2015 6.2 Trans stuck in 2nd or 3rd

6.6LTZ · August 4, 2017

Morning, folks.

I dropped my truck off about 4 weeks ago for some warranty issues, 1 being to update the trans and do a relearn.

FF to 1.5 weeks ago. I got an oil change and tire rotation at my buddy's shop. When I went to leave the check engine light was on. I got out and did a once over of everything. Buddy came out of the shop with his code reader. It was a trans code but I just had him reset and I was on my way.

FF again, to yesterday afternoon picking my son up from camp. Start the truck, put my seatbelt on, look down, check engine light is on. Oh well, right? It wasn't flashing so I'm going home. Put it in drive, SLAM. Son says, "Daddy, what in the world was that?" Put it back in park then back in drive, SLAM, again. This time I ease it in reverse, no problem. I ease it back into drive, SLAM. This time I let off of the brake but the truck would not move. I turned the truck off and waited a few seconds, only to slam back into drive. This time I gave it some gas. It revved up to about 2500rpm before it would move. I managed to get on the main road and headed for the dealership.

Long story short, I called the dealer on the way and they (thankfully) allowed me to drop it off and gave me a loaner.

After I got my paperwork one of the service reps was talking it over with me about the 8 speeds and the early builds. Another backstory, this truck had a trans put in prior to 14K miles when it was new, as I bought it with 14K miles. She told me that all they did was "tried" to fix it with a new trans before GM really knew what the problem was.

She said they would do everything they could on their end to warrant a new trans.

Thoughts? I have read countless trans posts, but has anyone been stuck in a gear and no shifting at all?

Wish me luck!

6.6LTZ · August 8, 2017

Back to this dead thread...

Dealership just called and said the battery caused all of this. Said a low voltage battery will cause the trans to go in a limp mode.

Anyone heard of this?

InertiaFaze · August 8, 2017

If you get a chance, let us know what the code is......

6.6LTZ · August 8, 2017

If you get a chance, let us know what the code is......

Ok. You think it will be on the paperwork?

GMC_SD_Ult_16 · August 8, 2017

My 16 6.2 8 speed went into limp mode (3rd gear) at 128 miles on the odometer. It was found to be the TCM not communicating properly. They replaced the TCM and went through the relearn process. Truck has driven better than when I first picked it up and now I have 26k miles on it. Good luck on the repair. It seems weird they would claim the battery since I would have thought the alternator would have been supplying most of the voltage by the time you put it into gear. But maybe something strange could be happening with it. Best of luck in a proper diagnosis.

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6.6LTZ · August 9, 2017

My 16 6.2 8 speed went into limp mode (3rd gear) at 128 miles on the odometer. It was found to be the TCM not communicating properly. They replaced the TCM and went through the relearn process. Truck has driven better than when I first picked it up and now I have 26k miles on it. Good luck on the repair. It seems weird they would claim the battery since I would have thought the alternator would have been supplying most of the voltage by the time you put it into gear. But maybe something strange could be happening with it. Best of luck in a proper diagnosis.

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Funny, this morning it feels like it is shifting a lot better prior to this. Couldn't really tell yesterday as I was driving through a heck of a rain storm.

What bothers me, is this same thing happened (check engine light) 2 weeks prior, but at that time I did not shift out of park before the code (trans code) was cleared. If the battery was an issue I would thing it would trigger the light before 2 weeks. Gonna go grab the paperwork here in a bit and I'll see what it says exactly.

6.6LTZ · August 9, 2017

Here is what it says.

Vehicle had many different low voltage codes in a couple of modules. Tested battery with GR8 machine to get failing results. Removed and replaced battery with new then test drove to verify repairs.

I asked for codes when I picked it up. Service said the tech did not put them in the system at the time.

We will see in the coming weeks if it holds up.

InertiaFaze · August 10, 2017

Their diagnosis and actions seem on the money. Low voltage could possibly send the TCM offline and if the TCM were offline other modules wouldn't receive important data from the TCM, like whether or not it was in park, etc. It would also possibly choose a gear not based on the TCM programming but rather based on the hydraulics and their design for this unlikely but possible situation.

elchilero53 · August 11, 2017

That sounds like an alternator problem, not a battery problem.

Heavy 5.7 · August 11, 2017

Battery cables.

6.6LTZ · August 11, 2017

I have noticed over the last 3 days that the voltage gauge has stayed pretty low, what appears to be around the 11-12 volt range.

The truck has a battery light. Why would an engine light come on vs. the battery light?

6.6LTZ · August 18, 2017

Well, all was fine until I left the house a minute ago. Engine light on again and stuck in 3rd. After this doc appointment I'm headed back to the dealer. I'm worried most about when I pull my boat, as we were headed to the lake this weekend. I'm not sure 3rd would pull it out if this happens all of a sudden. This is starting to make me angry.