Help Me Wire Up A Siren
 

I cleaned up this siren that was under the hood of my c20 when I bought it.
I am wanting to wire it up via a ford starter solenoid to a switch under the dash. How do I go about doing this in a safe manner that won't burn the truck to the ground?

(I am just getting into wiring so please bear with me if I ask a lot of questions)

Re: Help Me Wire Up A Siren
 

What kind of switch do you have for it?

Re: Help Me Wire Up A Siren
 

Run siren at wrong time, go directly to jail...

Re: Help Me Wire Up A Siren
 

There is a switch under the dash that has the stock dual horns wired into it. I believe the previous owner did this so he was able to switch from the stock dual horns to a large semi truck horn that used to be under the hood. Both horns were activated by pushing the stock horn button in the steering wheel.


Would I be able to hook the other lead of the switch to the solenoid on the siren and use the stock horn button to engage it? If so, which spot on the solenoid would I hook it to?


Here a poorly drawn wiring diagram that explains what I am thinking of doing...
Also, the switch has "1/2HP 125 VAC" written on it. Not sure what that means, but it might be helpful in determining if the switch can handle it...


EDIT: *I guess the question wouldn't be if the switch can handle it, but if the horn relay can handle it*

Re: Help Me Wire Up A Siren
 

Yep, I don't plan on running it on public roadways. Don't want to get into trouble.

Re: Help Me Wire Up A Siren
 

Disclaimer: I have not done this. This is what I would try.

Wear hearing protection, safety glasses, remove jewelry, wear gloves, and have a fire extinguisher handy.
Disconnect the negative terminal of the battery.
Begin with disconnecting the relay from the siren and use jumper cables to connect the battery to the siren and see if it works.

Is the Ford relay marked on the terminals I, S, BAT, and STR?
If so, connect the BAT terminal to the battery + with a 12 or 10 gauge wire and a 20 amp fuse or circuit breaker in line.
Do not connect I to anything.
Connect your switch output to S.
Connect STR to the siren + terminal.
Make sure the siren and the ford relay are screwed down to bare metal for a good ground.
If the siren has a ground terminal, make sure that is connected to ground.

Before connecting the battery, tap the terminal end against the post and make sure you don't get sparks.
Connect the battery terminal loosely and have somebody tap the horn button. If something goes wrong, pull that battery terminal off the post.

Re: Help Me Wire Up A Siren
 

I have verified that the siren functions when I connect a jumper wire from the positive battery post to the large post on the drivers side of the ford solenoid.

The ford solenoid is marked I, s, bat, and str.

Am I correct in saying that the ford solenoid will be carrying most of the current not the switch or horn relay?

I just want to make sure that the horn relay and switch won't get "fried" before I hook it up.

Re: Help Me Wire Up A Siren
 

Yes, that's right. The Ford solenoid will carry most of the current, from the battery + terminal to the siren + terminal or wire. The wire to the S terminal will only carry enough current to energize the solenoid, which is less than what it takes to operate a horn. The I terminal is not used at all. Disconnect the neg terminal from the battery while working on this stuff and that will make it unlikely for you to fry any wiring.

Re: Help Me Wire Up A Siren
 

The two big post on the ford solenoid carry the load & amps. The smaller two connections are what trigger it. A relay is not required.


#1- Battery + to 1 big connector of FS.
#2- The other big connector of FS to siren.
#3- ground siren to truck make sure you have it good and cleaned.
#4- If using the truck horn button and relay should be green wire ran to FS small connection. Then the other small connection to ground. The siren should work when you push the horn button.:chevy:

Re: Help Me Wire Up A Siren
 

Thanks for your and dmjlambert's help. I wired everything up but when I hit the horn button the siren barely spun then stopped. I am running 10ga wire from the positive battery terminal directly to the ford solenoid. I put an inline 20A fuse in that 10ga wire and found that it was blown. I can step it up to 30A but I am wondering if I actually need a fuse there....thoughts? I feel like it will blow the 30A f


The good news is that the ford solenoid is engaging when I hit the horn button.

Re: Help Me Wire Up A Siren
 

Does the siren have a amp rating on it any where? Try the 30amp even 40 who knows. :chevy:

Re: Help Me Wire Up A Siren
 

I recommend show us pictures of how you have it wired.

Re: Help Me Wire Up A Siren
 

The horn button in the truck grounds the truck relay which in turns makes the horn blow. So

Per your drawing you need the switch to go to one of the small terminals of the FS. The other small terminal should go to ground. You may want a diode on them also to keep it from welding the contacts.

Run a wire from your battery to the FS big terminal. Then the other big terminal should go to the siren. It should work if the siren is not messed up are shorted.:chevy:

Re: Help Me Wire Up A Siren
 

I'm going to disagree, I believe the terminal labeled "I" should not be connected to ground or anything else. When used with a starter, that terminal would be used to go to the ignition coil.

Re: Help Me Wire Up A Siren
 

This is how I have it set up currently. The output wire from the switch going to the solenoid "S" terminal. I have nothing on the "I" terminal. I also have a ground from the body of the siren to the frame.

