AJS & Matchless Owners Club

When I sound the horn, with the engine off, the ammeter alarmingly swings to indicating a positive (+) charge and the horn emits a rather strangulated and tinny "eek"! . Do I take it that the horn's terminals might be wired in reverse? When the engine is running, however, it emits quite a healthy "beep". A new fully-charged 6v battery has just been fitted, so it's not a low voltage that is causing the strangulated sound! Any ideas, please?

I am going guess there is a poor connection somewhere.

For arguments sake let's say the horn uses 1A at 6V (6W) but you are seeing the ameter hit 5A. Thanks to maths we can assume that the voltage being seen at the horn is 1.2V. So you may well have a route to the horn but it's loosing a lot of the voltage.

Do you get the same high power issue with the lights?

Horns are not polarity sensitive and it should be wired directly off the battery so shouldn’t cause a deflection on the ammeter.
If you disconnect the wiring loom connections from the horn and connect your new battery straight across the terminals i.e. negative to one terminal, positive to the other ( you only need a momentary connection for this as it will draw high current) does the horn sound immediately and with a good note? If so the fault lies in the loom somewhere possibly within the switch if it is combined with the dip/main headlamp switch.
If the horn sounds the same as when normally connected to the loom, then the horn is faulty or needs adjustment.

Reynard24 wrote: ↑Sun Oct 29, 2023 8:35 am Horns are not polarity sensitive and it should be wired directly off the battery so shouldn’t cause a deflection on the ammeter.

It may be that when you look at your loom, the horn power supply is actually taken off the battery-side terminal on the ammeter - which is 'the same difference'. (There are small differences in period wiring diagrams.)

Should be no discharge with the engine stopped, but you SHOULD see a movement to the charge side with the engine running much above tickover if you push the button. Same will apply for a brake light wired directly from the battery. This is caused by the dynamo (or alternator) being asked to compensate for the sudden load by the voltage regulator. (Useful tell-tale sometimes that a brake light bulb has failed . . .)

Somthing just came to mind....is it a 6V horn not a 12V.

Thanks fellas. I take on board what you suggest. But why on earth does the ammeter swing to positive? It doesn't happen with the lights, which act normally and registers a discharge. The wiring loom is fairly new and so I don't think there's a problem with that. Anyway, I'll check things out as you suggested. Honk!

Mckorkins wrote: ↑Sun Oct 29, 2023 10:16 am Thanks fellas. I take on board what you suggest. But why on earth does the ammeter swing to positive? It doesn't happen with the lights, which act normally and registers a discharge. The wiring loom is fairly new and so I don't think there's a problem with that. Anyway, I'll check things out as you suggested. Honk!

Because the lights are wired through the ammeter.
Anything not wired through the ammeter will create an 'unseen' drain on the battery when activated. The regulator is in constant 'discussion' with the battery and 'has its back' as it were, so it detects the drain and asks the dynamo (or alternator) to make up for it.
The regulator also asks the dynamo or alternator to compensate for the loads imposed by the lights as you say, but they are already recognised as a drain by the ammeter, so what you see is the needle swing positive as the engine revs and then towards negative as revs drop to tickover or the engine is stopped.

Mckorkins wrote: ↑Sun Oct 29, 2023 10:16 am But why on earth does the ammeter swing to positive?

Is that with the engine stopped or when it's running (at above tickover speed)?

Spriddler, it's with the engine stopped.
I dismantled the h/b switch just now and no problem found, as there's nothing to go wrong with it. The ammeter still swings + when horn pressed. All else is normal. A head-scratcher.

jackstringer wrote: ↑Sun Oct 29, 2023 10:10 am Somthing just came to mind....is it a 6V horn not a 12V.

Definitely 6v!