Need and idea of how to fix. At 6:00 I got up to go to work started my truck and the eng. light came on. The dash said ac turn off / eng overheating and the temp gauge went to 0. Ran the truck to see if it will stop and it did not. The radiator fan runs all the time but the truck is not overheating. What would cause this problem? Every one said to change the thermostat. I did this and it is still does it.