Fan Clutch: Description and Operation

Fig. 2 Variable-speed Fan With Coiled Bi-metal Thermostatic Spring:

Fan drive clutch is a fluid coupling containing silicone oil. Fan speed
is regulated by torque-carrying capacity of silicone oil. Fan speed increases with a rise in temperature and decreases as temperature goes down. The more silicone oil in the coupling, the greater the fan speed. The less silicone oil, the slower the fan speed. A heat-sensitive, bi-metal spring connected to an opening plate regulates the amount of silicone oil entering the coupling.