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.

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.

The simplicity of this install makes it something of a no-brainer as modifications to the original circuit are minimal: you simply piggyback onto it, using the original switch on the handlebar to signal the relay, which then switches the headlamp. The relay always has full voltage (in our case 12 volts), and any voltage drop at the handlebar switch becomes inconsequential, as 6 volts is enough to activate and hold most 12-volt relays. The Matchbox Relay is also available for 6-volt systems.

This is all it takes: a relay, a wiring diagram — just in case — and a handful of bullet connectors.

It only took us about an hour to install our Matchbox Relay, and the improvement in headlamp output was immediately apparent. Before installing the relay, we were only getting 11.31 volts to the headlamp from a battery showing 12.72 volts, an 11 percent drop in voltage. With the relay installed, we had 12.48 volts, or just less than a 2 percent drop.

Installing a relay pretty much guarantees brighter lights, making you that much safer on the road. As usual, we suggest having a good workshop manual at hand. Wiring can be a bit like spaghetti — it’s everywhere — and it’s handy having a wiring diagram for reference.

Start by removing the headlamp assembly. On our Yamaha that meant simply removing two screws positioned at roughly 8 and 4 o’clock on the shell, then lifting the headlamp up and out.

Before we installed the relay we measured voltage at the headlamp socket. With 12.72 volts at the battery we were getting 11.31 volts at the headlamp, an 11 percent drop.

We had to wire in two jumper wires from the stock wiring to the relay, one from the low beam circuit and a second from the high beam.

The only wire we had to cut was the high beam from the lamp socket so we could connect the socket to the relay. The wire from the stock loom now runs to the relay input to switch the beam.

It’s hard to get lost with this particular install as the relay has a basic schematic printed on the back. All we’re doing is rerouting the high/low beam circuits to the relay, then connecting the relay to the headlamp.

The relay requires a dedicated 12-volt power source. We ran 14-gauge wire (red for positive, black for negative) from the headlamp shell to the battery, securing it as needed.

To protect the relay circuit we spliced in a blade-type 15-amp fuse close to the battery, which we first disconnected before final wiring. We placed it here simply for convenience.

We secured the relay to the inside of the headlamp shell with self-adhesive hook-and-loop material, placing it high to ensure water won’t collect should the shell get wet inside.

With the relay fully wired in a final check of output to the headlamp socket showed voltage had climbed to 12.48 volts from 11.31. Simple and effective, this is an excellent upgrade for just about any vintage ride.

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