Transmission overheating

dsm1995gst · August 28, 2019

We’ll never know what the issue is, as they ended up replacing the whole transmission.

Thus far it seems to be fine judging by the much lower temperatures (and much slower climbing) driving around town. Will hook up the boat soon and see what happens.

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dsm1995gst · September 5, 2019

And today GM called me (?!) asking for an update, because they apparently have no clue what is going on and were not aware I’ve had the truck back since beginning of last week.

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joselp15 · July 22, 2022

mine did the same this past weekend was on freeway going 70 and sudenly no accelerator pedal and message on dash saying “transmission hot idle engine “ so i pull over and let idle but cant check temperature so eventualy i drive slow off fwy in protect mode and turn engine off and wait 30 minutes and finaly temperature shows 270 degrees my trans thermostat was stuck closed. only 73k miles and ever since them my transmission not the same my first gear is weak and has no torque when taking off from a stop

Gandalf · July 26, 2022

GM quietly admitted their mistake with the original automatic transmission by-pass valve (thermostat) in 2014-2018 gas trucks. A while back, they replaced the part we all have from the factory, and replaced it with a part that looks identical, but that opens to send fluid to the cooler at a much lower temperature.

I did try the “flip the pill” method, which was dead easy, but in my haste to complete the job, I neglected to press the c-clip into its channel fully. Ended up with a few litres of ATF coating my garage floor! In my fury at myself for missing such a simple thing, I got more angry at GM. So, I googled a bit and found the new part (86774933), which my local dealer had in stock.

For the cost of a $100 part and minor effort to install, the truck now operates at safe temperatures all the time. Before, the ATF heated to around 195-200 just driving to work (half hour on hilly roads). With the new by-pass, it never gets above 150-155, even during a recent 8-hour drive through mountain passes here in northern BC, fully loaded and - because of 35 inch tires - constantly gearing down early on even small hills, not to mention the many 1-6 km long hills.

As any ATF fluid temp/tranny longevity chart shows, fluid this cool prolongs the life of the transmission by app. 40,000 miles. So far, the best thing I’ve done to the truck (2015 AT new to me in March this year).