centrifugal fan clutch (sp)

There is no centrifugal clutch on the fan. It is temperature sensitive only.

Thank you. That is how I guessed it would be but I am loath to argue until I am sure. On an other BMW I have the fan could be spun easily by hand when the car engine is not running whereas the one I have now seems to drag and is not as free and of course I can hear it spinning when the engine is accelerating. Now I can argue and be sure of my fact thanks. pete

It should spin freely when cold, drag when hot. Like multi-viscoscity oil.

I knew I had the wrong name. It is not a centrifugal clutch at all. It is a viscious coupling. Thanks for jogging my senile brain:-( pete

Most modern cars with viscous couplings use the temperature sensitive sort

- but older or cheaper ones may not.

The clue is to look at the front of the coupling. If it has an obvious spiral spring, it's temperature sensitive.

Ummm... well... no.

See, even though multiviscosity oil has a higher number for the hot viscosity (2nd number), that number is comparing it to a straight weight oil of that viscosity at that (high) temperature. So, even a 0W50 oil would still be considerably lower viscosity (thickness) when hot than when cold.

OTOH, whatever magic stuff they use inside these fan clutches would have to actually increase in thickness as the temperature increases. This would have to be a pretty rare substance to behave that way...

In actuality, there is a thermostatic spring valve that varies the fluid's flow path depending on temperature.

Here's a decent lowdown of it:

Sure. Oil is *relatively* thin at low temperatures, but still shows the effect of its "viscosity modifiers". I thought this was the basis of the fan clutch, something filled entirely with this backwards fluid. But I like the spring idea. Thanks for the link.

Yes that is what I have on my 535i Dave. I am having to post in this way because I have temporary troll trouble. pete

That is an excellent link Malt Hound. Thanks. pete

I still don't think you get the point. Oil (and pretty much every other liquid in the natural world) gets *thinner* with heating. It can't be avoided.

The "viscosity modifiers" that you (and the oil purveyors) mention only makes the oil thin out less with an increase in temp than it would if they were a straight weight oil.

So that 0W50 oil we were talking about, starts out as thin as a 0 weight oil when cold but it still gets thinner when heated. However it doesn't get as thin as it would if it were a straight ) weight. It only gets as thin as a straight weight 50W oil would. But that is still thinner than the 0 weight (or itself) when cold.

Hope that helps clear it up some.