I bought this 1985 300DT 173K miles recently knowing that it had AC issues. I had done some reading online and found that often when people say that the AC blows cold then hot it is the monovalve sticking open. I was planning on replacing this, however, recently I wanted to turn on the HEATER to get newly added coolant distributed through the whole system, and when I tried to turn on the heater the AC came on and no matter where I put the internal thermostat (all the way hot or all the way cold) it stayed on AC and blew cold. Now, am I right in assuming that this blows the monovalve diagnosis out of the water?

I know it isn't the compressor, because I can hear it engauge (at times). I know it isn't freon, because it blows cold sometimes (not in any predictable way). So, what could it be? Is there a thermostat somewhere in the system that tells the car to blow colder or warmer air based on the position of the internal thermostat that could be blown? I still might change out the monovalve just to see if this helps. I invite ya'lls thoughts and inquiries...

-Chas.