MB Doc knows is stuff as many others do on this website. All I can offer you is my experience with my A/C System on an 88 190e - W201.
If you really do not know what you are doing I recomend that you spend the $ to get a proper diagnosis. From there if you want to do the work yourself, that it upto you since repairs for this sytem can be costly due to labor, not so much in parts.
I took my car to get diagnosed and what the tech did was verify vacuum through the (do not remember what it is called) but it is a central vacuum thing-a-ma-gig that is located behind the glove box. (access through the glove box) From there the tech told me which modulators/actuators would not holding a vacuum. There are three modulators/actuators located behind the dash. I do not remeber exactly but either one or two of these where the problem. Also the heater actuator/valve you call it was out. The Heater valve was the most expensive. The three behind the dash ran me about 20-30 a piece. I ordered all three that are located behind the dash since to change just one of them requires removing the dash so while we was in there, decided best to change all three and not ahve to take the dash apart again.
So in end, what I recomend is that you have the vacuum system checked through the main vacuum thning-a-ma-gig behind the glove box. Also that main vacuum thing-a-ma-gig needs to be checked because if it is bad then the other actuators do not function properly.
Also, you should have checked is the Klima Control Relay located (in my car) behind the batery close to the OVP relay. Now that I remember this was not functioning properly also. I changed it though I have read here on other threads that people have resoldered with good results. Also, the climate control push button can be resoldered with good results - at least from what I have read on other threads here.
Once again, if you really do not know what you are doing have it checked otherwise you can just be throwing $$$$ til you get it fixed and the labor on this repair is alot and some of the parts can be pricy too.
Insanity: doing the same thing over and over again and expecting different results.
2006 - Suzuki Gran Vitara (2.0 L fully equipped) Like this car so far except for trying to put on the seatbelt.
1988 - 190e - 2.3L - 172K miles (It now belongs to the exwife)
1999 - Chevy Blazer LS Fully Equiped - killed it June 2006
2001 - Honda Civic EX - 68K miles (sold June 2004)
1963 - 220S - Dual Carb 6 cyl. (sold)
1994 - Yamaha WaveRaider (fun to ride)