Install gone bad; Something blew up.

Not a train horn, but just an inexpensive air horn that I bought from a rival(?) site, just because it fit the price range, annnd well, it stopped working after two tries. This is the horn

And this is the diagram I made showing how I hooked it up.

If it matters im putting this in a 93 Toyota Corolla.

When it got hooked up, it worked for two presses, and then stopped. I didnt notice any smoke, but I thought I heard some sort of hissing, but I couldnt be sure I did. I plugged in the OEM horn, and then that didn’t work either. I opened up the fuse box and saw none of the blade fuses broken, I also took out the horn relay, but I’ve no idea if that was blown or not, it’s a coil of wire and I’ve no idea what I’d look for.

Everything else seems to be working fine still, and I haven’t gotten a chance to unhook the battery for a brief time to see if that resets anything, or take a volt meter to it to see if there is anything at the end of the horn connection.

Do you all have any recommendations? Is it possible that horn relay/coil blew? I noticed that the horn relay (there’s no horn fuse, correct? Or is it shared with something?) is a 20A(?) and I’m using 30A inline and the relay is 12v/30a rated (it was provided with the horn) could that be something? Am I using a wrong inline fuse?

But when plugging in the OEM horn, and it not working… that means something went poof… which isn’t good. Again, when checking through all the fuses that I know of, I see none broken, am I missing something? Any clue what it could be?

I thought at first I blew up the compressor, or the provided relay, so I swapped in the OEM horn, but then that didnt work, so I figure the compressor and relay are still fine, and something in the original wiring blew. But what? Im pretty sure I checked all the fuses, and not sure what else there is to it. Button > Fusebox > Horn ? Is there something else that could have blown?


if the stock horn doesn’t work, check the fuses… that is the only thing that makes sense…

all i can think of is fuses, relays, wiring that got cut off, or something broke on your steering wheel button

Did you tap the factory horns wire? and then run to a relay? Look under the hood for another fuse box. Some horns are integrated with something else. Do a voltage check at the horn to see if its present when the button is pushed. Horns have relay yes probably the typical Bosch style relay. If you wire again bring power into relay and horn signal into relay so there is not a big amp draw. The relay will act as a switch. So go back and check everything good. Take your time. It is possible something fried. I would take everything out. and take compressor to you battery and make a fused lead or if you want add a rated switch in. and run a ground wire. Now you can see if the compressor starts. Can check for voltage. If thats a go theres one problem eliminated. And work your self back. Make sure all connections are good. Grounds can cause a problem too.

Good Luck,


Had time today to take a look at things, still didnt work when I first tried, so I went back and unplugged and checked every fuse there is, nothing broken.

However when I tried it again, it worked… (the OEM), so thats odd?

I unplugged the OEM, and put the new one in, and it’s currently working, but when I went to a some what secluded area to test it out how long it could go to see if it would blow something up, it went fine the first press, but every press after that, the horn would sound by itself for 20+secs, when i tried it again after a few tries it didnt stop at all, so I had to pull over and yank a wire.

Any ideas?

Hard to test it on a weekend when everyone is home working, or driving around the streets, so im afraid to try it anymore today.

Now the little testing I did, it seemed to work normally when the engine was off, but when I was driving around, the horn wouldnt go off, could this have something to do with it?

Not real sure why this would be, everything is hooked up according to regulation, as far as I know, I might hav to put a kill switch in or something now. Im debating whether or not to keep it hooked up, ive a feeling id hit the horn on accident somewhere.

when it continues to honk after you press the horn button, that’s because of relay sticking. the ones supplied with the horns are usually crap that have small contacts that get welded together and stay turned on. whacking a stuck relay turns it off. get a better relay to fix that problem.

as for your other problem, look for loose wires. replace the horn with a bulb temporarily and have someone press the horn button while you move wires/plugs and see which one turns the light off when moved.

That’s what happened it seemed. When I touched the relay as I took the wire out, the horn stopped.

Thats odd that it only does it when the engine is on though?

Do you recommend a certain relay/seller?

OEM Horn

Eyehole connects to battery, with 30A inline fuse.

My lolcustom bracket for the horn.

The air ‘compressor’.

The wires.

Relay in.

Finished product.

Im hoping that the issue with the horn continuing to sound after a press is just a relay issue.

Any suggestions what relay and where to buy it from?


the relay sticks with the engine on because the engine is charging the battery = more voltage = higher amp draw for that kind of compressor.

I’ve never experienced stuck relays with the Bosch brand. I usually open up relays if I’m not familiar with a brand. you’d want the ones with real coiled springs in them and large contacts. the ones that use a bent piece of strip of metal as a spring is more likely to have welded contacts.

That makes sense.

Do you recommend a certain Bosch relay? This is the first time ive ever delt with relays so I dont know what im looking for. The one I have currently is a 12V/30A 4 post relay, does it matter if I get a 5 post one, and not have anything connected to the unused one?

I called NAPA, and all they had were 5 posts, but could order a 4 post.

Sorry for the newbiness, but if you have a direct link to one, that would be great.

Bosch is now known as Tyco?

Got to a auto supply with your relay and ask if they have Bosch/tyco relays. I have used them for years in audio and alarms. They work great.

Hope that helps

Hey thanks, thats the one I found googling, good to have it confirmed. NAPA wants 20$ for it lol :confused:

With the extra prong though (87a) what do I do with it? Can I just leave it unused, or do I have to ground it, or?

Those Relays can be wire in different ways. Look at the link I just gave you. Or try this one.

I bought a 5-pin relay, can I still use it as a simple on/off switch instead of a changeover switch?
-yes. simply leave pin 87a disconnected.


Cool. Hope all works out.



I got my new relay in, its a 12V 20/30A relay from Bosch and could feel the difference btwn the one that came with the horn, so good quality.

However… when I went to hook it up, it’s now blowing the 30A inline fuse. Great, so I hooked up the original relay (which was working fine aside from it sticking), and now thats blowing a fuse too…

Wtf? Really? Really starting to get pissed. Wasnt blowing a fuse before, but now it is. But in the OEM and thats still working fine.

So now the new horn wont work at all, and I have NO idea what could be causing the inline fuse to blow when it wasnt blowing before.

What could be causing this?

I just cant make sense of it, why is it blowing a fuse on both relays now, when it was working fine a few days ago? Should I push back to a 20A fuse? I bought a 30A inline holder, is it ok to use a 20A in that?

Did something happen in the ‘compressor’?

OK, false alram it seems, I started going over and thinking what causes a fuse to blow, so I figured a wire went loose somewhere, and there was, so I rechecked everything, buttoned it all up, and now everything works perfectly. For now!

But a road test and everything went great.

Thanks again for all of your help everyone.

Good deal!

Hate to admit it, but I had a small problem with a relay, it arc’d and kept one of the compressors running… also another thing I have noticed, it is really really important to make sure you have a good earth/ground at all times when doing any install… I used the bed of the truck as a ground but when the tanks were filling, my fuel gauge went to zero…the install now has two new gounds straight to the compressors!

All working hunky dory now!:smiley: