oem horn trigger to aftermarket siren?

I have a 2011 F250 Lariat with the factory remote start/alarm system.  When you press the "lock" button once it just locks the door and arms the alarm silently.  Pressing it twice honks the horn (like most OEM systems).

My question, can I use the "Horn Trigger" wire in the kick panel to power a relay to a siren for the soul purpose of using the siren when I arm/disarm the OEM alarm and when it goes off?

Here is the wire info:Horn Trigger     PURPLE / green     -     horn switch or BCM, black 52 pin plug (B), pin 33
     The BCM (Body Control Module) is the fuse box in the passenger kick.

I'm assuming there is a separate signal coming from somewhere that provides the 12vt signal to the horn relay separate from the horn on the steering wheel?

Any suggestions would be helpful.

THanks!

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Former MECP Master Installer (10 years ago).

Sure you could but everytime you honked your horn the siren would go off also. Your information says at BCM or horn switch for horn trigger.So every time you press your horn this wire is seeing ground.  the relay for the horn is probably under the hood.

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if there is dome light supervision, maybe you could have a relay triggered by the dome light reroute the horn wire to a siren. the main side-effect would be if the interior light was on at any time, when you honked the horn the siren would instead wail ...LOL

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yuk
quiet rural missouri, near KC.
If your system moves you physically and not emotionally, you have wasted your money.

I think one way you might do this is to make it so that the horn circuit also triggers/powers the siren, but ONLY when the ignition is off. That way, if you honk the horn for whatever reason while driving, the siren won't come on too.

The negative-trigger horn wire might only be active when you honk the horn by hand....you could test the wire, and either lock your doors twice or hit the panic button on the remote, and see if the wire turns to ground. My guess is 50/50 if it will.

So, I would just go out under the hood, and find the actual horn unit. There should be a positive-trigger wire there that powers the horn.

Then do the following with a relay:

86: Chassis ground
85: fused vehicle ignition (can probably find in/near underhood fuse box)
87: no connection
87a: postive-trigger horn wire at car horn
30: postive-trigger input to siren (put red wire here; ground siren's black wire)

HOW IT WORKS: When the ignition is off, the positive power that goes to your existing horn, also flows through the relay and powers the siren.

On the other hand, when the car ignition is on, the relay "kills" the siren so that it doesn't work.

NOTES:

1. The current-handling capability of your existing Ford horn wire is limited. There's no way to tell just how much current it can handle unless maybe you know some Ford engineers. Anyway, it should be able to handle the small load of an extra siren, but if in the future you decide to add multiple sirens (or anything else high-current), you'd need to use another relay triggered by the Ford's output.

2. Every time you lock the door twice, the siren will chirp too. Depending on how "fast" your horn honk is from Ford, your siren may be too quiet to hear, or so loud that it makes you crazy. No way to predict this without hooking it up.

3. When your alarm triggers, the siren is also going to turn on/off just like the horn. If you want the siren to stay on steady the whole time, you could learn about capacitors, or maybe play around with a timer relay like the PAC TR-7.

However, if you do this, the siren will also make a big long blast every time you lock your doors twice, so you're going to want to stop doing that, if you have relay/capacitor in place.