Press horn, lights go out!

[Revvers]

Hello all. As the title says, when I press the horn on my ‘65 Li Special, both the rear and front lights go out. It’s a non battery model. Any ideas please??

[Toddy]

I would say you have a bad earth somewhere have you checked anything yet ?

[Revvers]

No I haven’t. I’m thinking of taking the headset off on Sunday and hoping there’s something obvious!

[Toddy]

I would see if anything’s popped out or touching anything

[Revvers]

Will do. Cheers. Just checking for the Mot and noticed it

[Toddy]

Do the lights go completely out or just dimmer ?

[Fast n Furious]

Horns do pull a lot of current and will dim the lights on batteryless setups.
If it's killing the lights and not actually sounding off, then the horn is, in effect, creating a short circuit.
Are you certain that the horn is one for an AC model? ( if you are unsure, then just connect it across a 12V battery to find out)
Try a substitute horn!

[Revvers]

Do the lights go completely out or just dimmer ?

They go completely out

[Revvers]

Horns do pull a lot of current and will dim the lights on batteryless setups.
If it's killing the lights and not actually sounding off, then the horn is, in effect, creating a short circuit.
Are you certain that the horn is one for an AC model? ( if you are unsure, then just connect it across a 12V battery to find out)
Try a substitute horn!

It’s the original horn and switch and the lights go completely out

[Revvers]

I’ve had a look at my wiring. Sorry, I was wrong in my initial post, the lights don’t go completely out they dim quite a bit. I cannot see any breaks and the earths look ok. When I press the horn, the headlamp dims and the back light is almost out. The headlamp is a Jockeys one and I disconnected that to see if that was the issue but the rear lamp still dimmed.

[Fast n Furious]

Does the horn work when the lights are switched off?

[Revvers]

Does the horn work when the lights are switched off?

Yes it does

[Fast n Furious]

It's normal for the lights to dim when using the horn on an AC setup, especially at low revs.