E46 Radiator Fan Relay Temperature Switch

The lower radiator hose has a temperature sensor for triggering the auxiliary fan, the ECU is controlling the logic for powering the fan at X temperature, in other words it's a temperature sensor(negative temperature coefficient thermistor) and not a temperature switch, like E36's.  However, if you ground one of the sensor pins and wire the other pin to the ground side of a fan relay the resistance inside the temperature sensor drops low enough that it'll have continuity around 155-160* and will trigger the relay to ground.  The other "trigger" side of the fan relay should go to +12V ignition power.

Note: This is for race cars which have removed the factory primary electric fan and auxiliary electric fan and have moved to a single race type fan(SPAL or similar).