Trans fluid temp getting REALLY hot

[Andrew Hunter]

So I have a 2017 GMC Sierra 1500 5.3 CC. At 19,000 I had it at the dealer for trans issues, they tell me nothing is wrong. 24,000 same thing. 28,000 same thing. At 44,000 I take it on a cross country trip to Colorado and the trans fluid temp gets as high as 270*. Going across the flat mid west at 75 it stayed a constant 250-260*. The dealer tells me there’s nothing wrong. Then, on my way back to PA, I lose OD for about 600 miles. It’s the middle of the night with my wife and 2 dogs so I just run it. I plan on calling the dealer and complaining tomorrow but I don’t expect to get anywhere. Any thoughts other than a blocked trans cooler/cooler line? 

[1SLOW1500]

There is settings that go to a trans "limp" mode. So when you say 270* that thing is in dead levels. I would do a thermostat delete and see that happens. Sounds like it isn't opening at all. A trans relearn wouldn't hurt.
The default thought is you have a ton of slip causing heat and wear. Question is the issue stall or trans. Also large debris can cause additional damage. This is what in all trans post I recommend pans drop,filter and clean magnet.
Gm had many issues that are stall,tune,wire,driveshaft,and axles.

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