Electrical question - intermittent load = stalling engine


Hey guys, im at a bit of a stall point here (pardon the pun)

Car: 02 Mirage, 4G93 converted
Conversion was done before i got hold of it. Ive had the car for a couple of years now, and slowly getting on top of small issues that have bugged me (mainly tidying up stereo and under dash wiring)

The issue id taken the longest to sort was the most frustrating: car would stall if i was on idle and used the horn. Previous owner had fitted some "triple chrome" airhorns. You know, the Autobarn / Supercheap type with the little compressor and relay that you hook up to your standard horn wiring.
Mate of mine put a new relay inline, and all worked but still had the issue if it was more than just a quick "blip" you'd stall it; of course, when you need to toot someone for whatever reason (and youre stationary), itd stall on me. Revs, no worries, only stalled on idle

Thought we had it sussed that it was the load from the compressor drawing too much, and forcing a stall. So in the bin they went, and found a spare set of supertones. Threw them in, using the same relay we'd put in for the old horn arrangement. Still stalls it if you hold it for more than 2sec.

Battery: replaced last year, still got good charge
Alternator: assuming its the original from the conversion, tested and still throwing a good voltage
Load tested the horn circuit with light instead of sound (series connected a pair of H4 headlight globes) - no stalling

Whats puzzled me is nothing else stalls it. My amp/sub probably draws more than the horn, and that can be up near on flat out and not stall the car. That and the above load test on the horn circuit eliminated the horn relay / wiring (as the 2x \H4s draw more than a pair of supertones)

Am i missing something here?
In the scheme of things, its not a huge issue, its just really really REALLY annoying if you forget haha


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Sounds like this is a lot of fun, sometimes we overthink it and forget about basics, so when you said your mate put a relay in, Did he wire it up the relay to take power directly from the battery or somewhere else, if the answer is somewhere else, disconnect it and run wire directly to battery, see if it still happens, that will rule out one option to start with.
Does the car have a good earth connection?
If that doesn’t work might have to look at some type of feedback when air pump motor is actuated, hard to diagnose without seeing the setup, but you have to start somewhere
Second suggestion would be to remove air pump , wire up pump directly to battery, bypass horn switch, relay and see what happens then.
I may be a case of having to run sep horn button directly to batt if that works, battery should act as a capacitor to filter any feedback, so try that.
Basically go back to the beginning and check each step of reinstall from there.
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Personally I would measure the voltage across the battery with the engine at idle, then idle with the horn being used. Then with engine at 2k rpm, followed by 2k rpm with horn being used. This information is useful for further diagnostic. If the battery voltage always stays constant, above 13.8volts, you can rule out battery, and alternator as the cause of the fault.

I would more than likely suspect that the power supply for the horns is piggybacked off of an engine related actuator, like fuel pump or even just the ecu itself. Hell it, could even be something weird, like the horn pad circuit cutting the ignition on signal from the ignition switch.. Something not right..


Sorry for the delayed reply here

So the airhorns have been removed and replaced with supertones. Factory horn relay in engine bay fuse/relay box has been bypassed on one pin to supply the horn relay originally put in for the airhorns

We tested the power / earth independently to the horn relay / horn button, and the same stalling resulted (ie basically powered the relay straight via the battery). So does exactly the same as it did when the airhorns were installed. Thats what had us perplexed)

I know this much, it doesnt stall it with revs on (used to have to creatively heel/toe if i was at lights and needed to beep someone ), will have to do that secondary test and see

Thanks for the input so far, worst case i just have to keep improvising with revs when i require horn haha