Any codes in the ECM? I read somewhere that if the coolant temp sensor has a fault it will turn the fan on full as a safety measure. Mine is on a lot when running the A/C, I don't think I have ever heard it other than that, not even when pulling my camper.
I installed a 12' auxillary fan on the A/C condensor to help with that. This new electric fan comes on when the compressor comes on and will cycle off when the A/C compressor cycles off.
It even dropped the in cab temperature out of the A/C vents from 48 degrees down to 40 degrees. The main engine fan does not seem to come on as much as it used too. It will be interesting to see what it does while I am towing.
A lot of people seem to think that the cooler the better but your engine is supposed to operate at like 190 on 2nd gen and 200 on 3rd gen but many people put fans on to lower the engine temp but those engines have to run at a certain temp to burn the fuel correctly among other things.
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