Can anyone describe how the fan operates , yes I know it's belt driven but I'm assuming the viscous coupling cases a bit of slip in the Dan or something similar . The reason I ask is the driven home tonight my engine started to get noisy and it sounded like the fan was running flat out .

fan does lock as heat is passed over it , it could be your fans seized or engines getting too hot

When a viscous fan is cool the fluid doesn't 'drive' the fan, such that when you first turn an engine on, it should be easy to stop the fan with, say, a rolled up newspaper (or something that won't hurt you or it!). As the motor warms up and the viscous fluid also warms up the fluids viscosity changes and it starts to drive the fan, with some 'slippage' of the fan, but it would be much harder to stop. As it gets even warmer at some point it will completely lock and drive the fan directly and will be impossible to stop without damage.

You can test this by first thing in the morning the viscous fan should be relatively easy to turn by hand. There might be a very little drag, but notalot. If it's very stiff to turn and tries to turn the belt/pulleys then it's probably nackered.

Go for a drive and get the engine properly hot, so not just for 5 minutes or so, but at least half an hour, preferably in the sun and stop start, or motorway thrash. Stop the engine and try to turn the fan now and it should probably turn the belt first! If the fan turns the same as it does when cold it's probably nackered.

Hmm ok thanks,

So it's always locked the fan initially when I start from cold then after a few seconds it quietens down , however it seems to be fully locked which is making it very noisy at high revs . I'll redo that check in the morning but I'm guessing it's probably seized so I guess the next question is replace or buy electric ones ?

replace with another viscous, it should only feel locked when hot

If it is locked , is there risk of causing more damage?

not unless it vibrates, but wants changing soon

Cool thanks for advice