We placed the coolant temp switch in the side of the head. The coolant temp switch tells the Derale relay when to turn on the fan by closing the circuit in the switch. The Derale controller relay will kick the primary fan on at your programmed temp via a control on the box. The secondary fan will turn on 10 degrees hotter than what you set the primary fan at. This means if you set the primary fan to 190, the secondary will turn on at 200.