The best analogy for the electrical system on a vintage Honda motorcycle is that of a sandwich. The toppings might be the most exciting part of a sandwich but it is the upper and lower piece of bread that makes a sandwich what it is. In the same way, the ground circuit and positive circuit make up the structure that allows the electrical system to function. Without a knowledge of how they are set up, it would be impossible to successfully diagnose issues within the electrical system. In this article and video above we cover the ground circuit, or "bottom piece of bread," to better help you understand what is happening in the middle.

The Ground Circuit

(AKA the bottom piece of bread)

Electrical direct current (DC) flows from negative → positive, so the ground circuit is the beginning of the electron path on the motorcycle. Electrons flow from the negative side of the battery, through the frame and the harness grounds, through an electrical component (light, coil, horn, switch) out the positive side connection of that part, to the key switch, and ending at the positive side of the battery.

After understanding how current flows, it is important to think of ground as simply one large extension of the negative battery terminal. Once hooked up, any and all metal to metal contact becomes ground. The engine, front forks, triple tree, metal brackets, bolts, and dark green wires throughout the harness are all ground. Good ground connection can be easily tested by connecting a test light lead to the positive side of the battery and using the test light probe to touch and verify that certain pieces are grounding.

This is the main reason the negative side of the battery is the first to be disconnected and the last to be reconnected, it acts as the master disconnect for the electrical system.

The negative side of the battery should ALWAYS be disconnected while doing any work on the electrical system!

The dark green grounding circuit

In addition to the frame and metal grounding, Honda used a redundant dark green wire grounding circuit to reach parts that were not able to directly touch a grounded metal surface. If you search closely, you will have a small dark green wire ring terminal bolted directly to the frame that connects the dark green wire of the harness to the frame. This carries the ground circuit from the frame to the green harness ground circuit, which goes on to connect to parts that are often rubber mounted on the motorcycle. Parts such as the taillight, handlebar switches, gauge lights, and horn use this redundant ground circuit. Be sure your frame ground is properly connected and free of corrosion and rust.

*CB175 / CB200 Owners: The harness frame ground on these motorcycles is connected from the factory through the OEM Honda Rectifier. If your motorcycle is missing this part or you are upgrading to our modern regulator / rectifier combo unit, you will need to add your own harness ground. See our regulator / rectifier installation document and video on how to do this: Installing Your Regulator Rectifier Combo Unit

The Two Unique Ground Paths on a Vintage Honda

Front Turn Signals - OEM Honda turn signals do not use a green grounding wire but instead ground through the metal body of the turn signal itself. In the case of the front turn signals though, they are attached to the rubber-isolated headlight ears. The proper ground path for these is actually from the body of the front turn signal → the metal headlight ears → the metal headlight bucket mounting bolt → the nut inside the headlight bucket. These nuts inside the headlight bucket are unique and should have a soldered flange with green wire attached to them and be plugged into a dark green harness ground inside the headlight bucket.

Handlebar Ground - Like the headlight ears, the handlebars are attached to rubber isolated handlebar risers on the smaller twins like the CB175, CB350, CB360, and CB450. Because they have no direct ground path to carry the ground circuit from the handlebar switches to the frame, there should be a small wire that connects the right handlebar riser to the gauge bracket to transfer ground connection. Refer to the video to see what this looks like. Without this, the horn and starter button will have issues properly grounding to complete their circuit.

Q&A

What can prevent something from properly grounding?

Often rust, corrosion, or paint can cause a bad ground connection. This is a common issue with both the main ground wire and harness ground wire that are bolted to the frame. There needs to be a solid, raw metal on metal contact for ground connection to be carried over. We recommend sanding the contact surface with 120-220 grit sandpaper if the ground connection is not carried over.

My horn button and starter button are not grounding?

See the above unique ground paths section. If the small jumper wire is in place and properly transferring ground connection to the handlebar riser, check and see whether you have painted handlebars or chrome handlebars. Stock handlebars from Honda are chrome, which is able to transfer ground current through the chrome coating to the handlebar risers. If you have aftermarket painted handlebars, you will need to sand off some paint from both the part of the handlebar that sits inside the handlebar riser, as well as inside the switch body. This allows the metal switch body to ground to the metal handlebar and handlebar to transfer ground to the riser. You can also bolt a wire to one of the small interior bolts inside the switch, run it inside the wire bundle with the other switch wires, and plug it into an open green ground connection inside the headlight bucket. Email us with any further questions.

How do I ground the OEM style Honda turn signals?

Both the OEM turn signals and our replacement OEM style turn signals ground through the body of the turn signal. We go into more detail above on how the front turn signals are grounded. Rear turn signals grounding was done several different ways from the factory depending on whether they were bolted directly to the frame or to a rubber isolated. See the below list and find which style of grounding was used on your motorcycle:

Frame Bolted - Either bolted directly to the frame or to a metal bracket that is bolted to the frame. Ensure there is no corrosion or paint preventing a proper ground path.
Rubber isolated - Bolted to a metal bracket that is rubber mounted against the frame or rear grab rail. The metal brackets will have a ground wire soldered onto them that should be plugged into the dark green grounding circuit connections under the seat.

If you are using our universal bullet turn signals, they will have a dedicated ground wire that can either be connected to the frame or a green ground connection in the harness.