View Full Version : 12V Wiring Question

04-10-2013, 07:55 PM
I am new to 12V wiring and I am having trouble wiring up a dual trumpet Ongaro marine horn. Using my multi meter I am reading a solid 12V power source going into a push button switch from the ignition switch and a solid 12V power source coming out of the push button switch. If I wire only the first trumpet to the output of the switch the trumpet blows perfectly. If I wire only the second trumpet to the output of the switch the trumpet blows perfectly. If I wire both trumpets to the output of the switch both trumpets produce a barely audible humming sound. I have tried wiring serial and parallel with the same results.

I am hoping a 12V expert just read the above and is laughing at my obvious mistake!

Any suggestions would really be appreciated.

Thanks for the help,


04-10-2013, 09:57 PM
You need to run heavy wire from your 12v positive and negative bus bars and install a relay near the horn. you activate the relay with your push button switch.

The wiring from your push button switch is not heavy enough to carry multiple horns.

04-11-2013, 03:30 AM
Gotta agree with OHB. Electric horns draw a LOT of power and IMO are a constant headache.

04-11-2013, 10:32 AM
As usual, OHB and Wilie nailed it. To know how big your wires need to be, it would be good if you could get the amperage draw for your horns. If you're lucky, it's written on a tag. Next best would be to get in touch with the manufacturer. If you don't luck out with these methods, you can hook it up with an ammeter in the wire. Wire sizing depends on a lot of factors like temperature, wire ratings, wet vs. dry, run length, and whether you're inside or outside an engine compartment. Having said that, here is a rough and dirty guide for moderate length runs:

10 amps - 16 gauge
15 amps - 14 gauge
20 amps - 12 gauge
30 amps - 10 gauge

If you're between sizes, round up (note that gauge numbers get smaller as you go up in wire size). If your runs are really long, you can jump up a wire size. If your horns are close to your power source and your push button is far away, you can help matters by installing a relay. A relay is kind of a remote switch that uses a small current to switch a bigger current. If that's something you need, you can give a shout here and get some more advice on the particulars of wiring a relay. It isn't as hard as it sounds.

As OHB said, your problem might be that you're going off your ignition switch. That's a problem since ignition switch wiring usually isn't that big and has to share loads with a lot of other stuff. You'll do better with a direct run to your main panel. If you want the horns to only work when the key is on, the relay I mentioned above will do that for you.

04-11-2013, 01:31 PM
I think the horns in actuality draw more than the rating or even an ammeter is going to show. An electric horn is just a buzzer, and as such is cycling on&off at a fast rate. If it's "on" say 50% of the time, my guess is that during that time the amp draw is about double what the AVERAGE amp draw is. So if the thing is rated for 15 amps, it probably needs wiring for 30 amps.

I ditched mine after cussing it for months, it would work great connected to a battery but not on the boat; and installed a 12v air horn.

I've wondered if a big capacitor at the horn would help, but I've never tried it.

04-11-2013, 05:03 PM
First, welcome to the forum.
Second, remember you said ANY suggestions would be appreciated. Return the 12 volt horns, mount an air compressor and buy some air horns. You'll have better horns and will use your compressor for much more than just horns.

04-12-2013, 09:20 AM

Thanks for all the great suggestions! Dan, a neighbor made the same suggestion and even offered up an air horn/compressor that he had laying arround so I think I will go that route as my permanent horn solution. This weekend I think I might try everyone's advice and bypass the ignition, go straight to the bus bar, install a relay between the bus bar and the horn and wire up the button/switch to trigger the relay simply for the learning exercise! I am new to 12v and my 1978 has a rat's nest of 12v wiring issues so I figure I will need the relay training at some point so I may have to hit you guys up with more questions!

I will let you guys know how it goes...


04-17-2013, 07:53 PM

Thanks for all the help! I really appreciate it! I now have dual trumpets blowing! I bypassed the ignition, put a relay between the bus bar and the horn, tied the lower and upper helm horn switches to the relay and it worked perfectly!

I have another 12V question if you guys don't mind?

The upper and lower helm both have an on/off blower switch and both switches work but currently you have to turn off the blower from the same switch that you used to turned on the blower. Is this the standard way a dual helm blower switch system is configured? Is it possible to rewire so that you can turn on at one station and turn off at the other?

Thanks again,


04-17-2013, 09:06 PM
Don't know why you would want to but: You will need 2 new SPDT switches and you will wire them up the same as household "3way" light switches. The address below will give you a schematic. Can you hear the blower run or do you have indicator lights? You may have to add indicator lights cause switch could be in either position depending on where you were when you last turned the puppy off/on.


05-16-2015, 01:28 PM
Thank you for the*informative*posts.