Installing A Second Horn - Automotive Repairs - DIY Chatroom Home Improvement Forum

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Old 11-15-2012, 02:44 PM   #1
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Installing a second Horn

I am installing a second horn that will have a separate switch. The horn has one wire coming out of it. I was told I should use a relay. my relay has four prongs.

1 for ground
1 to power supply (fuse box)
1 to horn
1 to switch.

The question is, if I only have one wire coming from the horn, how am I to connect both the horn AND the switch to the relay??? It seems as though the horn has to go to the switch, and then the switch goes to the relay, but that leaves an open prong for either the horn or the switch, depending on which one I use.

What do I do??



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Old 11-15-2012, 08:08 PM   #2
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hook up horn wire to horn terminal on relay.
horn normally grounds by being screwed or bolted to a metal frame.
connect the rest of relay wires as marked. switch to switch, power to power, ground to ground.
you flip the switch, and relay sends power to the horn. relays it. just make sure you have good horn ground.


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Old 11-15-2012, 08:47 PM   #3
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Ok so by doing it that way I will only have ONE wire going from my switch to the relay, even though the switch has two terminals on it...

I admit that I don't know much about electricity but I always thought you had to have a loop of some sort. Just for curiosity, could you explain how having only one wire going from the switch to the relay can work??

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Old 11-16-2012, 12:29 AM   #4
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The metal parts of the car make up the negative 'wire' in most cases.
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Old 11-16-2012, 06:46 AM   #5
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This is for a Bosch relay.
A power wire goes to one side of the switch. The other wire of the switch goes to relay terminal 85. terminal 86 gets a wire to ground. 30 gets fused battery power. 87 goes to horn.
When you turn the switch on, a small current flows through the switch to the relay, completes the relay ckt, closes the contacts, and allows a larger current to flow from pin 30 to your load.
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