Many horns installed in vehicles may be adjusted when there is not enough current going through them and their sound is weak. Unfortunately, horns cannot be repaired once they fail. There are many things that can go wrong, preventing a car horn from doing its job. You can examine and troubleshoot a typical horn through a series of simple steps that may serve as a general guide for most circuits. You only need a basic knowledge of electricity or the desire to learn basic troubleshooting procedures.
Check the circuit's fuse if the horn is not working at all. If the fuse is blown, replace it and test the horn again. If the fuse is OK, go to the next step.
Open the bonnet of your vehicle, and ask an assistant to press the horn button on the steering wheel while you listen for a possible weak sound coming from the horn. Sometimes, the sound is so weak you won’t be able to hear it. Touch the horn with your hand, and try to feel a vibration while the horn is activated. If you hear sound, go to the next step. If not, go to Step 4.
Locate the adjustment screw on the horn and adjust it using a standard or Phillips screwdriver. If the horn still does not work, go to the next step.
Connect a jumper wire to a good ground on the vehicle, and ask an assistant to press the horn button while you bring the other end of the jumper wire in contact with the horn. If the horn works, fix its ground connection, making sure the horn makes good contact with the vehicle’s chassis.
Detach the horn from the vehicle, and connect it directly to battery power using jumper wires. If the horn fails to work, replace it. If it does work there, go to the next step.
Reinstall the horn to its circuit, and check for voltage at the horn with a voltmeter, connecting the red probe to the horn’s terminal and the black one to the horn’s body. Ask an assistant to depress the horn button at the steering wheel. If the horn is receiving voltage, replace the horn. If there is no voltage, go to the next step.
Check for continuity at the wire running from the horn to its relay. If there is no continuity, the wire has an open. Fix it, and test again. If there is continuity, go to the next step.
Check the horn relay, and make sure it is working properly. Test for voltage at the relay’s power-and-control circuit with a voltmeter while an assistant operates the horn button at the steering wheel. If the relay is not working properly, replace it and test it again. If there is no voltage reaching the relay, go to the next step.
Inspect the wire going from the horn relay to the fuse panel. If you find an open or short, fix it and test again. If the wiring is OK, go to the next step.
Ask an assistant to press the horn button at the steering wheel while you check for continuity at the wire running from the relay to the horn button and ground. If there is an open in the wire, fix it and test it again. If you don’t find the open in the wiring, replace the horn button.
Keep the horn circuit diagram for your particular vehicle on hand whenever you are troubleshooting its circuit. You can purchase a vehicle service manual at most auto parts stores or consult one for free at your public library.