It's this dew-hickey:



Mounted on the driver's side fender in plain view. Either the PTCM/ECM is telling it to run fast because a temperature sensor is indicating a HOT engine, or this module is going bad.

IF you have a little yellow radiator-looking light glowing on the left end of the instrument panel, that is an indication that the PTCM and Fan Module are no longer talking to one another. In those cases, the PTCM lights the light and the FCM runs the fan in emergency mode: 100% speed.

If the light is NOT on, the PTCM and FCM are talking, but for some reason, the FCM has gone bad.

Your most effective move is to replace the FCM. One easy snap-on connector and one metric nut holds it on. ANYONE can change it in 5 minutes. Search "MERCEDES SLK320 Fan control Module" and order one.