6 speed suddenly rough shifting

[EVRLET]

Have you attempted to pull codes? From your facts, going to M put you in sports mode and it may have temporarily got stuck in this mode, technically referred to as driver shift control quadrant. You would have more aggressive shifts when in this mode. The 6L is so heavy with electronics that I would start with codes. If you don’t have any, try going back into M mode and back into standard to see if the electronics can get you back to normal.

[1976gmc20]

Originally Posted by EVRLET

Have you attempted to pull codes? From your facts, going to M put you in sports mode and it may have temporarily got stuck in this mode, technically referred to as driver shift control quadrant. You would have more aggressive shifts when in this mode. The 6L is so heavy with electronics that I would start with codes. If you don’t have any, try going back into M mode and back into standard to see if the electronics can get you back to normal.

No, there were no lights on. All I have is a simple little code reader.

The truck was shut off and re started a bunch of times in town and it kept acting about the same.

I always use M in this truck going down that steep hill into town. I don't use the +/- buttons. I just pull it down after getting going out of the roundabout and by that time it is in 3rd gear per the display so it just stays there all the way down.

I haven't driven it since we got back except to move it 50' to its usual parking spot after we unloaded. It often sits a week or two at a time.

If it doesn't clear up I will probably schedule an oil change with the dealer in a couple weeks and see what they think about it. We've only put about 3000 miles on it since we got it but it's been almost a year.