Originally Posted by
Bush65
Test the viscous coupling with a paint brush, not something like your hand, which will hurt (worse than that) or something less forgiving, which will damage the fan.
At temperature below the opening of the thermatic valve, or if the coupling is faulty, you will be able to stop the fan by holding the bristle of the paint brush against the spinning blade.
With a viscous coupling in good order, when the thermatic valve opens and the silicon has effectively "locked" the coupling the brush can't prevent the blade spinning.
Some viscous couplings can be repaired by re-filling with silicon.