Im trying to run a relay on both for a light bar and an air horn. But they are 12v+ when idle and ground when engaged.

I'm assuming I'll either need to tap a wire after the computer or find a way to wire a relay so that 12v+ does nothing but it engages the relay when grounded?

 

What? Do you know what a relay is for?

Wiring is like this:
+12--------Device---------switch--------ground

To wire a relay in put it in place of the switch and actuate it similarly.

 

Alright I figured it out.

I wired the 12v relay as normal, but with the change in the trigger and ground wires.

Ground will now be the wire coming from the horn wire at the steering wheel, since it only grounds when the steering wheel button is pressed. I cut the wire leading from the engine bay so it would not get 12v. The wire leading into the harness is the side you will tap into. It might work without cutting the wire. But there is enough room to re-attach the wire, and I've installed breathalizer units in vehicles and cut factory ignition wires and tapped into them and then removed units with no issues, so I feel confident in this.

Your car might be different, so I suggest using a power probe if you can get your hands on one, just to be sure. (The power probe is probably my most useful tool for 12v)




12v trigger post on the relay will be bridged with the 12v+ input wire. (Which you should fuse)







12v output stays the same.

Here I demonstrate how the lock alarm still works with a $5 alarm siren swapped for the stock horn, and the horn button works with my added air horn.

 

yes i have 5 relays in my car ive added. But all were 12v+ activated. The steering wheel and high beams are constant 12v and ground when activated. I managed to figure it out and have posted a write-up above. Basically you can swap the ground on a relay and it will work. Never done it this way.

 

A relay doesn't care what polarity triggers it. Most DC circuits are switched on the ground side to avoid arcing.
Evidently, you don't have a good grasp on DC electronics.

There should be either an internal or added diode on the relay