The fan behind the radiator (between rad & engine) is activated by a viscous clutch, as Arthur said. With the engine off, you should be able to spin the blades by hand but notice some resistance, like it's covered with syrup. when the engine runs at lower temperatures this resistance will spin the fan with some slippage. Sort of a low speed mode. When the fluid inside the clutch heats up, it expands and engages a mechanical clutch "hard". No slippage. Sort of a high-speed mode. but that fan should be spinning any time the engine is running.

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Norm in NJ
Next oil change at 230,000miles