More than likely, like most of the rest of us, one or more of the diaphragms in the vacuum pods that actuate the flaps in the acc system are going/gone. My 1981 SD has worked flawlessly until yesterday when the footwell vacuum element failed. Since you have to tear into the dash to get at all this stuff, I'm just going to replace all the diaphragms at once. My failure has caused only cold air to blow out the rear vents, due to the nature of the design. I have studied the diagrams carefully and have determined that with the failure of my footwell flapper, the air backs up and flows out the rear vents before ever reaching the heater core.. It really is a complex system. Oh yeah, I have yet to hear of anyone replacing the switchover solenoids, the vacuum elements seem to be the weak link. Could happen though.