Install Switched Relay Dual Horns

If the horns on your vintage bike are like most blasters installed back in the day, chances are they don’t put out much more than a feeble beep when you hit the horn button. While the main culprit might be the horns themselves, a contributing factor to poor horn performance is often the simple fact that prior to about the mid-1980s, most horns were activated and powered by the horn button instead of being wired through a relay, as is common practice today.

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The problem is that switches — especially on vintage bikes — can be poor conduits for voltage. That’s because they typically involve a pair of contact faces that wear against each other every time the switch is activated, producing residue and attracting grime that diminishes the contact area and reduces the voltage passing through the switch. It’s not at all unusual to discover that a powered switch with 12 volts fed into it is only letting 9 or 10 volts through. That kind of voltage drop can play hell with your bike, and it certainly doesn’t encourage optimum performance in critical components like horns, lights and ignition systems.

A switched relay, however, handles the actual voltage on/off function, with the button only producing a voltage signal to the relay to tell it to open or close a circuit. Further, voltage drop in the switch becomes a minor concern as even a 6-volt signal will activate and hold most 12-volt relays. And while a relay still has a pair (or more depending on how many functions it controls) of contacts, they’re housed in a sealed cartridge that typically lives under the gas tank or inside the headlamp shell where it’s protected from rain and other undesirable elements. That alone tends to improve their life span considerably.

Honda horn

This short How-To was inspired when one of the horns on Q & A man Keith Fellenstein’s 1980 Honda GL1100 decided it’d worked long enough, leaving him with a single horn emitting a low decibel, barely audible beep. Like many bikes of its era, Keith’s GL has dual horns, one high toned and one low. Together they work pretty well (Honda claimed 103db), but knock one out and it’s a different story. So it was time for a new horn, and since the GL wasn’t originally equipped with a relay, this was a perfect opportunity to install potentially louder horns and upgrade to a relay-activated system.

A dead horn gives the perfect opportunity to wire in new horns switched on a relay. 

Making this an even better project is the simple fact that this is a cheap upgrade. We bought a pair of horns with a 4-pin relay from Harbor Freight for the grand sum of $10. Yes, a pair. With a relay. For $10. OK, so we know they’ll never pass concours muster, but they’re perfectly serviceable units with a rated output that exceeds 110 decibels — “extremely loud” according to decibel charts. And while the not-necessarily-accurate decibel app on our smart phone showed only 100 decibels, these units are definitely loud.

On the GL, this is a very simple and effective upgrade, aided in no small part by the GL’s horn wiring, which is gloriously easy to access. Every bike is different, but following these basic steps will get you going down the right path — louder.

Here’s our starting point. The bottom connector of the bank of three is the horn circuit. The light green and white/green wires coming in at the left are the factory isolated ground and power lines, respectively.

With the wires identified we disconnected the horn wiring and removed the horns. This is the right side horn. The wiring from the horn itself is black/power and green/ground. The GL’s horn button completes the isolated circuit, grounding it to activate the horns.

We bolted our horns to the stock mounting points. Here’s the right side horn again. The green wire is our ground, which is no longer isolated, hooked to one horn terminal (doesn’t matter which), then to the mounting strap for a solid ground to the GL’s frame. The brown/green wire is our power feed.

Identify the relay connections. Our 4-pin relay has battery power in, switching power, ground, and power out to the horns. We didn’t want to cut the factory wiring in case we ever went back to stock, so we made up short connectors we could plug into the stock wiring connectors.

Next, connect the circuits to the relay. The paired brown/green wires are our power out to the horns. At this point we’ve hooked up the switching circuit from the horn button but not the main relay power we’ll run from the battery to a fuse, then to the relay.

Here’s Keith running our relay power circuit to the battery. Note that we forgot to disconnect the negative battery cable before we started, which should be the first thing you do when working on electrical systems. We know better, and now you do, too.

We ran our power line forward from the battery to the relay. We wanted it to be fused, so we spliced in a dedicated blade-type fuse and then zip-tied the fuse holder to the stock fuse block where any owner or rider should be able to find it. We also replaced the supplied 25-amp fuse with a 10-amp fuse.

Almost done, we zip-tied the relay to the main wiring harness and tucked all our wires out of the way.

Finally, we connected our relay power circuit to the battery and zip-tied the power line to the frame to keep things tidy. Power on, it was time to test the horns, which worked even better than we anticipated, regardless of what our decibel meter said. Your bike will be different, but this is typically a pretty straight-forward upgrade.

Published on Feb 28, 2013

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