The purpose of the 12 volt relay (in an RV refrigerator) is to give smaller gauge wires and switches the ability to control high amperage 12 volt heat elements. Pictured above is a common relay. Posts 85 and 86 represent a small electromagnet circuit that, when activated, snaps the switch between posts 30/51 and 87 together.

A large, positive 12 volt wire (8 or 10 gauge) is connected to the 30 or 51 post, bypassing other controls. The positive 12 volt heat element wire is connected to the 87 post. Post 85 is grounded to the refrigerator. When a remote switch supplies 12 volt to post 86, the electromagnet is energized and the switch between 30/51 and 87 is snapped shut. This allows power to flow to the heat element. When the power to post 86 is stopped, the switch snaps open, breaking the circuit to the 12 volt element.

Testing the relay is best done with a 12 volt test light or meter. Simply be sure that 12 volt is present at 30/51 and that when 12 volt is applied to post 86, post 87 has 12 volt present. Also be sure that when 12 volt is removed from post 86, post 87 has no power.