How is the radiator fan speed controlled on the BPU?

The radiator fan speed on the BPU is controlled by a viscous clutch. This component utilizes a silicone fluid to manage the engagement and disengagement of the fan with the engine's pulley system based on temperature. When the engine temperature rises, the viscous clutch responds by allowing the fan to spin faster, thereby increasing airflow through the radiator to promote cooling. Conversely, when the engine cools down, the fluid in the clutch reduces the fan speed, allowing it to disengage or operate at lower speeds. This system ensures efficient engine cooling while also minimizing unnecessary energy consumption and noise when the cooling demand is low.

The other options are not suitable for this specific function. An electric motor typically drives electric fans that can be controlled via a different mechanism, while a hydraulic system is generally used for power steering or similar applications, and manual adjustments would not provide the automatic response needed for varying engine temperatures.