My a/c on my 2004 E320 works fine as long as I am driving (moving) and not idling. My compressor appears to be working. The auxiliary fan behind compressor appears to be turning and the main interior fan is blowing. I had freon added, but unless I am driving or moving, no cold air. any idea what I might check or is compressor simply not performing as it should?

You have air flow?? but not cold.

The compressor design is variable & is controlled to allow MORE refrigerant at lower speeds. Sounds like the signal to the compressor from the controller is incorrect.

Possibly a fault in one of the many sensors, sounds like a trip to dealer.

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The aux fan "appears" or actually really is running?? Important difference right?

CMIIW, but if you're driving with cold air and you pull over immediately, the aux fan should be running low speed. Later, if you're not moving and the air is now outside air temp, also look to see if the aux fan is running- it should be.

Or, jump the aux fan to check that it is functional, to rule out if it is the or part of the problem.

There are indie shops that can work on it as well.