The taillight and parking light circuits are some of the more unique circuits on a vintage Honda motorcycle as compared to others out there. With wiring running through both the front and rear brake light switch, the ignition switch and the headlight switch it can seem a bit overwhelming. With a good battery, a test light, and a little bit of knowledge you can diagnose and fix any issue in this circuit. In this video, we are going to help you understand the basics of the factory Honda taillight and parking light circuit wiring and how to individually test each component to discover what is at fault and what works as it should.
If you have watched the ground circuit explained video, you will know that a circuit starts at ground. Ground can be a few different spots on these motorcycles, but in this case specifically ground starts with the wiring harness ground that connects directly to the ground wire going to the taillight. Once at the dual filament taillight bulb, this circuit gets interesting and splits into 2 separate circuits, with the running light circuit itself splitting once again.
The Running Light Circuit: Current goes through the taillight bulb on the dimmer of the 2 filaments and exits via the solid brown wire. The solid brown wire travels to the keyswitch where is is connected to the brown wire with white stripe when the key switch is set to the first position (ignition). Once on the brown/white wire, it makes its way into the right handlebar switch to connect to the black 12v positive wire when the headlight is turned on. This can differ slightly on some later motorcycles from 1975 on as Honda switched to an always on headlight for safety regulations. In these models the brown/white wire is joined either directly with the black 12v positive circuit in the headlight bucket and turns on with the keyswitch or is connected to the black 12v positive wire inside the right handlebar switch.
The Parking Light Circuit: Its best to think of this and the running light circuit as the same circuit as the both go through the dimmer filament on the taillight bulb and out on the brown wire. The parking light circuit differs when getting to the ignition switch. The ignition switch actually has a 3rd position on the switch that is used to connect the brown directly to the black 12v positive. This allows only the taillight to be left on when street parking. While uncommon today, this feature existed on most japanese made motorcycles from the 1970s.
The Brake Light Circuit: From ground, current goes through the brighter of the 2 filaments on the taillight bulb and exits on the green wire with yellow stripe. This wire travels to both the front and rear stop switches on the motorcycle. When either the front or rear stop switch is trigger, its then connected to the black 12v positive wire to make its way through the key switch, back to 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 taillight bulb
- Remove the lens of the taillight then push the bulb in towards the socket and rotate it out of its bayonet connector.
- The outside of the metal bulb base is negative and the 2 small contacts on the bottom of the dual filament bulb will be the positive connections.
- Use a large jumper to clip around the outside of the bulb and connect the other end to a ground point on the frame.
- One at a time, touch each of the positive contacts to the positive post on the battery. Be careful to not touch the ground side of the bulb to the positive post.
- Each filament should turn on with one being noticeably brighter than the other.
Potential issues:
One or both filaments not turning on: The bulb needs to be replaced due to a blown filament.
Testing the taillight housing
- Install a known good taillight bulb into the taillight housing.
- Unplug the green/yellow and brown wire from their main harness connections.
- One at a time, touch the green/yellow wire and brown wire to the positive side of the battery. You might need to use a jumper wire to extend the reach.
- Each filament of the bulb should turn on with one being noticeably brighter than the other.
Potential issues:
One or both filaments not turning on: Either a bullet connection is bad, there is a break or corrosion in the wire, or there is corrosion in the bulb socket that needs to be fixed.
Testing taillight ground
- Test your test light by connecting one side to ground and touching the probe to the positive terminal of the battery.
- Unplug the taillight ground wire from the connection to the main harness.
- Connect the test light alligator clip to the positive side of the battery.
- Touch the test light probe to the ground connection that the ground wire was plugged in to.
- The test light will illuminate if you are getting the proper ground at this connection.
Potential issues:
Test light not turning on: Refer to our understanding ground circuit video linked above. There is either an issue with your main harness grounding point of a break in the ground wiring somewhere.
Testing 12v positive power to the taillight
- Test your test light by connecting one side to ground and touching the probe to the positive terminal of the battery.
- Unplug the green/yellow wire and brown taillight wires from the main harness and connect the test light alligator clip to ground.
