6L80 hard shift 1-2

[Plymouth Barracuda]

Hey, 2011 Silverado 6.2L with 6L80, RWD.

It started late march when I suddenly noticed harsh shifts from 1st to 2nd, even in the morning on a cold engine. The transmission fluid was changed in March 2024, so it was fresh.

• In normal driving: strong jolt through the body.
• If I add about 15% throttle: no jolt, but feels like slipping and a slow shift.
• Flooring it: shifts perfectly.

1) I reset the adaptation using my scanner.

It got better: now it only happens when the fluid is warm, around 32°C (90°F) per the gauge, so no issues when cold.

• In normal driving: noticeable jolt through the body.
• If I add about 15% throttle: shifts fine, not like before.

2) Took it to a local GM center.

The transmission tech drived it and said it’s a common 6L80 issue. He suggested starting with the valve body. He swapped in two used valve bodies, but the issue persisted. He reset adaptation each time, drove it, then tested my valve body with a vacuum test, checked seals, etc., and reinstalled it. He didn’t check the channels since the used valve bodies made no difference. My fluid was clean, magnets looked fine. Ended up paying for labor with no fix. That day, it was still shifting harshly, same as before. When I picked it up, I drove in all modes, especially flooring it in S mode.

3) The next day, the jolts were gone.

I think some special service adaptation kicked in. Drove without issues for 2 days. But now I noticed harsh shifting from 2nd to 3rd. It’s a paradox.

4) Decided to change the fluid and filter.

They used a machine to flush and fill 12 liters of AcDelco Dexron VI, in addition I added BG ATC Transmission Conditioner, which is supposed to help with harsh shifts. After a couple of miles, the issue came back: harsh jolt from 1st to 2nd in normal driving, but fine when adding throttle.
The shop now suggests replacing the clutches, piston, and all parts labeled for 2-6.

They didn’t check the TCM. The tech said it’s impossible to test, but as far as he could see via the scanner, there were no error codes or other issues.

Could you please your thoughts and where to search next ?

[richard wysong]

On my 4L80 a bad input turbine speed sensor caused that problem but it showed as a code, possibly it's still functioning enough that it hasn't thrown a code