Transmission running hot - Overheating message

Shane Huffman · November 22, 2020

I have a 2015 Silverado with 105,000 miles on it. Did a full transmission flush about 3 months. Driving with no trailer for more than 2-3 hours at a time specifically in hill country the tranny temperature is running between 220 and 260 obviously very high. After a few hours if I am not babying it I will get the message " Transmission is hot let engine idle" If i let it idle for 60 seconds it drops 5-10 degrees and on the road again however the temp still remains way to high. If I am pulling a trailer of any kind flat bed, toy hauler it will over heat within 2 hours or so and again really having to baby it. I ran the computer on it and receive a Lost communications with TCM OBD-II trouble code. Now before i start swapping parts such as the TCM I am hoping for some ideas or suggestions with what is going on with it. Also occasionally maybe 3 times in the last year the rear tires with lock up in first gear for a 10th of a second just long enough to screech the tires. Once in a while there will be a hard shift doesn't matter if the truck is hot or cold and is rare but non the less happens.

Any ideas before I take it into the shop?

truckguy82 · November 22, 2020

Could be stuck trans thermostat, that’s what happened to my truck.

It’s supposed to open up at 190-200F. This is how to diagnose.

The thernostat is basically attached to the trans on the outside. It allows flow to the trans cooler. When your trans is hot, feel both of the cooler lines. In my case it would allow a little bit of flow regardless of the temp. I don’t know if they are all like that. So my drivers side line was hot, burning hot. But the flow was so slow that it was ambient temp by the time it left the trans cooler. So I had one line hot and one line cold regardless of temp.

After I had the thermostat replaced. It was the same exact thing, only when I reached 190-200, the passenger side line was hot as well.

Both lines should be very hot when it’s flowing. Like too hot to hold onto.

The dealer was incapable of diagnosing this, I basically had to self diagnose and explain it to them.

I suppose it could also be low on fluid.

i would advise getting the trans fluid replaced as well

Eddie 70 · November 23, 2020

Lots of guys on here have done the transmission thermostat "Pill Flip" mod and have seen the trans temps drop to around 160 going down the road. Myself included. Takes about 20 minutes to do at most.