2000 Audi S6 - Problems engaging reverse gear when cold

Question

I've been having some problems with my S6's auto-box lately where it doesn't seem to engage reverse gear when both the car and the outside temperature are cold.

No error or warning lights come on and the DIS shows the car as being in 'R' but there is no drive, and there is no "thunk" felt/heard that is normally present when the 'box engages and meets the resistance of the parking/foot brake, this seems to only affect reverse - engaging 'D' or another forward gear works perfectly.

This seems to only occur literally on first start up - if I let the car idle for a few mins or drive off in another gear then it goes away and reverse behaves normally.

I'm not seeing any sign of the clutch slipping or anything like that and it seems binary - either reverse engages normally and works perfectly or it doesn't at all, there's nothing in between.

UPDATE: Following GdD's advice I've had my specialist garage change the gearbox oil, they reported that when they did the change the level seemed to be low (it took more oil than came out of it) and that the oil hadn't drained completely which is to be expected since some of it will be sitting around in various parts of the system. Because of the fact that it will take a little time to circulate the fresh oil completely they have suggested that I see whether the situation improves over the next couple of days after I've put some miles on it. The problem recurred first thing this morning but I'll be doing a good 70+ miles today so that should hopefully be enough for it to take effect and I'll see what happens tomorrow.

UPDATE 2: I'd discussed doing another fluid change/flush with my garage and said we'd book it in in the new year when everyone was back from the holidays. Unfortunately the car decided to take matters into it's own hands last week and the gearbox spat it's dummy out, having trouble selecting gears and reporting fault codes on both 2nd and 3rd gears. It's now with the garage waiting for their transmission specialist to have a look. I think it is going to be an expensive month!

Answer 1

I would say the most likely culprit is old transmission fluid, or the level is low. Transmission fluid breaks down over time and exposure to heat, becoming less effective. According to your question and the comments you don't know when it's been last changed, so that's where I'd start. You probably want a flush, simply draining the transmission fluid only gets about half out of the system, the rest is in the lines, the cooler and the torque converter. A flush requires special equipment, if you are a home mechanic you can get a similar result by changing the fluid and filter, then doing it again a few days or a couple of weeks after depending on how much it's been driven.

Apparently Audi are saying that the transmission fluid should last the life of the car (for the A6 at least), so it's possible that it's never been changed. 151k miles is a long time for fluid to last, especially in a performance car that's probably been driven hard, so I would suspect fluid even more.

Answer 2

Well following my last update (see "UPDATE 2" in the question) I finally have the answer to this one - turns out the reverse problems were the initial stages of the complete failure of the gearbox due to oil pressure regulation issues apparently. Something in the A-Clutch pack went pop and that's what eliminated most of my forward gears and the reverse issues were down to the problem affecting the functioning of the F-clutch piston. The 'box is now undergoing a full rebuild (including using the up-rated A-Clutch parts that are a known weakness) while my wallet quietly weeps.

Many thanks to @GdD for his helpful answer while I was trying to get to the bottom of this!

Answer 3

Try this! After driving and wanting to reverse put foot on brake and put into park. Release brake then press again and put into reverse. Works everytime but if I put from drive into reverse straight away the engine just revs.