420SEL viscous fan runs nearly continuously
 

1987-420SEL with 169k miles

I've noticed in the past few weeks that the viscous fan clutch is engaged nearly continuously at freeway speeds with ambient temps above 60F. Between 50-60F the clutch will cycle on/off every couple minutes cruising @ 60mph. Below 50F the clutch stays disengaged. Engine temps are ~82C on cool days, ~85C on warmer days when clutch stays engaged all the time. I've always use MB antifreeze since I got it in May-2005.

Seems like the fan clutch is working right, do I need a new radiator?

If the car is not getting hot, why mess with it?

I want to fix it because the noise bothers me, and the fan belts start slipping above 3500rpm when I get on the freeway. The fan belts are tight.

When these viscous clutches fail, they either lock up completely, or they free-wheel constantly and the car overheats in stop and go traffic. Sounds like yours is on it's way out.

Spin the fan by hand and see if if there is a slight resistance. Check the coolant level as well. You can always add an electric fan with a temp sensor that pushes into the radiator.

I replaced the fan clutch early last summer cuz the old one had locked up. The clutch engages and disengages, just depends on what the outside temperature is. When it's 60F or hotter outside it is engaged continuously. I think I'm gonna drain the coolant and pull the upper hose and take a peek inside the radiator.