Did you get this figured out? I have a similar problem (2011 GMC Sierra). If I turn on the A/C shortly after a cold (or hot) start, the high speed fan is triggered after ~30 seconds, regardless of coolant temperature. It will run high speed for several minutes before eventually reverting to low speed (correct) operation and when running at high speed, the battery voltage will drop significantly. In addition, the fans will continue to run high speed even at 60 mph, but will eventually switch off as they should. Refrigerant charge/pressure has been verified correct. I also replaced the high pressure A/C switch with no change in operation. Any help would very much be appreciated.