Geoff Hubbard

Sounds like you might have a clutch issue. Here's the deal with what I did. Mine, not so much like yours but would slam into gear right as I'm completing a stop. Took it in after it did it 5 times. They unloaded new software for the transmission. Now. Here is the catch. After getting the upload I found out the it was in fact a bulletin that GMC put out for that transmission. Not considered recall. So you won't get it until you have an issue. They uploaded the new software and things were fine for about a day. Then it happened again. So I went on YouTube and watched a guy from allison transmission returned a transmission. The real catch to this is PATIENTS. Now I'm not a patient person when spending money like that but he's right. THE TRANSMISSION MUST LEARN YOU DRIVING HABBITS AGAIN. So I was patient. Its been almost two months and drives perfect. No issues. Ask for the updated bulletin on that transmission. See if it works. We as owners now have a part that we have to do thanks to software changes. Once you get the unloaded driving done you will need to go through it again when you pull a heavy load. Good luck

The problem is just before the stop. It has actually slammed into the lower gear bad enough to make my front tires skid. I have made sure that I was in normal mode. The dealer does recognize that there is a problem. I am taking it back again Monday. I will try tow/hall mode and see if it does it at stops. It does not do it while in manual mode at all. So. Yes its electrical causing a mechanical malfunction to include premature ware on components. Not good. Although they say its learning your driving habits. BS

OK. Took the truck in on 30 Nov 20 and they did a transmission update. Hmmmm OK, so the truck has been running and shifting fine, just like new. So today 2 Dec 20 the transmission started shifting into that first gear violently again. Did it consistently for the next four stops. Called the shop and they actually said that the transmission is still learning and it will for the next 1000 miles. BS. It should not do this at all. So now I cant get in until Monday. Great. Will see what they do this time. So they really expect me to believe that the transmission is still learning.

I have a 2020 GMC AT4 2500. (Diesel) Currently at 7040 miles. Problem started 28 Nov 20. Just prior to a full stop the transmission will violently slam into it's lowest gear. I thought at first it was just a hiccup but not the case. I came to three more stops and it kept on doing it. So I have been driving in manual mode until I can get it to the shop tomorrow. Had similar problems with my 2016 GMC 1500. They sort of fixed that problem. Changed fluid, did something with the computer but I was back in again in about 6 weeks. I am somewhat confident that this is a signal problem of sorts. Sense it does not do it in manual I would assume that this is an electrical component when only in drive mode. We will see. Haven't even got my first oil change. How sad.