I am assuming that your car is stationary when it overheats, as the air flow across the radiator when driving is more than sufficient to cool the engine.
The fan will have a thermostatic switch, this should look like a small plug, usually placed near the top of the engine (on an associated cooling system part) from this there will be a couple of wires terminating in a plug. To test the switch, remove the plug, measure the continuity between the 2 pins when the engine is cold, it should be open circuit. as the engine reaches its trigger temperature the switch should become close circuit thus making the circuit to switch on the fan.
Hope this helps.
lee, January 2009
