Install a Headlamp Relay

Reader Contribution by Mc Staff
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In the March/April 2013 issue, we showed you how to install relay-switched horns on a 1980 Honda GL1100. While we replaced the GL’s original horns, the thrust of that How-To was installing a relay to operate the horns. That’s because your horns will work better powered and switched by a relay rather than a horn button, which can wear and get dirty, impeding voltage. 

Much as your horns can perform poorly powered through an old switch, so can your headlamp, but with potentially more dire results. That’s because headlamp output drops exponentially with voltage drop. According to, a 9006 halogen headlamp bulb rated at 1000 lumens at 12.8 volts drops to 510 lumens at 10.5 volts, a 49 percent drop in output from an 18 percent drop in voltage!

On an older bike operating without a headlamp relay voltage drop to the headlamp is typical, the result of a worn and dirty headlamp switch and sometimes made worse by a long wiring circuit. Fortunately, you can ensure proper headlamp voltage by installing a relay.

Motorcycle Classics Magazine
Motorcycle Classics Magazine
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