I have a 1990 E350 Diesel. The blower motor seems to have a mind of its own, when turned to any of the speed selections nothing happens, then after a few miles (different each time) it will come on and run for a few minutes then go off again. I metered for power at the switch and found it ok but none at the blower. Any ideas, I haven't had time to realy look into it yet but hope to this weekend as it is starting to go below freezing in the mornings now and the windows fog up.
Thanks

 

Blower speeds are controlled by the use of a resistor pak in the ground circuit. This resistor pak is located in the evaporator/heater case in the engine compartment. Look for a three-wire connector in that location. This resistor pak is usually where the problem lies.

 

I believe that the resistor pak has a four wire connector.