When I used my '78 6.9 two weeks ago, it was blowing very hot air regardless of the setting of the temperature dial. I checked the resistance values of the rheostat there via the 10 pin test connector (pin 9 and 10) and saw a resistance of 550 Ohms at 65 F selected and 1700 Ohms at 85 F selected. I then took the car for a test drive.

Now, it only blows cold air all the time, regardless of temperature setting. So I wondered if the servo is stuck. The car has an aluminum bodied servo, and I thought those were more trouble free. Feeling the auxiliary pump I noticed that it seemed operational (slight hum), the lines to the servo were hot, but the lines from the servo were cold.

Any suggestions as to where I might look next? Thanks!

Robert