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The power is supplied by a transformer, and when the thermostat makes contact between the 24 volt power and one or two of the other wires, a relay back at the heating/cooling unit activates the corresponding heat/fan/cool function of the unit(s).
The thermostat (or thermal switch) is simply the control device. The thermostat in your cooling system controls water flow through the rad depending on engine temperature. The fan thermostat controls the fan, turning it on when needed and off when not.he fan shroud is very important. It's job is to force the fan to pull air through the entire radiator. Without it, the fan will only pull air through the area directly in front of it's blades, as well as the side between the blades of the fan and the radiator. All this does is churn hot engine bay air.