Thread: Viscous fan

What do you call hot? Fans don't couple up (apart from cold starting) until the air coming through the core is hotter than about 90C. It has to operate the coiled bimetallic spring mechanism on the front of the fan clutch.

Having said that, I replaced a viscous recently showing exactly the response you had as the engine wasn't being kept below danger point. It didn't work when properly hot.

Hi BeeUtey
The hot I am referring to is at idle after a 15min+ run at operating temp. From what you are saying it shouldnt activate(couple) until normal operating temperature is exceeded - which isnt the case for me as I have had no indication of any over heating. But then how are you meant to check if these things are working properly then? Surely you dont wait for overheating symptoms.

They are meant to save fuel, so they don't actuate until they are needed, i.e. lots of heat coming off the radiator. To test a fan you leave the engine idling until the rad is too hot to touch at the inlet and outlet then the fan should start to couple up. But unless it is overheating it is probably doing its thing. Next time the engine temp rises over normal listen for the fan, especially if you rev it after a period of idling.

W they have been still for a whileeg overnight the viscous fluid falls to the bottom and they are stiff and roar for about 500metres.

When the engine warms up the fluid is disbursed and they need very little to turn.
when the temp of the air through the radiator gets beyond a certain point say 95C, the bimettalic strip on the front expands /turns and orifices inside the hub are closed to stiffen the fan.
IMHO, the best test of a VC is to closely examine the front and rear seal and see if fluid has escaped and left gunge, if not they are probably OK, but bets are off if the thing has been untouched for 20 years and the engine bay cleaned.
Regards Philip A