CarTalk.com Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Installing a second horn

Ok, I am still trying to find an answer to this…

I am installing a second horn that will have a separate switch. I bought the horn used. It currently has ONE wire coming out of it that is connected to a switch. The switch then has a second wire coming from it (that includes an inline fuse). (The horn will be bolted to the frame grounding it.)

I was told I should use a relay. My relay has four prongs.

1 for ground 85
1 to power supply (fuse box via add a fuse) 30
1 to horn 87
1 to switch. 86

The problem is:

If I connect the horn to a relay prong and the switch to a relay prong, then I only have one wire coming from the switch. The switch has two terminals. Do I not need to use both terminals?? From my understanding a current needs to flow in a loop. But every time I see this question online the answer is always to just connect the switch to the relay. How can that possibly work? What does the other terminal in the switch need to be connected to?

Thanks!

Wire the other side of the switch to a low-power wire in the car, preferably switched power. The proper way to do it is to use an unused fuse slot in the internal fuse box, or to run a slave box off of that box if you don’t have one. A cheater way to do it if you’re in a hurry is to cut the wire behind the cigarette lighter and use a large wire nut to wire all 3 (from cig lighter, to cig lighter, to horn switch) together, then use electrical tape to secure the wire nut and wires to each other further. Be careful playing around back there - there will be sharp metal edges.

The reason you want it to be switched power (this means it’s only powered when the key is in and turned to “on”) is that it’s not your main horn, so you don’t need it in an emergency when your car is turned off, and you don’t want it draining the battery if the switch gets borked.

I’m assuming you’re wanting to wire this horn in to operate separately from the car’s original horn wiring harness? Just curious, because if it’s not tied into the car’s factory harness there really should be no concern over using a relay at all.

Does the factory horn contacts not work and that’s the reason for this? Just wondering because if the factory horn works then I would wire it into a relay that is triggered by the original horn wiring harness.

I figured it was one of those musical horns or something that he doesn’t necessarily want sounding off when he honks someone. He mentioned a switch, which made me think he wanted to activate it separately.

I remember seeing a 1939 Chevrolet Master DeLuxe coupe that had, as factory equipment, a switch on the dashboard. One position was for “city horn” and the other position was for “country horn”. If this is what the OP has in mind, perhaps going to J.C. Whitney online might give him some ideas.

If the OP wants to use a relay then the relay wiring as given is correct with an addition. The 86 terminal is the ground terminal for the relay trigger circuit. This means terminal 85 receives power from the switch. The remaining terminal on the switch should have a wire routed to a power source.

The amount of current required for the trigger circuit in the relay is almost non-existent so the OP should be able to tie in to just about anything with battery voltage.

It sounds like you know a bit about circuitry. See if this image helps. I drew it fairly quickly, but it is a simple switched relay circuit for your horn.