It can be easy to overlook the simple circuits on a vintage Honda. The horn circuit may be unassuming, but a little detail goes a long way in better understanding every last inch of your motorcycle. When it comes to the 12v positive side of the circuit, things are pretty standard but we find a lot of people are stumped with how the horn connects to ground when pressing the horn button. In this video, we are going to help you understand the basics of the factory Honda horn circuit wiring and how to individually test each component to discover what is at fault and what works as it should.

The Simplified Honda Horn Circuit Diagram

Current always begins with negative so it is important to start at ground when tracing any circuit. If you have watched our ground circuit explained video, you will know that most metal on these motorcycles is ground. In this case specifically, ground starts with the metal frame and travels up to the handlebars via a small grounding pigtail that connects to the bottom of the handlebar riser. From the handlebars, ground travels to the left-hand switch box to make contact through the switch box body to the horn button. The horn button is soldered to a light green wire that travels through the handlebars down to the light green connection on the horn. Current leaves the horn on the 12v positive black wire connection which as we know, goes back to the key switch and then the positive side of the battery.

Important: Ensure you have read our positive and ground explanation articles and watched the videos before diving into this troubleshooting. You will need a known good battery and test light to troubleshoot your ignition system wiring. 

Understand the Positive Circuit on a Vintage Honda Motorcycle

Understanding the Ground Circuit on a Vintage Honda Motorcycle

Warning: Only briefly turn the ignition switch on with the kill switch set to the OFF position to test it. When turned on with the kill switch set to on / run they are active and charging. If left on too long the coils will overcharge and be ruined. 

Testing the horn

1. The horn is located at the front of your motorcycle, usually under the frame rails and tank.
2. Unplug the light green and black wires going to the horn and connect the black wire to a jumper wire.
3. Connect the other end of the jumper wire to the positive post on the battery. Be careful to not allow the jumper wire to touch any ground point on the motorcycle.
4. Touch the light green wire to the engine block or another ground connection to test it.
5. The horn should hon loudly until you stop touching the green wire to ground.

Potential issues:

Horn not honking/honking quietly: The horn can have bad internal contacts and need to be replaced. Sometimes a horn has a small screw on the rear to make adjustments to the volume but it is more likely that it just needs to be replaced. 

Testing 12v positive power to the horn

1. Test your test light by connecting one side to ground and touching the probe to the positive terminal of the battery.
2. Connect the test light alligator clip to the engine fins for ground.
3. Turn on the ignition switch.
4. Use the probe to touch the 12v positive black wire that was plugged into the horn.
5. The test light will illuminate if you are getting the proper 12v positive power at this connection. 

Potential issues:

Test light not turning on: Check the understanding positive connection video linked above as you likely have an issue with the 12v positive circuit on the motorcycle. The bullet connector could also be full of corrosion so try poking the probe into the wire itself if the test light does not turn on.  

Testing ground connection to the handlebars

1. Test your test light by connecting one side to ground and touching the probe to the positive terminal of the battery.
2. Connect the test light alligator clip to the positive post on the battery.
3. Touch the probe to the handlebars.
4. The test light will illuminate and indicate the handlebars are grounding properly.

Potential issues:

Test light not turning on: The most common issue is that you might be missing the small jumper wire that transfers ground from the bottom of the handlebar riser to the frame for ground. The handlebars are rubber mounted in the risers so this wire is crucial for transferring ground connection for both the horn and starter button. 

If this is in place, the other issue is you could have painted or powder-coated handlebars. The metal switch boxes must make contact with either bare metal or chrome underneath them to be able to transfer the ground connection properly. Either sand a bare metal ring in the handlebar paint underneath the switch box or run a ground wire from the frame, up the handlebars, to a bolt inside the handlebar switch boxes. This can be done nicely if you run the wire in with the existing bundle of wires going to the switch. 

Testing the horn button

1. Test your test light by connecting one side to ground and touching the probe to the positive terminal of the battery.
2. Unplug the light green and black 12v positive wires from the horn. 
3. Connect the alligator clip on the test light to one of the wires and touch the probe to the other.
4. Turn on the ignition switch.
5. Push the horn button.
6. The test light should illuminate when the horn button is pressed down and turn off when you let go. 

Potential issues:

Test light does not turn on:  The light green wire could be broken somewhere in its run to the handlebar switch. The momentary contact for the horn button in the switch or spring could have corrosion or be missing. The wire solder joint could have broken off of the horn button contact inside the switch.

Parts to Buy: 

Test Light

Replacement Horn

Horn Button and Spring

CB350 / CB450 Left Handlebar Switch

CB360 / CB550 / CB750 Left Handlebar Switch