I have a '78 E-150 with a dead A/C fan. Last year it would run a while on high and then quit. After a little while and time to cool off it would come back on. I started running in the third position instead of high and it ran fine. This year I tried the third position and it quit for good. I read about the Thermal Resistor and searched for it on the firewall as stated but never found it. If it is the fan I dont have a problem with replacing it but I would like to know for sure. As you can very well tell electrical gremlins are not my forte'.