Technical Information K20 Fan Switch
K20A Engine Swap Radiator Fan Switch
The radiator fan is normally activate by a thermo-switch in the bottom of the radiator. The ECU can also activate the fan if necessary.
Do not rely solely on the ECU switching on the radiator fan - this is using the sensor in the cylinder head, which is the wrong way to switch on the radiator fan. The fan should be switched based on the water temperature on the return from the radiator - not the exit from the cylinder head.
For engine swaps without a thermo-switch in the bottom of the radiator you can place the fan switch in the return hose from the radiator to the engine.