air temperature coming through the radiator is sensed by the bi-metal
thermal spring on the front of the thermal fan clutch. It expands and
contracts with the change in air temperature operating a valve inside
of the clutch. When cold, the silicone drive fluid is pumped from the
working area to the reservoir. When hot, the valve opens allowing fluid
from the reservoir to be transferred to the working area thereby
increasing the fan speed. The clutch disengages as the air temperature
decreases, closing the valve and allowing the silicone fluid to be
pumped back into the reservoir.
A thermal fan clutch is engaged on a cold startup
because the fluid drains into the working area when the engine is shut
off. The fan clutch will slow down shortly after startup as a result of
a pumping action produced by a difference in speed between the shaft
and the body of the clutch.
Most models are
designed to duplicate original equipment performance. Some
Chevrolet/GMC truck models are specifically designed to engage at lower
temperatures than the original equipment parts that they replace.