Jeff's DIY

Advice on doing your own diagnosis and repair

Advice from the book Can I Do It Myself?

Horn Not Working or Horn Always On

Using the relay socket to diagnose circuit issues. (see Auto Electrical Basics for more on relays)

First find the horn relay. Your owner's manual should show where it is. If you don't have a manual, you can usually find one online. Your relay may have 5 terminals, but only 4 are used in typical operation. The four are shown in the figure below. Two are used to energize the relay coil that closes the connection between the other two terminals. These two are usually on opposite corners, so that the relay can be rotated without consequence. (The relay is designed to close regardless of the polarity of the power applied to the coil terminals.)
Circuits may be designed in various ways to energize the relay coil and close the main circuit.
In the case of the horn:

Here is a figure of a typical horn circuit:

Typical horn circuit wiring diagram

To diagnose the circuit using the horn relay socket, pull out the relay and use a 12 volt test light or voltmeter to probe the socket terminals. A DC voltmeter is best, but a 12 volt test light will do.

  1. First, touch one meter or test light lead to chassis ground, such as a grounding lug or unpainted surface of the frame and probe the relay socket with the other lead. Two of the terminals should show battery voltage or light the test light brightly. If not, check the fuse and/or wiring.

  2. Next, touch one lead to battery positive and probe the relay socket again with the other lead. This time, there should be only one terminal that shows partial battery voltage or lights the test light dimly. This is the horn or "load" terminal of the relay socket. If there is another terminal that lights the test light brightly or shows full battery voltage, your horn button is stuck on or the wire from the button is shorted to chassis ground. This is the horn button terminal. If there is no terminal that shows partial voltage or lights the test light dimly, there is a problem downstream of the relay--such as a broken horn wire, horn ground wire, or horn. For the final test at the relay socket, have someone press the horn button while you continue to probe the socket. You should now see full battery voltage on one and only one terminal. This is the horn button terminal with a properly operating button and wire.

  3. If there is no partial voltage terminal in step 2, replace the relay, find the horn(s), and remove the connector(s). The horn is usually near the front grille but may be elsewhere. Probe the connector with your voltmeter while someone pushes the horn button. This should close the relay and supply voltage to the horn connector. You should hear the relay click. If the relay does not click, swap in another relay for test purpose and try again. If the swapped relay clicks, replace the old relay. If neither relay clicks, go back to the previous step, as the correct voltages are not being supplied to the relay socket. Now, assuming the relay clicks, you should get full battery voltage at the horn connector. If not, either the positive wire from the relay is bad or the horn ground wire is bad or disconnected from chassis ground. To figure out which one is the problem, connect your meter or test lead to battery positive and probe the horn connecter while someone presses the horn button. If one terminal shows voltage, the ground is good. Move your first lead to chassis ground and probe the connector again with the other lead while someone presses the horn button. If neither terminal shows voltage, the wire from the relay is bad. If you are getting batter voltage to the horn connector when the horn button is pressed but the horn was not working, replace the horn.

Horn repairs:

If the horn button signal is not getting to the relay socket or is constantly at the socket, even when the button is not pressed, you will probably have to remove the steering wheel to affect repairs. This process is different for every car, so I recommend you find the procedure in a manual for your car either online or at a library. It is important to follow the manufacturer's procedure, because most cars have an airbag in the steering wheel, which is quite dangerous. Having the wiring diagram is also helpful in identifying which wires are which. With the wheel off, check the button. If you can, test it for continuity with an ohmmeter. With the button pressed, the terminal should show continuity. When not pressed, there should be infinite resistance. If not, replace the button. Of the two wires that connect to the button, one should be grounded. Test both wires for continuity to chassis ground. If neither wire is connected to ground, you have a broken wire or ground termination. One of the wires should also connect to the button terminal in the relay socket. Check the continuity of both wire to the relay button terminal. If neither wire has continuity there, you have a bad horn button wire. To repair a wire, buy wire of the same gauge as the existing harness wire and splice new wire in using a soldering iron. Cover the splice with electrical tape and route the wire along the harness using cable ties.

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