Understanding how your motorcycle’s tail lights are wired is important for a couple of reasons. First, if you want to add an auxiliary brake or running light, you’ll want to know where to hook it up.
- Check the Wires
Start by gaining access to the lighting wires in the rear end of your bike. You may need to partially disassemble the rear cowling of your bike to find the wires, but usually you just need to remove the passenger seat and have a peek inside.
Here’s a good example to follow. This is the tail light wire plug (three wires) and the license-plate light (two wires) By comparing to the turn signal wires, we know the black with yellow stripe is ground. Because the red wire is found in both circuits, it must be the power. Blue must be, by the process of elimination, the brake light.
Once you can see the wires coming from the lamp housings, take a moment to write them down.
Carefully note whether wires are striped with two colors (white with a black stripe is different from black with a white stripe).
Left turn signal:
Right turn signal:
License plate light:
*Your brake/tail light is CAN-BUS if it only has two wires. In such bikes, the tail light and brake light signals are contained within a single wire. You will need to source a switched +12v at another location (eg: your accessory plug or power management system).
- Determine Ground
One of the wires attached to each lamp is the ground or negative wire. The ground wire is the same color for every lamp. Look at the list above. The color that is attached to every lamp is ground. Write “ground” next to the appropriate wire color above. The ground color will vary by manufacturer, and could be black, black with a yellow stripe, orange, or green. Regardless, it will be the only wire “common” to all lamps.
- Determine Tail Light Positive
The license plate positive wire is the one that’s not ground. This wire will almost certainly be the same color as your tail light positive wire. Write “tail” next to the appropriate wire color above.
- Determine Brake Light and Turn Signal Positive
You’ve already determined ground and tail wire colors in your 3-wire brake/tail light. The one remaining wire color is your brake light positive wire. Likewise, the one color other than ground in each of your turn signals is the positive turn signal wire.
- Determine Turn Signal Left and Right Positive
This is a little more difficult. But if you can see the wire harness coming from the individual turnsignals, you can determine which is right and which is left. The ground wire color will be the same as for the rest. Watch out for the fact that some motorcycles don’t distinguish left and right colors in the short harness from the tail section to the light itself; that is, it changes color once into the main harness. Always trace back to the main harness to be sure.
A simpler way is “poke and test.” Once you know the positive wires for each turnsignal, fit the PosiTap, hook up your device and test. You have a 50/50 chance of getting it right the first time.
Try this out while looking at the bike. It’s a lot easier to do this with it in front of you than to try to remember.