02 Grand Am SE 3.4 tranny swap now shifting HARD!

As the tittle describes. I have an 02 grand am SE 3.4 with the 4T45E trans. We’ve owned it since new and it’s always been stored in the winter. Last year the tranny began to start slipping. I was able to track down a “so called” low miles replacement to swap. I even put in a reman torque converter while at it. After getting it back together it actually shifts really good. No slipping with swapped trans. The shifting is factor new for a few miles until the trans goes into “full line pressure” mode. Shifting extremely hard as if I had a shift kit.

Strangely enough…… if you pull over turn off the car then restart the engine the trans clears and you’ll get another 5-10 miles of smooth shifting until it starts shifting hard again. Pull over, shut down, restart, good for another 5-10 miles??

The fact you can re-start and get a few trouble free miles instantly leans me to believe I’ve missed something externally?? If it were a bad TCC solenoid or shift solenoid I gotta believe the issue would be there all the time and a engine shut down wouldn’t correct anything??

I fully understand that if the MAP,MAF,IAT,TPS sensors aren’t happy in this car you’ll get hard shifts. GM had Bullitens regarding IAT and MAF connector pig clips. Indicating they might look good but fail internally over time.

Even though I’m not tuning up any codes on the scan tool I replaced both the IAT and MAF connectors. Cheap place to start but no change. I also have 2 different ECMs for this car. One was flashed stock and my second ECM was a mail tune from PCM of NC. BOTH ECMs are giving the same hard shift result so that eliminated one more possibility

Keep in mind hard shifting was a non Issue before tranny swap with all previous existing parts.

One thing I was sure of during my research was to get the correct trans as a swap. Ex: I needed to use a 4T45E from a SE model as it had my matching gear ratio. You would I have all kinds of issues and others have if you swap in a GT trans into a SE model as it has a different gear ratio. The ECM goes crazy and spits out a Trans 07.. code indicating wrong gear ratio

I’ve ruled that out cause the shifting is perfect, spot on, in addition no trouble codes. Which I really wished it would throw acids
to at least point my to the correct haystack then I could find the needle.

I’ve seated, reseated and cleaned all trans electrical connectors. This trans is 100% electrical with no vacuum. Engine does not appear to have any vacuum leaks. Not sure what i could be missing? These 4T45E trannies, look at them the wrong way and they get cranky! Time to turn the brain off for a while