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Old 11-18-2019, 05:03 PM   #1
Superhart
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Default Is a Horn Relay Necessary?

I have a 1930 Model A street rod converted to 12 volt. I want to add an Ahooogah horn with its own dash mounted horn button. Is it necessary to install a second horn relay to operate this horn or can it be wired directly? I plan to leave the modern horns connected to the steering wheel horn button operational. Anyone have any insight about this? I figure there must be a reason why manufacturers use horn relays.
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Old 11-18-2019, 05:13 PM   #2
Kurt in NJ
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Default Re: Is a Horn Relay Necessary?

Originally power went to the horn, the button made the ground.
Stock horn has place for 2 wires, , since you have a modified car we can only guess as to how it is wired now
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Old 11-18-2019, 05:28 PM   #3
duke36
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Default Re: Is a Horn Relay Necessary?

My original Ford 6 volt horn has a relay for the 12 volt wiring system.
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Old 11-18-2019, 06:24 PM   #4
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Default Re: Is a Horn Relay Necessary?

Are you planning on running a 6 volt horn? If so, you'll want to reduce the voltage. 12 volt into it will NOT give you the correct sound, with 12 V it will sound frantic! Voltage reducers can be bought off of ebay quite inexpensively. As far as a relay is concerned, not necessary if you run a switch and wire that's rated for the current draw.
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Old 11-18-2019, 06:42 PM   #5
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Default Re: Is a Horn Relay Necessary?

The little 6V motors will really spin some rpm on 12-volt. The horn power came direct off the generator output terminal on a standard model A set up and that was about as hot and close to the horn as you could get. I don't think a relay could do it any better unless a person used heavy gauge wire for the whole horn circuit and put the relay in the horn can. That would be kind of an overkill though.

Horns that used relays were generally the vibrator types and they take a lot of in rush current to get the vibrator coil going. The early ones had a big capacitor in there to help do that before they started using a relay. The motor drive types aren't all that bad for current draw.
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Old 11-18-2019, 06:52 PM   #6
larry harding
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Default Re: Is a Horn Relay Necessary?

my 2 cents worth, i had converted my a to 12 volts and while doing some maintenance i accidently burned up the field coils in the 6 v horn. i measured the old wire size and counted the turns on each coil. i ordered some magnet wire 1/2 the size of the old and wound back twice the number of turns on each coil. some electrical engineer will tell me i'm nuts and completely wrong.... but it works and sounds right on 12v.
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Old 11-18-2019, 10:10 PM   #7
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Default Re: Is a Horn Relay Necessary?

No a relay is not necessary but keep in mind that the horn draws several amps and the switch would need to handle the current. on the other hand the coil of a relay needs very little current and a smaller switch can be used to activate the relay and the relay contacts handel the larger current for the horn.
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Old 11-18-2019, 10:12 PM   #8
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Default Re: Is a Horn Relay Necessary?

Quote:
Originally Posted by larry harding View Post
my 2 cents worth, i had converted my a to 12 volts and while doing some maintenance i accidently burned up the field coils in the 6 v horn. i measured the old wire size and counted the turns on each coil. i ordered some magnet wire 1/2 the size of the old and wound back twice the number of turns on each coil. some electrical engineer will tell me i'm nuts and completely wrong.... but it works and sounds right on 12v.

It ain't wrong if it works (I saw that on a T shirt).
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Old 11-19-2019, 10:47 AM   #9
larry harding
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Default Re: Is a Horn Relay Necessary?

a quote i heard somewhere goes like this; according to the best of aeronautical engineering the bumble can't fly but the bumble bee knows nothing about aeronautical engineering and just goes on flying anyway.
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Old 11-19-2019, 10:59 AM   #10
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Default Re: Is a Horn Relay Necessary?

