I have a question for some of you engineers. I get the mechanics of a fluid/viscous coupling, like a torq converter, but how does temp make the fan clutch lock up? Is heat conducted through the shaft? if so how does this lock it? I've wondered about this and would like to know.
Jim

 

i think the heat makes a spring expands and then pushes the clutch together and it is locked.

 

Here this pretty much explains how are fan clutches work.
Fan Clutches

 

Interesting - I learned something there - Did'nt know how they worked either. Anybody ever had to replace theirs ?

 

Thats really cool, so it has a spring that acts like the water pump thermostat. The only thing I don't like, is that it only lasts about as long as the water pump. Hmmm, have to keep an eye on it.
Thanks for the replys guys, Now I know.

 

It's actually wound like a clockspring from what I remember. It expands and engages a cog or pin which locks the fan blade hub to the fan pulley for direct drive so the fan blades match the pulley rpm and give that distinctive whoosh-roar sound.

 

So in the end its a mechanical lock up. The viscous part is to make the fan spin at a lesser rate than the pulley to save power and fuel. If I ever need a new one, I'll have to take apart the old one to see it.