Transmission goes THUNK

[redcab]

Hi, wondering if I can get some help diagnosing...

2001 996 w/ Tiptronic. Only 38K miles.

Here's the scenario: Driving freeway speeds I floor it. Really mash it. The car downshifts. Sometime (but not always) I hear a big clunk as the transmission downshifts hard from 5th to 3rd. Is this normal?

It doesn't happen every time. I haven't paid close enough attention to say it happens when I'm initiating at a certain speed. If I accelerate more moderately it doesn't happen. Only when I mash it.

If I force the downshift from the steering wheel buttons it will not happen (in either M or A mode).

Any ideas? Is this normal?

I've owned the car for 8 years but only recently started driving it regularly on the highway (commute!) so maybe it's always been this way, but I just started noticing.

Thanks!!!

[goldenwarrior1]

If your car is NOT a C4 some of this won't apply... I would put the car up on jacks or a lift and inspect the complete drive train to start with. All 4 drive shafts (if C4) and especially the flex coupling between the Cardan shaft and output shaft of the transmission. Also check your transmission mount for a cracked or deteriorated rubber bushing. I doubt if engine mounts are the culprit but you can check them as well. If all of this checks out then you most likely have somthing internal to the transmissoin that is causing the clunk. You have a differential inside the transmission and something could have broken or is on its way out.

[redcab]

This morning I paid a bit more attention to what causes the thunk ((btw, it's a C2 cab)

Definitely only happens when mashing the pedal at freeway speed. Always a 5 - 3 shift.

I did it perhaps 30 times and only caused the problem only twice. Starting speed was between 65 and 75. When I tried it from a slower speed that allowed a 5 -2 shift, it was always smooth.

The two times it happened... on the first the starting speed was definitely 65 (I was checking) and the second time it was probably 68. Both times it happened were in perhaps the first 50% of trials (so maybe it is a warm up thing?)

The second observation was that it was a two - phase thunk..... if I had to guess the first part related to disengagement of a gear and the second related to the engagement phase because that second part was in sync with the rear tires biting for traction and the beginning of acceleration. (But that's a guess).

I guess I could just stop mashing the pedal, but isn't that half the fun?