Install a Headlamp Relay

| 8/27/2013 12:22:49 PM

Install a Headlamp Relay 

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.

BikeMaster There are different relay kits on the market to accomplish this. One of the simplest we've seen is the Matchbox Dual Headlamp Relay from Cu Layer ($45.95;, which has dual relay function providing high/low beam circuit control from a single unit. It's also small (about the size of a box of matches, hence the name), so it fits easily into just about any headlamp shell.

We installed our Matchbox Relay in a 1974 Yamaha TX500, which, like most bikes of its era, uses a simple handlebar-mounted powered switch to toggle between low and high beam. Installation was easy, even accounting for removing the gas tank so we could run the necessary dedicated power circuit to the 12-volt relay. The only wires we cut were the wires from the headlamp socket so we could run jumper wires to the relay. In the case of our Yamaha, which uses a standard three-prong headlamp socket, we could have kept the original socket unmolested by substituting an off-the-shelf universal replacement, splicing it into the circuit instead. The Matchbox Relay is not waterproof, so it's important to mount it where it will stay dry, either in the headlamp shell or tucked up high under the gas tank.