I have a 2008 E350 Sport. The A/C started to not work when idling. And, got progressively worse. A shop, to which I drove an extra distance because they are “a/c specialists” told me it needed a compressor. I was unable to use that shop due to distance and mounting issues without being able to use the car but, ordered a compressor and took it into my regular shop who swapped out the compressor. This shop is an independent but a good one with the correct test tools. The compressor would not cool the car. He found a TSB and scoped the system and found there is a hose that regularly fails. It was failed. We put one in. Still No cooling. The control unit and all sensors seem to be functioning within tolerance. So assumption made the compressor must have an issue. Bought a different open at a local shop. Put it in. No cooling. Computer says the control unit is asking for 70%. There is voltage at that wire to the variable valve on the compressor. So the tech, who is a past Mercedes tech, wants to get a Mercedes compressor from Mercedes. While I am mechanically inclined enough to have a suspicion and opinion that its not the compressor, I have no evidence to say he’s wrong. I bought a icarsoft MB reader overnight and checked the system myself. No faults from the control unit. And it is asking for 70%. I now have 2 compressors to return and a non functioning A/C. The tech is going to start sourcing compressors from the dealer in the am. So if anyone has info on this it would help a lot. We suspect the original compressor was not faulty. But it is now long gone.

 

I haven't looked to see yet, but if your car has an expansion valve, and it's stuck open, it will not cool. So, what's your high side and low side pressure readings? Sometimes if you have excess oil in the AC system, it'll act like you have correct pressures, but it won't cool, and you have to pull the system apart and flush the individual components.
Mercedes AC systems aren't magic. They operate under the same principles as all the other systems.

 

What brand compressor did you have them install? There's a lot of crap Chinese compressors out there for very cheap.