I can hear the solenoid engaging when I push the horn button so it seems that all works fine.


The issue now is that the 20amp inline fuse blew when I hit the horn button. What amp fuse should I have on the main wire coming off the battery positive? 50amp? This siren is basically a mini starter motor so I assume it pulls quite a few amps...

Re: Help Me Wire Up A Siren
 

Here is a picture of how it is wired.


1- 12V+ coming from battery + terminal. This is the wire that has the inline 20A fuse that blew. (Yes I know its black, I am out of red wire at the moment)

2- "S" Terminal on solenoid. This is going to the output on my switch.

3- 12V+ going into siren motor when solenoid is engaged

4- Ground running from siren body to truck frame (I believe it already has a good ground to the inner fender that it is mounted to but I ran another one just in case)

Re: Help Me Wire Up A Siren
 

That looks good. Only thing is, without actual documentation such as current rating, or horsepower, wattage, or some other data plate (does the siren have a data plate with info stamped on it?) we can't tell how much amperage of a fuse to use. 12 gauge wire should be good to go about 20 amps and 10 gauge wire up to 30 amps. Higher currents are OK if the wire is short.

If you bypass the fuse and go ahead and let it run for a minute or so, does the wire between battery and solenoid get warm? If so, get bigger gauge (smaller number) wire or double up the wire and repeat the test. You should probably put circuit protection on it if leaving it wired up, such as a fuse or circuit breaker. Circuit breaker is probably best if going higher than 25 amps. You may need an 8 gauge wire or 2 12 gauge wires, and 40 amp circuit breaker.

What gauge wire is it you have running from the battery to the solenoid and how long is it? We may be able to measure the current with a voltmeter. Then put appropriate permanent wire on it and a circuit breaker that will be good. Need to measure the voltage along the wire with siren running, and know the length of the wire, then with some math figure out the current. Then we would just know instead of guess.

Re: Help Me Wire Up A Siren
 

Unfortunately, there is no data plate or any markings on the siren. I currently have a 10 gauge wire running from the battery that is about 7.5 feet long. Do you know of any manufacturers that make a circuit breaker for an automotive application like this?

To measure the voltage along the wire when the siren is running I just use volts DC setting on the multimeter right? After I find the voltage with siren running do I use ohm's law to calculate the amps?

Re: Help Me Wire Up A Siren
 

Yes, that's what I was thinking.

Put the + or red lead of the voltmeter on the + terminal of the battery, and put the - or black lead of the voltmeter on the + terminal of the siren. No need to connect volt meter to ground. Turn on the siren and make sure the wire isn't getting too warm. See what the volt meter reads.

I=V/R

7.5 ft of 10 gauge wire is .007 ohms.

So, for example suppose you read .28 volts, .28/.007=40 amps. In that case I would get a 50 or 60 amp circuit breaker. I'm going to be surprised if that siren draws that much and if 10 gauge wire gets hot. But I've been surprised before. If it has really high current and it is convenient for you to move the siren closer to the battery, that will help shorten the wire and that will handle higher current with less voltage drop.

It may be a good idea for you to run an equal gauge wire from the - terminal of the battery to the ground of the siren, so you don't rely on the ground provided through the engine block and the small ground straps between engine and truck body.

If the wire gets hot, I would consider the measurement and calculations just a rough estimate, and would probably be a good idea to go to bigger wire and re-do the test and calculation.

I suppose you could use a short piece (6 to 9 inches maybe) of fusible link wire at the battery + terminal instead of a circuit breaker or fuse. The fusible link wire should be 2 wire sizes smaller, which is 4 numbers because electric wire is just sold in even sizes. So if you have 10 gauge wire to the siren that means you would use 14 gauge fusible link wire. That is a wire with a special non-flammable insulation. Fusible link wire is supposed to burn up first, protecting the main wire. That is why you buy fusible link wire that is smaller (bigger number gauge) than the wire you're protecting.

Here's a resistance calculator for wires of various lengths and sizes:
https://www.cirris.com/learning-cent...lculator-table

Here's an example of the type of circuit breaker I was thinking about:
https://www.wiringproducts.com/50-am...-parallel.html

Chart for figuring out the right wire size. This is for a max 2% voltage drop and your application is not sensitive, you may be able to fudge on this a little and just have a higher voltage drop and the siren will work fine.
http://info.waytekwire.com/blog/auto...e-gauge-guide/

Re: Help Me Wire Up A Siren
 

Thank you so much for all of the info. I'll do the test soon (when I can get the truck out to the country to run the siren) and let you know what I find. My city neighbors won't like the siren blaring lol.

Re: Help Me Wire Up A Siren
 

Curious minds here! Update?

Re: Help Me Wire Up A Siren
 

Sorry for late reply, I still haven't been able to take the truck to an area where I can test the siren. I ended up running 8 gauge power wire from the battery to the solenoid on the siren. Also installed a 50 amp self resetting circuit breaker in that wire.

Siren seems to work fine and nothing gets hot.


I still want to calculate the current properly, but I will have to wait as I am currently rebuilding all of the suspension.