I installed a 4L80 trans out of a 99 dually in my 05 GTO a few years back. It was shifting perfect in both normal driving and under WOT for at least 2 years. I removed the trans to install a new engine in the car this past winter and ever since then the trans will not shift under WOT. I have checked all the wiring, replaced the speed sensor at the rear of the trans. and still nothing.
Playing with the tune I found figured out that if I reduce everything (Speed and Throttle) by 50% it would shift. Example 1-2 shift set at 18 mph @ 5500 actually shifts at 42 mph @ 6700
I did some searching but came up dry. Any guesses?
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