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 ….
The numbers represent the terminals that come out of the bottom of the relay, and the wires that attach to them. If you look at the bottom of the relay, the pins will also be numbered. 85 and 86 are the power and ground source that activate the relay. 30 is the input from the circuit the relay completes, and 87 is the output from the relay once the circuit is complete. Simply add power and ground to terminals 85 an 86 and then check to see that continuity between 87 and 30 now exists.
The diagrams almost always look exactly like this. It is easy to remember what is what, because the diagram makes sense. The small rectangular box to the far left is a battery, with power and ground supplying another slightly larger box which has a line across it, the electromagnet. When the electromagnet has power and ground, it generates a dashed line, the magnetic field. The magnetic field reaches over to the line going out of 30, the actual switch, and pulls it over to 87 to complete the circuit. It’s abstract, but simple and effective.
Now a simple test light and a few jumper wires will tell you a vast amount about the electrical circuit you are trying to diagnose. You can check for ground at the relay, and look for the switched power that activates the relay (usually it’s switched power, can be constant power with a switched ground, but that is are). You can look for the constant power coming into the circuit, pin 30 (can be constant ground, but is rare, just check for ground if there is any doubt) You can activate the relay to ensure it works, or You can also jump the wire that feeds 30, to the wire that comes out of 87 and see if the desired electrical component works (thus eliminating the relay from the circuit). You can also use the relay as a test site for current ramp, and voltage drop testing. It’s a one stop shop of electrical diagnostic wonders.