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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.

Ideas ?...
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.....
 

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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.....

Used to be you could touch the brake pedal and the transmission would unlock. Very noticeable by holding a steady speed then touching the brake with your left foot. You would get an instant rpm rise of a few hundred rpm then it would drop back down as it locked up. I been towing quite a bit lately on hilly terrain. Been towing a max load 14,000 lbs dump trailer loaded with firewood. I figure the trailer weight was around 7,000 or 8,000Lbs. What I have noticed is 2nd gear in manual mode actually go from slipping to lockup. I seen a 300 to 500 rpm drop. I have found this is noticeable in any gear but first. I haven’t tried putting it in manual mode and starting from a stop going up hill to see if I get the rpm drop.
On the older lockup transmissions it was very noticeable when the transmission would unlock as you pressed down gently on the go pedal. Or you would notice an unlock just before a down shift...

But this transmission does not unlock by touching the brake. It stays locked. My guess would be in the old days there were some problems with lockup transmissions stalling the motor in a panic stop. So they added the unlock feature whenever the brake lights came on.
And now the unlock feature is probably connected to the antilock brakes.
I also never see a unlock before a downshift. Doesn’t seem to matter if the downshift is invoked by throttle or by brakes.

Try this
While climbing a hill gently press down on the go pedal, if it’s locked there will be no increase in rpm beyond the normal increase caused by increasing speed.
But I have never seen this. It seems to hold lockup until a downshift at witch time it may be unlocked and then lockup again depending on the load. Easier to reproduce with a large load as in a heavy trailer.

Without towing I have never seen high temps.
But keep in mind that the trans cooler is in the rad and the hot water in the rad will actually heat the transmission fluid. If the coolant in rad is 180 than that heat will be transferred to the transmission fluid....

Rob
 
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