- To test the brown wire, touch the probe to the inside of the brown connector on the main harness that the brown wire was connected to. Turn the ignition switch to the run position. Turn on the headlight if you have a headlight on/off switch on your motorcycle. The test light should kick on and signify a good 12v positive connection.
- Leaving the probe inside the brown wire, turn the ignition switch to the 3rd parking position. The test light should kick on and signify a good 12v positive connection for the parking light position of the switch.
- To test the green/yellow wire, touch the probe to the inside of the green/yellow connector on the main harness. Turn the ignition switch to the run position. Reach for the front brake lever to sqeeze it. The test light should kick on only when sqeezed and turn off when you let go. Repeat the same test with the foot brake pedal. The results should be the same with the test light kicking on only when the pedal is pressed and turn off when you let go.
Potential issues:
Test light does not turn on when testing brown: Besides a wire being broke or corrosion causing an issue, there are 2 bigger areas that could be preventing 12v positive power from reaching the connection. First is the key switch, if you have 12v positive power on the black wire and brown/white wire at the keyswitch then you will need to replace the keyswitch or disassemble it to clean the contacts. If brown/white does not have 12v positive power then you might have an issue inside the right handlebar switch. Its also common for the wires that run inside your handlebars to be shaffed and broken where they enter and exit the handlebars.
Test light does not turn on when testing green/yellow: One or both of the brake light stop switches could be broken or not adjusted properly. Move on to test each individual stop switch.
Test light does not turn off when testing green/yellow: Its common for the rear brake light stop switch to be adjusted too tight, causing the brake light to be on all the time. One of the brake light stop switches could also be broken.
Testing the rear brake light switch
- Test your test light by connecting one side to ground and touching the probe to the positive terminal of the battery.
- Unplug the stop switch from the circuit.
- Connect a jumper wire from the black 12v positive wire on the stop switch directly to the positive battery post.
- Connect the alligator clip of the test light to a fin on the engine to ground it and push the probe into the green/yellow wire connection.
- Push the brake pedal down.
- The test light should turn on when the pedal is pressed down and off when let go.
Potential issues:
Test light does not turn on: The rear stop switch needs to be replaced or adjusted so the brake pedal pulls it enough to activate it. The switch can be adjusted up or down by turning it in the mount on the frame to tighten or loosen the tension on the spring.
Test light does not turn off: The stop switch is adjusted too tight and needs to be loosened. The switch can be adjusted up or down by turning it in the mount on the frame to tighten or loosen the tension on the spring.
Testing the hydraulic front brake light switch
- Test your test light by connecting one side to ground and touching the probe to the positive terminal of the battery.
- Unplug the stop switch from the circuit.
- Connect a jumper wire from the black 12v positive wire on the stop switch directly to the positive battery post.
- Connect the alligator clip of the test light to a fin on the engine to ground it and push the probe into the green/yellow wire connection.
- Sqeeze the front brake lever.
- The test light should turn on when the lever is sqeezed and off when let go.
Potential issues:
Test light does not turn on: The pressure switch has gone bad and will need to be replaced.
Testing the front drum brake switch
- Test your test light by connecting one side to ground and touching the probe to the positive terminal of the battery.
- Unplug the stop switch from the circuit.
- Connect a jumper wire from the black 12v positive wire on the stop switch directly to the positive battery post.
- Connect the alligator clip of the test light to a fin on the engine to ground it and push the probe into the green/yellow wire connection.
- Sqeeze the front brake lever.
- The test light should turn on when the lever is sqeezed and off when let go.
Potential issues:
Test light does not turn on: The drum brake switch has likely gone bad and needs to be replaced.
*Early Drum Brake Models with Combination Front Brake Stop and Brake Cable: Very early models from Honda actually used a front brake cable with an internal front brake stop switch. This was quickly phased out to a separate drum brake cable and stop switch fitted inside the right handlebar switch perch. If you are using this early cable and the stop switch is not working, you will need to pick up a later style switch or our separate drum brake lever perch to switch to a separate drum brake cable and drum brake stop switch.
Parts to Buy:
Hydraulic Brake Light Pressure Switch