Just having issues with my viscous fan on 560sel. I put a new one in 3 years ago and only have 7000 miles on it and it is gone again. From experience this is how the fan SHOULD work:
CAR COLD- on start up the fan will appear to be locked up and howl for a few seconds when you rev it - then it slows down and spins at idle speed even if you rev the engine again and again.
ENGINE HOT - when your temp. guage shows about 85C or higher - the fan's bi-metallic strip will bend from the heat and allow the clutch to engage so the fan will howl when you rev the engine. That is normal.
If the fan howls all the time when you rev the engine -hot ot cold - it is defective. If the fan does not howl when you rev the engine when hot (my case) it is defective. My new viscous clutch arrives tomorrow so I can play with the old one to find out why it failed. Mine was working great until one hot day when I noticed the engine temps climbing to 110C on the highway. Revving the engine in neutral caused the temp to climb to 115C and the fan would not lock up - I took a gloved hand and even turned it in the opposite direction at idle. Normally - if the fluid leaks out - this is the result. However if the fluid had leaked out of mine - it would not spin and howl for a few seconds at cold start up - which it does - then slows down and never speeds up again. I suspect a bi-metal problem but don't want to play with it until I have a new one installed. I will post my findings if the old one can be fixed.