Adding a larger electric horn to a P-series Vespa is not all that hard, especially if you are already running an auxiliary battery for lights. I am, so these notes will assume that you have a second battery - however, there is no reason the same wouldn't work for a bike without one.

I bought twin electric horns from Wolo - they have the look of air horns, but no air compressor is required. I found these horns at Pep Boys. I also used a 1 2volt, 30 amp relay, to give me more juice to the horns. In addition, I used 14amp wire and various wire connects. See the opening section of the Wire Your Lights essay with information on how to connect a relay.

I wanted to disconnect my stock horn and be able to trigger the new horns from the switch on the left handlebar. The stock horn has two connections: one is a double white wire, which provides positive power to the horn, and the other is a pink wire, which connects to the handlebar switch and ultimate the bike’s ground. The stock horn is always powered, it beeps when the ground circuit is completed by pushing the contact on the handlebar switch, which connects the pink wire to the Vespa’s black ground wire.

On a relay, plug the double white wires into one of the sides marked 85 or 86, and plug the pink wire into the opposite side. Check your relay - if it mentions a particular number for ground (for example, mine said to ground pole 85) use that scheme, and plug the pink wire into that pole. You now have your switching set. To provide power to the relay, run a wire from the battery to the pole marked 30. Finally, run a wire from the pole labeled 87 (it’s opposite pole 30) to the positive (probably red) wire of the horns. Lastly, run a wire from the horns’ negative (probably black) to the battery negative. You’re good to go!

I have the relay and wiring tucked into my horncast. The wires leading to the battery leave the horncast through a screw hole that helps connects the horncast to the legshield - I’ve removed the screw obviously, and have found that the wires pulled taut do a fine enough job of keeping the horncast secure (it still has the two screws at the top of the legshield, beneath the Piaggio plaque).

Lastly, if you’re not running an aux battery, I don’t think there isn’t any reason this wouldn’t work with the stock battery. You’ll just have to run longer wires from the relay to the battery positive and from the horns to the battery negative, and deal with how to protect and disguise them.

If you have questions, email jj@modchicago.com