The fan assembly 1999 Dodge Durango

It lets the fan free wheel when it isn't needed to save engine power and improve fuel mileage. As heat begins to blow from the radiator onto the built-in thermostatic clutch, the fan will begin to spin faster. The roaring noise will decrease at the same time.

When you stop the engine when it's still cold, the fan will coast freely to a stop and you will be able to turn it freely by hand. When it is warm, it will be harder to turn. If you're brave, you can hold onto the fan blades when a helper starts the cold engine, and as it warms up over a period of a few minutes, you will feel it tug harder and harder. If you want to let go before the engine is stopped, do so quickly. If you dilly-dally around, the blades will bang into your fingers as they pick up speed. When I do this as a demonstration, I always use a piece of string to hold the blades because there is always the chance the clutch could seize and shred fingers.

This only works with belt-driven fans, not electric ones.

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