How is the radiator fan controlled on a BPU?

The radiator fan on a BPU (Basic Power Unit) is controlled by a viscous clutch. This type of system uses a fluid-filled chamber to engage and disengage the fan based on temperature. When the engine or coolant temperature rises, the viscous fluid inside the clutch thickens, creating a grip that connects the fan to the engine. As a result, the fan spins faster and helps to cool the engine by drawing air through the radiator.

This method of control is advantageous because it allows the fan to operate more efficiently and reduces noise when cooling demand is low, as the fan can disengage or spin at a lower speed. This design allows for automated control based on temperature rather than relying on a manual control mechanism or an electric motor, which can introduce complexities or may not respond as effectively under varying conditions.

Additionally, while other options like an electric motor or manual lever could theoretically operate a fan, these methods typically do not provide the same automatic temperature-responsive functionality that the viscous clutch system does in practice.