Thank you Coloradorob. I was wondering which gears locked up, and have looked, unsuccessfully. Not actually questioning you, but is there a piece of GM lit that says so? Your description of trans temps jibes with ours. We can push 215-220F towing a ~6000# trailer up and down slow hills, near max truck pay load. But with just the 2 of us, we max out at 110-110F above ambient. Now, I've heard anywhere from 225F to 250F as "stay below" temp, but have also checked thru GM lit, unsuccessfully for this. Any documented guidelines? FYI, if I find some, I'll stay under with modified driving habits. No J.C. Whitney Mickey Mouse BS under my hood.....I don’t understand where the heat issue comes from.
At least in 5th or 6th gear it should be in lockup.
So if your running fast enough to be in 5th there should be no slippage and very little heat.
If your pulling a trailer on a back road and the transmission is in 1st to 4th and or shifting a lot then yes it’s will be hot as it will not be in lock up. And the torque converter would be hot.