986 2.5 Tiptronic: 2nd to 3rd problem after a mini service

[norbs]

Posted Elsewhere trying here

Hi guys,

Hoping for some collective wisdom on my 1999 2.5L Boxster. It has developed the classic flare and thud on the 2nd-to-3rd upshift.

This was happening before I touched the gearbox. I did a full fluid and filter service using Febi ATF (LT71141 spec) hoping it would be a miracle in a bottle fix, but the thud has decided to stick around.

The Symptom: Under light throttle (the pootling to the shops range), the revs flare up by about 300-500 RPM mid-shift, and then 3rd gear engages with a heavy thud as the clutch finally bites.

The Data (via ThinkDiag): I’ve been logging the data, and it seems my Tiptronic has some serious trust issues. The adaptation value for "2-3 shift, lower rpm range" is sitting at 1.39 bar, and I’ve watched it climb as high as 1.49 bar on a hot run.

My Theory: My stab in the dark is that the TCU learned this high-pressure correction to overcome the old, tired fluid. Now that the fluid is fresh and slippery with a new filter, the car is still using that massive bar pressure to slam the clutches shut. I've noticed that if I drive it with more purpose (steady 20% throttle on once on the relearn drive ), the pressure dropped to 1.29 bar and it behaves much better. The moment I go back to being a sensible citizen at low revs, it spikes right back up.

Next Steps: I’m in the process of trying to get the car to relearn. My ThinkDiag doesn't cover a full adaptation reset, so I tried a manual process, but I'm not sure it worked as the values haven't changed. I haven't really pushed it into the medium or upper rev ranges because the thud is so pronounced, but I guess I could try once the trans is fully warmed up to see if it helps.

I'm in Oxfordshire and I think I need to give this TCU a lobotomy so it forgets its old bad habits and starts fresh with the new fluid. Any specialists or anyone mobile area with a PIWIS who can do a full adaptation reset? Also curious if you guys think a hard reset is the cure, seen this before or if I should be looking at the solenoid with values this high.

Details:
5-Speed Layout: The log shows "Selected gear" values plus more ranging from 1 through 5, plus Reverse.
Adaptation Buckets: The "lower, medium, and upper" RPM range adaptation categories are a signature of the Bosch TCU logic used to control the ZF 5HP19.
Any thoughts or advice would be great.
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Ok a couple of updates....I hooked up the ThinkDiag while the car was stationary and idling. The 2-3 lower rpm shift pressure is sitting at 1.39 bar without the car even moving. Probably confirms the value is hard-coded in the adaptation memory and isn't just a live calculation......

I also have a DTC 108 (Supply Voltage) showing up, the battery has been a bit low lately. Even though my live logs show about 14.0V while driving, I'm wondering if the low voltage during startup is stopping the TCU from correctly saving any new adaptation data or if it’s just making the computer a bit grumpy