2020 Transmission issues

MyDefectiveGMC

New member
Sep 9, 2022
3
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I have a 2020 AT4 and absolutely loved it up until March. Now I'm having constant transmission issues. Started out. I'd be running down the highway at 80 and it felt like you ran over a speed bump. Come to find out. For some reason the transmission is downshifting but immediately shifts back up. At first they were trying to tell me it was caused by the regen for the def but I finally got them to take it for more than a 2 mi test drive and they realized it was not that. Then they changed a valve body that didn't fix it. Flew in a GM engineer. He said it was a module took almost 2 months for that to come in change that didn't fix it. Now they've just replaced my transmission. Didn't take it for a test drive afterwards and that did not fixed either. Reading a lot. People say it's a torque converter.

Does anybody know what I can tell my dealership to do to fix this?

Proud Chevy Truck legend. Life time GM owner.
 
I've seen more trans stories over anything lately with these trucks. Even mine has a weird downshift/shutter when accelerating at higher speed (50+ MPH). I know this doesn't help you. but I would push for them to replace anything they need to.. hopefully they have been good to work with, some aren't so lucky.
 
You would imagine they would have replaced the torque converter with the tranny, but then, knowing GM, maybe not. It could very well be the torque converter but they could verify that simply by hooking up the scanner, driving it and commanding the converter to lock up. I had an issue with my Caprice with the 6L80E where the torque converter clutch would continuously engage and disengage, seeing the RPM bounce around 200-300 RPM while cruising. However, my issue was a tune problem where the tuner messed up with the pressures, shifts and TCC lockup. I returned the tranny tune to stock and it was fine. I figure it went into some sort of hysterisis when trying to lock up.

After eliminating a mechanical issue with it, they should look into the tune. Obviously the regular dealer tech won't be able to do this but their engineer might have to dive deep into it.
 
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