Air Horn circuit problem

I do a lot of interstate driving and after a couple of B-double trucks moving into my lane without hearing my horn I have been fitting a Stebel Nautilus 12v Compact air horn to each of my vehicles.

The wiring setup is very conventional, taking power directly from the battery and protecting any feedback to the trigger source by using a 1a 12V diode (1N4001).


The vehicles I have used the above setup include 5 BMWs and 3 Audis.

Now, for the first time, I have a problem.

The issue is that while the air horn works fine when tested (brief blast so as not to annoy neighbours), when used in anger it works fine for a second and then goes flat (a farting level noise) and the stops working at all.

If I leave the car overnight and try again, it works fine again for a second and then goes flat and stops working again.

I have tried replacing each of: the diode, the wiring connections, the relay and the horn itself but the problem persists.

Any suggestion as to what might be causing such strange behaviour?

Just a thought - how old is the car’s battery? Sounds similar to an issue I had with a car alarm that ran directly off a car battery that didn’t hold much charge.
Have you tried driving for a few kilometers after the first test of the horn (to let the battery charge up) and then testing the horn again?
If you’ve tried replacing everything else to no effect, the battery would probably be the next thing I’d try swapping out.

Thanks for the thought Nick
The car is only five months old and the battery is fully charged.
Even driving for an hour or more does not change the problem.

check to see if you have the relay wired correctly
you may just be running power from the standard horn through the diode through the coil and out to the air horn…this makes sense to me…
failing that try the below
i would put a rectifier diode across the coil of the relay…

cathode to pin 86 and anode to pin 85…an 1n4004 would most likely do the job

also the diode going to pin 86 i would put at least an 1n4004 1 amp 400 volt or even change this to an 1n5404 would make a difference depending on the coil resistance…
dont forget that the diode will give a .6 volt drop…

if the relay is finicky…it may be casing a problem…

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Thanks Brian, I will try your suggestions

Brian, confirming this is your suggested circuitair horn circuit

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that should be ok yes …

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Brian, a belated (delayed due to accident) thank you - that solved the problem

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