Based on a bimetal sensor located at the front of the viscous fan the sensor expands or contracts, depending on the air
temperature flowing through the radiator.
At Cold Temperatures the bimetal sensor contracts closing the valve, so the silicone fluid remains in the reservoir chamber.
At this stage, the viscous fan coupling is disengaged and turns at around 20% of the rotation speed of the engine.
At operating Temperatures the bimetal sensor expands, rotating the valve and allowing the silicone fluid to move through
the entire chamber to the outer edges. This generates enough torque to drive the cooling fan blades at engine operating
speeds and remain at a consistent temperature. At this stage, the viscous fan coupling is engaged and turns at around
80% of the rotation speed of the engine.
Source: BG Automotive