A relay, in the most technical terms, is an electro magnet that acts as a switch. As the video shows, when power and ground are applied to the relay, the magnet becomes active, and the switch is completed. The infamous “clicking” one hears when a relay is activated, is the switch being magnetically drawn over to complete the circuit (please see video).
Ok, so a relay completes a circuit when power and ground are applied to it. How can we test if a relay is good in order to diagnose an electrical problem?
Most relays have a diagram of the relay right on the side of them, such as this fine relay here ….