2004 Suburban 2500 with the 8.1 and the 4L85E. Fundamentally the same as the 4L80E.

Ever since I got it, every time the transmission shifts from 3rd to 4th and the converter was either previously locked in 3rd or the tables say it should immediately lock in 4th, the shift will happen, the converter will lock, unlock, and then lock again and be fine. The transmission and torque converter are mechanically fine and not slipping. No temperature issues or any other odd behavior. It does this with the stock tune. The converter appears to be a stock replacement rather than an original, and I am not sure if the stall speed is stock or not.

When this occurs, everything is good in 3rd gear, the 3-4 shift occurs (and feels nice and quick), and as soon as this happens the TCC slip shows ~-200 to -300 in the datalogs (I believe it still is using 3rd gear for the maths). TCC state then goes to "off due to slip" and the converter unlocks. As soon as it unlocks, TCC slip goes positive again and the converter re-locks. The shift error time roughly corresponds with the ECM thinking the 3-4 shift is "complete" when the converter unlocks and the slip number goes positive again. The 3-4 shift has a very large time error (sometimes as large as 5-6 seconds and always more than how long the shift actually takes).

I have seen one other person report this exact problem, but they never posted a solution and it was nearly 5 years ago now.

Any ideas? If the converter in this truck is higher stall than stock, can that cause the PCM to not "see" the shift? How does the PCM know that the shift is complete? Is there anything that EFILive has control over (desired shift time, shift stabilization ratios?) that would actually impact this?