What indicates a properly functioning fan clutch?

A properly functioning fan clutch adjusts the fan speed according to engine temperature, which is crucial for efficient cooling throughout the operating conditions of the engine. When the engine is cold, the fan clutch typically allows the fan to spin at a lower speed or disengage entirely to reduce drag on the engine and improve fuel efficiency. As the engine temperature increases—indicating a greater need for cooling—the fan clutch engages and allows the fan to spin more quickly to draw more air through the radiator. This ability to modulate fan speed based on temperature ensures that the engine maintains optimal operating conditions and helps prevent overheating.

The other concepts presented do not align with the expected operation of a fan clutch. For instance, if the fan speed varied solely with engine speed, it wouldn't account for the engine's cooling needs based on temperature, which is not ideal. A fan that continues turning rapidly after engine shutdown indicates a malfunction, as it should ideally slow down quickly when the engine stops to avoid unnecessary wear and noise. Viscous drag being present even when cold suggests a malfunctioning clutch that doesn't disengage properly, leading to unnecessary resistance and inefficient operation.