Main Cooling Fan
The radiator is equipped with a main cooling fan that is either electrically powered or engine driven via a belt, the water pump pulley, and a viscous clutch.
The viscous clutch reduces fan speed when cooling requirements are low. The clutch contains a small fluid coupling partially filled with a special silicone oil. When engine cooling requirements are high (during high-temperature operation), more oil is injected into the fluid coupling. This causes more power to pass through the coupling, so that fan speed increases. When cooling requirements are low (for example, during cool-weather operation), oil is withdrawn from the fluid coupling. Less power passes through and fan speed drops off.
Even when engine cooling requirements are high, there is still a small amount of slip (The coupling does not lock up). This allows the oil to continue to circulate through the fluid reservoir and the coupling.
The amount of oil in the fluid coupling, and the resulting fan speed, are controlled by a thermostatic strip. The strip bows outward with increasing under-the-hood temperature. This motion allows a control piston to move outward. As the piston moves outward, more oil is forced into the coupling, causing the fan speed to increase.