I have a 78 300D with an A/C problem.
6 months ago the A/C idler pulley bracket would not tighten any more due to a stripped hole. Prior to this the A/C worked fine. I replaced the bracket and pulley and the air worked fine for about 1/2 an hour then the fan and compressor stopped. I switched to defrost and the fan and compressor both came on as usual except the air never went to high heat as it does when on defrost. This worked for another 10 minutes with cold air on defrost then everything stopped again. Came home and the blower fuse and the "A/C switchover/idle control fuses were both blown.
Replaced them, next day same thing all over again.
I also replaced a leaking trunk diaphragm at the same time as the pulley as the vacuum leak would not allow the fan to blow through the front panel vents. Not sure if this would have anything to do with the problem or not.
Any advise on this would be great, it is 94 degrees here in FLA and I just know the power windows will decide to quit pretty soon too!!!

78 300D A/C Problem
 

The A/C problem seems to be solved.
The blower fan motor was on it's way out and was causing
an overload and blowing the fuse. Since the fan was not working, some kind of safety would not let the compressor come on.
I replaced the fan and it is all working fine.