I agree about the monovalve issue. Early last year in the winter I had an issue with my car's climate control system. It was winter and I had no heat at all. It turned out to be a rip in the monovalve's diaphragm. It was a very,very easy 15 minute fix with a new monvalve insert kit.
The strange thing was that at the same time that the monovalve diaphragm ripped from old age, my car would start to only blow air out of the defroster vents. I found out that by turning the car off and turning it back on, my CCU would reset and all would work well for about 5 minutes and then back to the defrost mode. After doing research on this I learned that the CCU units in the 86 on 126's have a default mode that when the CCU fails, it automatically reverts to defrost only. I also learned that a failing aux water pump ( which sits attached to the monovalve) can draw more current than the CCU can handle. A lot of people have put a 1 Amp inline fuse in the power wire to the aux water pump to keep it from drawing too much current and burning the CCU. We are somewhat lucky in our 86 and on models that when this happens in our cars, the built in circuit breaker in the CCU will trip and then our cars will have defrost only.
I then looked at my aux water pump and it was toasted. I replaced it but still was having difficulties with the CCU to work correctly. I hoard parts that I have found in wrecking yards and had another CCU from a different car, and installed it and all is well now.
There are companies that rebuild these units if yours has failed. I believe Programa has a website that actually has a troubleshooting section that you could use to check your entire CCU system. I did this with a simple ohmmeter by disconnecting the two large wire harnesses on each side of the CCU and then using the wiring harness of the car to check all of the assorted sensors that send signals to the CCU. It really was not as difficult as it sounds and the website has the instructions on which terminals you use to test the different sensors.
By the way, just for your information, on the PRE 1986 cars, when the aux water pump seizes and draws too much current, you will actually burn up the circuit board in the CCU. Search this site and there are many, many posts on this occurring.
Good Luck and keep us posted on your progress.

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John
1986 300sdl
1985 380se