
Android stereo & reverse camera
All brake discs & pads (EBC Redstuff).
S4 steering wheel mod to use paddle shift
Cam cover gaskets
Injector seals

Annoyances still remaining:-
Sometimes gearbox error on cold start
Backlight of dash LSD panel does not work when very cold (making it harder to tell if gearbox has gone into limp mode until trying to drive forward).

Sounds like you've got a few hours in to it then :) B6 / B7 steering wheel?

When my F125 was acting up last winter I took it to bits and cleaned it - the rivets can be drilled out and replaced with M4 bolts. Its still working fine +++

B6 wheel, looks the same as original wheel (and uses original air bag), just has the paddles instead of the thumb buttons. Bit of simple re-wiring in the wheel required.
Does it look like standard type micro switches in the F125? Might be able to replace those. Bit of a pain to remove with the car on ramps though?

I was going to do the same with a TT wheel - sounds like its much the same part. Decided to do a manual swap instead though :D

The F125 is rotary contacts moving over embedded tracks. Its about 3 hours work to remove and replace on a lift. Probably a bit longer on ramps, but still do-able. The main thing is lifting the gearbox slightly so you can detach the passenger-side gearbox mount. I'll see if I can find the post I made when I did mine :)

Found it: http://forum.a8parts.co.uk/showpost....&postcount=112

The inside looks like this: