I don't know what you have done but I would like to add my experience. I purchased my 82 300D and the air did not work, no compressor. I by passed the pressure limiter switch to see if the system was low on coolant, sure enough it was. Very little pressure is required for this switch to function so I knew my system was flat. I charged the system and found the components to function, one line hot one cool, yet cabin temp air was still very warm to hot. I then removed the opperating portion of the mono valve to verify it opened and closed on command which it did but the rubber diaphram was shot so I ordered a new one. When new valve body was installed the cabin air was very cool. In subsequant reading I found out Mercedes uses heat to manage temperature in cooling mode and the refridgerant to manage temperature in heating mode. These cars do not use external air to "temper" the system. Once I replaced the valve the cabin air was cool. Perhaps this will help you, I hope it does.

Rick