The relay in your car is simply a remote control switch, which operates through magnets. From a distance, this component controls various electrical circuits that are used for operating different accessories such as car lights, wipers, fuel pump, air-conditioner, and radiator fan. Unfortunately, this electrical component uses mechanical contacts that may eventually wear out or burn. When it comes to this, the current to the circuits will be blocked. When one of the accessories (horn, headlights, heater, or blower) suddenly stops working, you may run a few tests to check.
Ways to check if the relay is bad and already needs replacement
Step 1: Locate the specific switch that has to be checked or tested. This depends on the circuit it controls. It may be found under the dashboard or situated inside the engine compartment, somewhere in the junction block. You may also look into the fuse panel under the hood or inside the car. Use the owner's manual as a guide to find the right part to check.
Step 2: Switch on the ignition so that power will run to the circuit that you want to test.
Step 3: Attach the alligator clip from a test light to a ground on the vehicle. The wire coming out of the relay and going to the component should then be probed with the test light's tip. When the bulb glows, this means that there's voltage and that the electrical component you're testing is working fine.
Step 4: Use a test light to check if there's incoming voltage from the wire/s to the switch. (See step 3 for this procedure.) If the light didn't flash, then checking the voltage source is the next step.
Step 5: Turn off the ignition and disconnect the switch from the electrical connector. As you unplug this electrical part, be careful not to destroy or tear the locking tabs.
Step 6: Find the power and control terminals of the specific automotive relay you're checking. Look for a circuit diagram that you can use to locate the terminal. Some relays have this on top of the case.
Step 7: See if there's continuity between the power terminals. Use an ohmmeter to test this. If reading from the ohmmeter shows that there's continuity, then the switch must be replaced at once.
Step 8: Make a jumper wire connection between one of the control circuit terminals and the battery positive terminal. The other control terminal should then be connected to another jumper wire for grounding. You'll have to reverse the connections if ever you don't notice or hear a click when you make the second connection. If there's still a click, then the switch must be replaced.
Step 9: Connect the jumper wires between the power terminals. After this, use an ohmmeter to see if there's any continuity. The continuity between these power terminals means that the relay is working fine. If there's no continuity based on your ohmmeter reading, then consider replacing the busted electrical component.