Hello from twinke74...I also have an Armstrong ac unit mod# TR21-12.on my 1974 overlander...it seems to work well at most of the time...there are two different fans ,one is the squrell cage blower (evaporator),and the other the outside( condensor) blower. the ac compressor has a thermal overload/limiter switch.my understanding is that the compressor cycles on/off depending on r22 refrigerent temp/pressure in relation to inside dailed temp setting. once the inside temp is reached say 76deg the hole unit shuts off! after a few min's it recycles on.ALTHOUGH...sometimes the compressor does not engage!
as we are going into fall,I can live with it at this time.I will be looking for parts,diagrams as I am one of those DO IT YOURSELF types. wish me luck!