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The cab temperature sensor is located on the dash to the right if the steering wheel. Behind the small slotted openings.I have a similar problem in the hot Florida sun. I did some experimenting with the a/c and think I understand why it appears to not work that well.
Being a Software Engineer by trade, I blame it on the temperature control unit/software.
Usually I have mine set at 72. When I first get into the hot truck, it does not seem to cool as expected, however the air is cold enough. Eventually it will get comfortable. Setting down to 65 (or Max) seems to kick in the fan to a higher speed and cool down quicker.
So my theory is that the temperature control is just not working like other cars. I'm not sure where the temperature sensor is located, but if it is checking the output (not the ambient temperature), it may not be compensating for the radiant heat of the console/seats/dash.
I just turn mine down to 65 or so, and wait until it's comfortable. Then I reset to 72 and all seems well. Give this a try before putting in more freon or taking it for service.
Yes I are a Rocket Scientist
No matter how low I have the A/C on, my truck sounds like a damn 747 taking off. That fan runs so loud ALL THE TIME.