Electric fan thermostatic switch location; radiator or intake?

[my67c20]

Regardless of the debate, it should be in a position that is going to be the highest point that coolant will collect. If the sensor is in the head, you might not be reading the warmest temperature, since heat rises. The intake manifold will guarantee this, unless you are extremely low on coolant, have a very large bubble, or are upside down. Likewise the location on the radiator should also be at the highest point, but not higher than the lowest point on the upper hose inlet.

The water flows down the radiator and cools as it goes, hence the large surface area of the cooling fins. The purpose of the thermostat is to control the temperature of the engine and also acts like a valve to purge out the hot water, and prevent it from returning. If there was a big enough radiator the fan would have no purpose, but since this is not a perfect system the water in the radiator needs to be cooler than the engine for convection to work properly, otherwise when the thermostat opens, the water could flow in either direction. The hot water is not propelled out of the upper hose only because of the flow created by the water pump, but because the hot water is trying to get away from the cooler water from the radiator.

When the car is turned off, the surface area of the radiator helps to cool the water, but the water in the engine continues to heat up, until the thermostat opens up. The natural convection action helps to cycle the water and cool the engine down, another reason to have it in the radiator and the fan to run after the car is turned off.