Brian

Thank you for these comments. One more question for you.

I have the same problem with my 83 300sd and have replaced the monovalve insert and the cabin temperature sensor with no result. In my case I get swings of very hot air with periodic blasts of hot air out of the dash vents and no hot air at any time out of the foot well vents.

makes no difference if the CCU is in AC or ECONOMY setting. Temp still randomly roasts occupants. At speed will get sometimes get hot air out of dash vents even with CCU switched off.

Is this more likely to be a monovalve, CCU or a vacuum problem or can't you tell without further diagnosis?