Ok, first: Stop taking your car to the dealer. Find a small independent shop known for expertise with MB products; taking a Mercedes to a Chrysler dealer is not productive if the dealer has no "Crossfire Tech".

Second: "USA" is not a location, if you change your profile to tell us where you are, we may have a shop for you to go to where you can get your car fixed.
For example: I live near Atlanta - if you are within 3 hours of ATL, I'd tell you to simply make an appointment with Buckhead Imports.

The radiator fan is SIMPLE:

The PTCM communicates it's desire for the fan to run over a single wire, bidirectional, Pulse Width Modulated data stream. When the PTCM wants the fan to run, it "tells" the Fan Control Module to speed it up. There is a two-way pulse sent back and forth, the PTCM 'pings' the FCM and the FCM pings back, indicating all is well.
If the communications break down between the FCM and the PTCM, two things happen:

1) The PTCM alerts you to a possible cooling system failure by turning on a small yellow indicator to the side of the instrument panel, it looks like a little radiator.
2) The FCM goes into "emergency mode" and spins the fan at high speed.

If you do NOT have the little light, my guess is that the FCM is bad. The modules run about $140 if I remember right.
If you DO have the light, you have a bad connection between the two modules or a broken wire.

The PTCM COULD be bad, but this would be a "first" that I've heard of in over four years on this forum. FCMs go bad kinda often.


But it IS that simple.