viscous fan question

[mhb]

does anyone know how the viscous fan works, ie does it kick in when the car is slow or stopped(ie no air flowing through the rad) or is it heat operated

thanks

 

[C220GJS]

See here for an explanation of it's workings ; http://mb.braingears.com/124_DISC1/Program/Engine/602_603/20-310.pdf .
Hope this helps , Geo.