Quote:
Originally Posted by larry harding View Post
my 2 cents worth, i had converted my a to 12 volts and while doing some maintenance i accidently burned up the field coils in the 6 v horn. i measured the old wire size and counted the turns on each coil. i ordered some magnet wire 1/2 the size of the old and wound back twice the number of turns on each coil. some electrical engineer will tell me i'm nuts and completely wrong.... but it works and sounds right on 12v.
You aren't nuts. There's a site somewhere that describes how to do exactly what you've done. PIA though. Much easier to install a 6-amp 12V-6V buck converter.
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Old 11-19-2019, 03:29 PM   #11
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Default Re: Is a Horn Relay Necessary?

To clear things up a bit, my car is 12V and so is the horn. It is 12v motor driven. Strangely enough, when I disassembled the horn to inspect, the motor in it is identical to the can type motor used in a couple of my HO Scale model railroad locomotives. Obviously, this horn is an aftermarket repro 12V unit. I have decided to try and measure the current draw when the horn operates and if 1 amp or less I will just go with the push button switch. I did find a 30 amp horn relay with socket on eBay for just a few bucks. I plan to mount the horn in the original Model A position on the headlight bar and use the chrome conduit like original and route the wiring through the firewall to an illuminated momentary switch on the dash.
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Old 11-19-2019, 06:30 PM   #12
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Default Re: Is a Horn Relay Necessary?

The relay will save your button. Buttons and switchs dont like amprege. 12v is harder on them. The harbour freight horn can be made to sound ok with a 6v ballast resistor. it will start to smoke if you hold the horn prolonged, I noticed that when tuning it. i had my horn hooked up no relay and it was working then I noticed my button failing sometimes. needed tapping. switched it to relay problem solved.


i used the standard off the auto parts store shelf sqaure black relay.

Last edited by mike657894; 11-19-2019 at 06:36 PM.
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Old 11-19-2019, 07:56 PM   #13
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Default Re: Is a Horn Relay Necessary?

Thanks, Mike. Just what I wanted to know. I will definitely spend the $8 for a relay.
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Old 11-20-2019, 11:59 AM   #14
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Default Re: Is a Horn Relay Necessary?

I got this one from Snyder's, fits inside the horn cover. Thought I got a bad one, they immediately sent me another. Turns out the female connectors were too tight to insert the bullets completely, but resolved with sufficient force. I also ran a wire from the horn through the conduit to the hood latch on the left frame rail. (I don't trust the conductivity of nuts and bolts.)

https://www.snydersantiqueauto.com/P...s/model-a/horn
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Old 11-20-2019, 08:40 PM   #15
Chuck Sea/Tac
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Default Re: Is a Horn Relay Necessary?

Quote:
Originally Posted by mike657894 View Post
The relay will save your button. Buttons and switchs dont like amprege. 12v is harder on them. The harbour freight horn can be made to sound ok with a 6v ballast resistor. it will start to smoke if you hold the horn prolonged, I noticed that when tuning it. i had my horn hooked up no relay and it was working then I noticed my button failing sometimes. needed tapping. switched it to relay problem solved.


i used the standard off the auto parts store shelf sqaure black relay.
Why is 12 volts harder on switches?
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Old 11-20-2019, 09:24 PM   #16
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Default Re: Is a Horn Relay Necessary?

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Why is 12 volts harder on switches?
more heat more melt more bad. 6v cars dont have horn relays. 12v cars do.
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Old 11-21-2019, 10:47 AM   #17
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Default Re: Is a Horn Relay Necessary?

I wasn't thinking - the Snyder's relay is 6 volts. Will probably work for a time, but not right for 12V.

It's current that burns stuff, not volts. 12V loads generally draw half the current of 6V ones. Of course connecting 12 volts to a 6V horn is a bad idea.
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Old 11-23-2019, 12:09 AM   #18
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Default Re: Is a Horn Relay Necessary?

If the horn draws 5 amps, then make sure the switch, wire, and relay (if used) are rated for at least 10 amps. If you use a relay, then the switch and wire to the relay can be rated for less amps than the horn draws, but the wire feeding the horn still should be able to handle at least twice what the horn uses.
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