Has anything been added to the truck recently. If it goes dead it should show a draw. Does this have dual generators?
The only thing I can suggest is take the large wire off the alternator or both if you have that system and see if it does the same thing if it does then the alternator is leaking through the diode bridge. Then pull starter relay and see if it still does it then get it scanned see if anything pops up.
Friday, November 14th, 2014 AT 12:19 PM