So it seems that you know that the mechanics of the fan system works fine,it is the trigger mechanism is not.Do you have access to a Pro-link or some other diagnostic equipment?A breakout box could come in handy too.You need to find where things are going wrong,is the ECM getting input from the engine to tell it if the engine temperature is too high or is the ECM just not signaling the clutch to work.If I were you I would disable my fan and run the engine to the signal temperature and see if the ECM receives the correct information,note that what dash gauge and what goes in to the ECM may not be the same information,this is where the Pro-link comes in,you can see what the ECM sees.Also do you know the repair history of that truck?Did it have a fan clutch installed in the past?If it did was it the correct one for the truck,some fan clutches are air apply and spring release while others are air release and spring apply.If the wrong clutch is on the truck when it gets to the right temperature it will disengauge and stop working but if the fan is constantly running it may never reach the correct temperature to stop because the engine coolant temperature is not hot enough,thus a constant running fan.Just some of my thoughts on a Sunday morning,hope they might be of help.
