Rad Fan Manual Control

Simple circuit to manually control radiator fan operation:

The radiator fan is wired such that its operation is controlled by a temperature sensor which is mounted in the radiator itself. This sensor (which is a thermistor, which means it is a resistor which changes resistance with temperature) constantly monitors the temperature of the coolant passing through the radiator. If the coolant becomes too hot, the radiator fan is activated.

A simple way to check the operation of this circuit is to disconnect the temperature sensor in the radiator while the car is running. You will notice that when you do this the radiator fan runs continuously. By disconnecting the circuit you are essentially creating an infinite resistance situation, which means no feedback to the ECU. With no feedback the ECU assumes the worst-case situation (which is coolant boiling) and runs the fan as a sort of  “limp-mode”.

Therefore, a simple way to manually turn on the radiator fan is to wire in a switch in the coolant temperature sensor circuit. By flipping the switch you effectively disconnect the sensor from the ECU and the fan will turn on. I wanted this functionality as who knows whether the coolant sensor (now 25 years old) is still calibrated properly.