Only blowing cold air through heater

Hello my name is Dave.

If you are sure that the blend door is working properly there are a few other likely suspects, one of the most commonly overlooked culprits is the cabin air filter, if you aren't getting a lot of air blowing from the vents this is most likely the cause. Another thing I have encountered with your car is purging the system as they can be extremely stubborn about releasing trapped air in the system which could cause the issue you are having. The other thing I would make sure is that the control valve is functioning properly and if it is I would check for a clogged heater core by turning the heat on after the car has reached operating temperature and feeling the how hot each of the hoses are going to he the heater core. If one hose feels a lot cooler to the touch and your control valve is open then the core is clogged. If the hoses feel relatively close to the same temperature then you issue is most likely related to the blend door inside the duct work, provided that the air is blowing out strongly as I mentioned referring to the cabin filter. Please keep us up to date on your progress and thank you for using 2CarPros!

It is possible for there to be an issue with the vacuum system when it comes to any of HVAC controls, this is why I asked if you were sure the blend door was moving. Unfortunately that is quite an elaborate system and the first suspect has been eliminated if at least one door is functioning and that is the vacuum supply to the system. I have attached a diagram of the vacuum system for the HVAC in your car, and if you can get to the switch over valve block then you can test for vacuum to the blend door when you switch it from hot to cold. From the diagram it looks like it shouldn't be that difficult to get to. The other thing you could do is supply vacuum to the hose and see if you can tell if the door is moving. What I would do if I didn't have a vacuum pump is get a piece of hose, attach it to the hose going to the blend door and suck on it and see if it moves, I would also put my tongue over the hose to see if the actuator hold vacuum. If alternately if you have a vacuum pump or can get one (around $25.00 at Harbor Freight) and use that o supply the vacuum and see if the gauge stays steady after you supply vacuum. If the actuator doesn't hold vacuum then it will need to be replaced. The other issue that concerns me more is that you are having what sounds like inconsistent temperatures in your cooling system. If the coolant going to the core isn't hot you won't get heat. That being said I again lean toward trapped air in the cooling system which could definitely cause the temperature fluctuation you are describing. I have quite a bit of experience with Mercedes and they can be extremely stubborn about releasing trapped air. The best way to purge the system is to use a anti spill coolant funnel. I have seen them on Amazon for around $20.00.

https://www.amazon.com/OEMTOOLS-87009-No-Spill-Coolant-Filling/dp/B01A2CQSU6/ref=sr_1_5?keywords=coolant+funnel&qid=1573560466&sr=8-5

Starting with a cold engine attach the funnel and fill it approximately 3/4 full with coolant and start the car with the heat on high and let it run until it reaches operating temperature, maintaining the coolant level. Once it reaches operating temperature allow it to cool completely again maintaining the coolant level in the funnel. On these cars I usually go through these steps 3 times as I have thought I had all the air out of the system and then they suddenly start having symptoms like you are describing. I feel that will get to the heart of your problem. Please keep us updated on your progress.