Horn circuit weird fault

[Jay Peg]

Hi there,

I have a strange fault on my horn. When I turn the ignition key to on, the horn works after starting the engine and engine is running the horn is dead.

Anyone have any ideas what this could be?

Thanks

Jason

[Robsey]

That is really odd, as you have a very early "simple" wiring set-up.

From what I can see the horn is wired almost directly to the ignition switch terminal "15" via a fuse.

Fuse 12, which looks like it also supplies the reverse lights.
Are these affected too.?

Just before the fuse, the feed wire from the switch also goes to the ignition coil.

So the only way my brain can seen why it would stop working with the engine running, would potentially be if there is a poor / dirty earth at the horn, meaning too much resistance for that circuit to work, when something else (coil or reverse lights) are also in use.

First suspect on these 40 year old vans is to check and clean all the earth points, both low down on the horn itself...
And because faults come in threes, it is worth checking the earth crowns on the left a-pillar above the fuse box.

I did consider the ignition switch, but then a lot of other electrical items may have been affected.
Such as an intermittent starting problem due to the coil supply being disrupted.

In simple terms, the only way to know, would be to trace the whole circuit and clean all connectors and contacts as you go.
I still suspect the earth on the horn itself as it is exposed the elements.

[TONYT25T25]

Did it previously have an alarm system that triggers the horn maybe remnants of it causing problem.

[Jay Peg]

Thank you for your replies, useful! I will definitely follow that advice! It didn’t have an alarm but the horn use to work. It makes sense what you’re saying I will try that.

Best

Jason