I recently experienced the same issue with the radiator fan for my 2006 C230 as it was going full speed when the key was in the ignition alongside the engine being on as well. This occurred after seeing I had stretched my serpentine belt while driving and caused the alternator, power steering pump and all the other components the belt is in charge of from functioning in the moments of the drive. I replaced the belt within the the same hour of the old belt tearing and when I turned on the engine to see the belt run , I also heard the jet noise of a sound the radiator fan can make ! I simply unplugged the fan and drove the car in two different intervals of the day for 5 minutes each. The next morning , I plugged the radiator fan back in and everything was back to normal as I figured the computer would know what to do this time !