1990 Mercury Marque Air conditioning headache

I have a 1990 Mercury Marque that the air conditioning blows very hot air part of the time and cold air ( 36 degree) part of the time. I suspect that its some how in the vacumn system that opens and closes the plenum doors, but the answer eludues me. Coolant is good, compressor is good and pressures are fine. Before I pull the dash, I was hopeing someone had come across this kind of problem before and a better solution than I have at the moment.

Thanks. Cantrelm I agree with you that the trouble must lie in the operation of the plenum doors, or their vacuum connections. Try checking the fuses. My A/C did this and it turned out that a fuse controlling it had gotten loosened somehow and I popped it back in and it worked like a charm!