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Do you have the automatic HVAC? when you have it at 78*, what is the temp outside?
If you put it on auto and the temperature is above 40 or so degrees, it will use the AC compressor to dry the air. You will notice the little snowflake on the top of the display when it is being used. I just press auto, and then quickly press the AC button to turn off the AC because I don't really want it running unless I need to cool the truck. If you turn off the AC fast enough, it won't even have time to kick on.