What device creates pressure for the transmission?

[Hy Octane]

Curious as to what the mechanism is that supposed to control the tranny psi for keeping the TC's clutch locked up even when revs are low. Anyone shed some light on this?

[skiwi]

there is an concentric oil pump driven by the input shaft. this also works the tcc (torque converter clutch)

[Hy Octane]

Is it possible to increase the pressure to counteract the clutch becoming unlocked when you coast in gear at low rpm?

[skiwi]

not really. the unlocking of the tcc is a function of the tcm based on input conditions. it generally works fine. if in doubt as to whether it is according to spec, log the values and check. you should find that the tcm will manage the tcc slip ratios, lock and unlock when you expect it to, although it sometimes allows quite large slip values.

if what you are chasing is the dreaded zf "hunting issue", there is a growing pile of emperical evidence gathered by my good friends on the aw s8 list (my last home) that a significant contributor to the "hunt" issue is the inputs to the tcm, rather than the transmission itself...

[Hy Octane]

Actually, its not the gear hunting, but recently, I noticed that if I am on the freeway for example, and in tip mode in 4th gear at 65mph, the rpm is around 2200 or so.. if I let off the gas and coast for a few seconds, when I apply throttle again there is a moment where the tranny has to relock up again before we go forward. Its a slippage where the revs go up a few hunderd rpm before locking up again .. This happens repeatedly in 3rd, 4th and 5th mostly anytime the rpm drops down around 2200 or below and I take my foot off the accelerator... If I keep it in a lower gear with higher revs, it does not do this. Thats why I was wondering what dictates the rpm at which the TC clutch is programmed to lock up and if it could be modified to eliminate this. It didnt use to do this until a few months ago, so if its not adjustable, what then? I just had a tranny service done at my 35k last week and it has made a big difference in smoothness of shifts and it did help the above symptoms a little.. I strongly recommend doing a tranny service before the 70k recommended time..after all, if the timing belt only lasts 35k, the tranny fluid and filter could use a change as well..