Most of the 12v motorcycles made by Honda in the late '60s to the early '70s fell into 2 distinct categories of electrical design. There are what we call the "Early" motorcycles, which primarily came before 1973, and the "Late" motorcycles from 1974 and on. The headlight circuit is no exception. In this article, we will cover the early style headlight and gauge light circuit motorcycles that used a single 15 amp fuse on their electrical systems. We will cover how this circuit is generally set up on these motorcycles and how to troubleshoot every part and piece within it.

____________________________________________________________________________________

Compatible models with this instructional
CB175 / CL175 / SL175
CB200 K0 / CL200
CB350 / CL350 / SL350
CB450 / CL450
CB500K
CB750 K0-K2
Most Honda models before 1973.

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. 

Explanation of the circuit path

In this explanation, we will be going over the circuit in reverse. If you have seen our ground circuit video, you will know that a circuit starts at ground. While the current flows from negative to positive, it is not always the most simple way to explain it. In the case of the headlight circuit, starting at the single connection to 12v positive in the right handlebar switch and branching our way to the various splits that make up the headlight and gauge backlight circuit on your vintage Honda will be easier to follow. 

12v positive to the right handlebar switch: Current begins with the black 12v positive wire that runs into the right handlebar switch. The right handlebar switch on these motorcycles acts as a junction for several different functions. With the switch in the off position, the black 12v positive wire connects to nothing. With the switch set to the L(low beam) position, the black 12v positive wire connects both to the white wire and the brown wire with white stripe. With the switch set to the H(hign beam) position, the black 12v positive wire will connect to the blue wire and the brown wire with white stripe.

The white (low beam) wire: The white wire runs directly from the right handlebar switch to the low beam filament on the headlight bulb. The headlight bulb then grounds through the green harness ground connections inside the headlight bucket.

The blue (high beam) wire: The blue wire exits the right handlebar switch and splits into two sections. The first runs directly to the high beam filament on the headlight bulb. The second will connect to the high beam indicator bulb either inside one of the Honda factory gauges or in the top of the headlight bucket. Both the headlight bulb and high beam indicator bulb will connect to ground through the green wiring harness ground connections inside the headlight bucket.

The brown wire with white stripe: The brown wire with white stripe is activated in both the low beam and high beam positions of the right handlebar switch to turn on both the gauge backlights and taillight running light. To troubleshoot the taillight portion of this circuit, see our taillight troubleshooting and basics technical article. For the gauge backlights, the brown wire with white stripe from the right handlebar switch will connect directly to the brown wires with white strips that run to each gauge backlight bulb inside the speedometer and tachometer. All of these backlight bulbs ground through the green wiring harness ground connections inside the headlight bucket.

Troubleshooting Tests

Testing the headlight bulb

1. Remove and unplug the white, blue, and green connections that go to the headlight to remove the lens and bulb for testing. 
2. Using a known good battery, connect a jumper wire from the ground post of the battery to the green ground wire going to the headlight bulb.
3. Connect another jumper wire to the positive side of the battery and touch the other end of the jumper to the white wire to test the low beam on the headlight. Be careful to not let the positive jumper wire touch any ground point on the motorcycle or headlight. If the low beam filament kicks on then the low beam filament works properly. 
4. Touch the positive jumper wire to the blue wire to test the high beam on the headlight. If the low beam filament kicks on then the high beam filament works properly.

Potential issues: 

The low beam filament or the hi beam filament on the headlight does not turn on - The headlight bulb needs to be replaced. If you are still using the stock sealed beam headlight lens, you will need to upgrade to our H4 compatible lens that allows you to use brighter H4 halogen and LED headlights and replace just the bulb down the road. 

Testing the right handlebar switch

1. Test your test light by connecting one side to ground and touching the probe to the positive terminal of the battery.
2. Locate where the right handlebar switch connects to the main harness on your motorcycle. For most motorcycles, this should be inside the headlight bucket. 
3. Connect the alligator ground of the test light to a known good ground point.
4. Test the black wire(s) going to the right handlebar switch by touching the test light probe to the inside of the black connector. Turn the ignition switch to the run position. The test light should kick on and signify a good 12v positive connection. 
5. Unplug the white wire coming from the handlebar switch and touch the probe to the inside of the white connector. Turn the ignition switch to the run position. Turn on the headlight to the low (L) position. The test light should kick on and signify a good 12v positive connection. 
6. Unplug the blue wire coming from the handlebar switch and touch the probe to the inside of the white connector. Turn the ignition switch to the run position. Turn on the headlight to the Hi (H) position. The test light should kick on and signify a good 12v positive connection.
7. Touch the probe to the inside of a brown/white connector inside the headlight bucket. Turn the ignition switch to the run position. Turn on the headlight first to the low(L) position. The test light should kick on and signify a good 12v positive connection. Switch to Hi(H) position. The test light should kick on and signify a good 12v positive connection.

Potential issues: 

The test light does not turn on when testing black - Refer to our positive circuit guide.

The test light does not turn on when testing white, blue, or brown/white - You could have a broken or torn wire in the handlebar switch wires inside your handlebars, dirty contacts inside the switch, or a broken solder joint inside the switch that connects the wire to the internal contacts.

Testing the high beam indicator

1. Unplug the blue and green wires that go to the high beam indicator inside the headlight bucket. 
2. Connect the blue wire to a black 12v positive wire. 
3. Turn the ignition switch to the run position.
4. Touch the green ground wire to a known good grounding point. The blue high beam indicator light should kick on and signify a working bulb, socket, and wiring going to the high beam indicator.

Potential issues: 

The high beam indicator bulb does not turn on - Either the bulb is burned out, there is corrosion on the bulb socket or sockets, or there is an issue with the wiring going to the bulb socket. If you are running an LED bulb, refer to our note on LED gauge light bulbs at the end of this article.

Testing the gauge backlight bulbs and socket

1. Unplug the brown/white and green ground wires that go to the gauges inside the headlight bucket. 
2. These will be tested exactly like the high beam indicator. Connect the brown/white wire to a black 12v positive wire. 
3. Turn the ignition switch to the run position.
4. Touch the green ground wire to a known good grounding point. The gauge backlights should kick on and signify working bulbs, sockets, and wiring going to them.

Potential issues: 

A gauge backlight bulb does not turn on - Either the bulb is burned out, there is corrosion on the bulb socket or sockets, or there is an issue with the wiring going to the bulb socket. 

A Note on LED gauge light bulbs

If you are running an LED bulb, try switching the positive and ground connections for testing so plug the green wire going to the socket into ground and touch the brown/white (or blue) to ground. The factory incandescent bulbs can have current running in both directions. This means they can be plugged in reversed and still light up. Because of this, Honda did not always keep a specific color as the positive and ground for the gauge bulb sockets. It was often up to whoever soldered the bulb socket on the assembly line that day because it didn't matter. Modern LED bulbs are only one way, so unlike the older incandescent bulbs, they will not light up regardless of which wire is positive and which wire is ground. If you find that your LED bulb works with the connections reversed, then that means essentially the factory wired the bulb socket “backward” and the green wire will be your positive wire, with the other color wire being your ground wire.