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The fan clutch is a small fluid coupling with a thermostatic device
that controls a variable-speed fan. The fan clutch ensures that the
fan will rotate at just the right speed to keep the engine from
overheating, and reduces drive to the fan when it is no longer
needed.
The fan clutch has a fluid coupling partly filled with silicone oil
designed for just that purpose. If the temperature of the air passing
through the radiator rises, the heat alerts a bimetal coil spring to
"uncoil" or expand. When it expands, it allows just a little more oil
to enter the fluid coupling, so the fluid coupling starts to rotate
the fan. If the air coming through the radiator is cool, the opposite
happens; the coil spring contracts, the oil leaves the fluid coupling
and the fan slows. Slowing the fan when it is not needed reduces
fuel consumption, makes less noise and saves engine power.
Sometimes a flat bimetal strip spring is used instead of a coil
spring; it bows out and in when the temperature rises and drops,
letting oil in and out of the fluid coupling.
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