second air horn in fj

Hey gang, I need to install a secondary air horn (read semi legal siren) in my FJ. Now I need to have a switch, that could select between stock and air horn, and it has to be very discreet. I'm thinking of converting one of the blanks to a On/Off switch (so i've been advised) to a switch...but I have no idea how to go from there. Anybody care to try to explain it? Another way is having an on-on switch but that is usually a rocker switch which is noticeable in the scenario if i get pulled over. Any ideas?

The switch type you need is a SPCO it interrupts the (-) horn trigger in the steering column and feeds this to a second relay to drive the alternate sounder. Find a matching switch? Good luck. Anyway, cops will pop your hood and there it is, why would they even look for a switch? wth is an FJ?

Howie i think hes talking about a toyota FJ cruiser. dont know if you guys have those over there but its toyotas rendition of a Hummer. you can use any regular on/off switch and a SPDT relay to make your horn switch. might need another relay if the factory ground for the horn isnt strong enough to activate the air horn. good thing about this is that the switch would only make the air horn live, you still need to blow the regular horn to make the air horn blow. you might be able to buy a factory switch that is meant to go in one of your blank spots, possibly a fog light switch, and just use that as your activation point.

howie ll wrote: The switch type you need is a SPCO it interrupts the (-) horn trigger in the steering column and feeds this to a second relay to drive the alternate sounder. Find a matching switch? Good luck. Anyway, cops will pop your hood and there it is, why would they even look for a switch? wth is an FJ?

hi Howie, It's Toyota FJ cruiser, not available in Europe. Compressor and horns will be stealth fitted at well behind the bumper. Problem is I already have the damn thing and have a serious itch to put it on for fun:)

soundnsecurity wrote: Howie i think hes talking about a toyota FJ cruiser. dont know if you guys have those over there but its toyotas rendition of a Hummer. you can use any regular on/off switch and a SPDT relay to make your horn switch. might need another relay if the factory ground for the horn isnt strong enough to activate the air horn. good thing about this is that the switch would only make the air horn live, you still need to blow the regular horn to make the air horn blow. you might be able to buy a factory switch that is meant to go in one of your blank spots, possibly a fog light switch, and just use that as your activation point.

Toyotas rendition of a Hummer? That is an insult and now you owe me that diagram :D no it is a rebirth of Toyotas legendary Land cruiser BJ40 way before hummers existed Siren definitely gets its own relay since compressor is around 120w, but I just don't have the knowledge what goes where and cannot figure it out myself, so if anybody would be so kind to draw me a diagram? I'll try to make one up myself so you guys can see if I'm wrong. Thanks.

gromov wrote: Land cruiser BJ40

Sorry, FJ40, BJ was earlier than that

Ground to on-off switch matching a regular switch, e.g. a fog light switch, then to the relay mount under hood, preferably coated in water retarder, terminals underneath as follows. Switch to 85. Battery via 15amp fuse to 86 and 87. 30 to horn or siren.

Sorry, last post was incomplete. (-) horn wire in steering column loom to switch then to ground:- Find the original horns, two wires one will be either (+) or (-) constant the other will be open circuit and only go to (+) or (-) when you press the horn button. If it's neg (-) take this to 85 of the relay then wire as above. If it's pos (+) more likely take it to 86 then; 85 to ground. 87 to 15 amp fused to battery. 30 to compressor or siren. This will give you regular or both. There's yet another way, up your original horn fuse to 20amps keep the switch and wiring as above then take the horn lead that goes (+) when you press the horn directly to your siren/compressor grounding the other side. If the lead at the horn goes neg (-) again take that to the compressor and take the other side to the battery via a 15amp fuse, no secondary relay required.

you can try to interface at the horn like howie said or you can go straight to the source and keep everything inside. the factory horn wire on your FJ is GREEN/ red, meaning green with a red stripe. its polarity is negative, meaning when you press the horn this wire will show ground on a voltage meter. the wire is running along the steering column in the main harness and its probably really small gauge. you should take the plastic panels off of the steering column to get to this wire. once you think you have found it you need to test it with a digital multi meter, not a test light or anything else because these wires will be ran with data wires and airbag wires. if you dont have a multi meter dont try this until you have one. if you do have one then the wire will show around +12v at rest and when you honk the horn it will change to ground and show no voltage, when you release the horn it will go back to showing +12v. once you are 100% sure its the right wire, you need to cut it and probably extend both sides so they will reach where you will mount the small SPDT automotive relay. when you cut the wire you will have two sides, the side that runs toward the steering wheel will be connected to the relay terminal marked "30". the other side you are left with will be connected to terminal "87a". when you are done with that part, honk the horn to make sure it still blows. terminal "85" should be hooked to an accessory wire. the accessory wire should be WHITE/ green at the ignition switch which is where you put the key. this wire will only show +12v with the key in the run position and will also turn off while you crank the starter. when you find this wire dont assume its the right wire, test it and make sure. terminal "86" of the relay needs to be screwed to ground and ran through whatever you will be using as your on/off switch. the last terminal on the relay should be "87". this will go to the negative side of the air horn (not the compressor). the air horn should have two wires on it and they might both be the same color. this means that they arent polarized and it doesnt matter which one is + or -. the wire that you have left at the air horn after you connect one side to the relay's 87 can either go to 12v accessory or 12v constant. which one you hook it to depends on if you want the horn to be able to blow when the car is turned off. lastly, the compressor will have two wires. the ground (usually black) will go straight to ground. the red NEEDS to go to a high current accessory, dont hook it straight to the battery because if there are any leaks at all it will just sit there and run all the time until the battery is dead. to do this you will have to run it through another relay because the accessory circuit probably wont have enough to power it directly nor would i chance it blowing a fuse in the car. use THIS DIAGRAM to wire that relay. the "remote input" will go to the car's accessory and the "fans" will be your compressor +12v feed. run the +12v from the relay straight to the battery using some 14 gauge wire or thicker and you must fuse this wire close to the battery. look at the spec sheet for the compressor and it will tell you how much current it should draw and use a fuse that is as close as possible to what it is rated for. hope that wasnt too confusing, its not as hard as it might sound but i also didnt cut any corners either.

Soundandsecurity, nice points but there are 20 ways from here to Sunday to do this. But air horns AND compressor with wiring? Also every compressor I've ever seen was polarised, why you want it to blow not suck. Lastly, by ISO convention, 85 is the coil NEG, you'll run into trouble using 86 as the NEG if you ever come across a relay with built in diode.