A conventional thermal fan clutch is filled with a viscous silicone fluid that is characterized by high shear strength. The fan clutch has two internal chambers: 1) an open fluid reservoir and 2) a fluid coupling possessing densely spaced, interlocking, concentric rings. Half of the rings are fixed to the drive hub that attaches to the water pump, the remaining rings are fixed to the driven support for the fan blade. The two fluid chambers of the clutch are separated by a bimetal thermostat controlled valve. The bimetal and valve together manage fluid distribution within the clutch. The greater the volume of viscous fluid within the fluid coupling, the stiffer the clutch lock-up to the limit of the clutch capacity. Hence, it is actually the presence and distribution of viscous fluid between the interlocking rings that couples the drive hub to the fan.
Now, when an engine is off for many hours, the viscous silicone fluid within the fan clutch redistributes by gravity to partially fill the fluid coupling. As a result, the fan clutch locks up and the fan becomes difficult to turn. However, once the cold engine starts and continues to run, the silicone fluid in the fluid coupling gradually circulates back into the reservoir and the clutch disengages the fan. The clutch won't reengage the fan until the bimetal senses sufficient heat to open the valve, allowing the fluid coupling to recharge. Consequently, it is normal for a viscous fan clutch to temporarily engage the fan while the engine is cold. Heavy-duty fan clutch options exaggerate the phenomenon.
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