Talk Morgan Forums Morgan Sports Cars Classic Morgans (pre-CX) Horn Button

If anyone raises an electrical query on the Citroen Specials forum the first reply is always "it's the green wire" as Citroen largely used one colour...

Peugeot wiring used to change colours when it passed through the bulkhead! That and the wires were identified by numbers printed on them so small you needed a powerful magnifying glass to decipher them.

The wiring itself was strange in that it used to suffer from oxidisation which result in a black deposit on the wires which resulted in much scraping with a razor blade to get it clean enough to solder.

To get back to the original post, on my 1971, you can't sound the horn if the headlights are on main beam! Not sure what, need to investigate!

Arwyn

Likely a high resistance connection in a shared circuit such that the extra current from the headlamps increases the volt drop (E= I x R) depriving the horn from a great enough voltage to allow function. A voltmeter would prove this and a rewire would cure it.

ETA: just checked the wiring diagram 1969-1973 and it shows the horn and rad fan sharing a fused supply taken directly from the starter solenoid in turn close to battery positive. It was common practice at the time for the horn to receive this direct supply bypassing the ammeter. My Velocette is wired so giving the added advantage of proving the dynamo is working as it shows current flowing to battery to compensate when horn operated.

Advice remains the same regarding voltage checks but also confirm wiring is ex factory as above, and if so check this dedicated wiring to horn fuse. The electrical path from battery neg to chassis is also common to headlamps and horns as well of course so worth checking including possible shared ground connection. Finally although unlikely, it could be a near dead battery with the headlamps taking most of remaining current when engine not running or at idle.

A common problem with radio control models, too.

And in boats which is why we always use tinned cable these days.

Just wanted to tidy up a loose end here.

I traced the wires back and the purple black wire from the horn switch doesn't go directly to a horn as I thought it would but instead to a transformer of some kind

The transformer also has a yellow cable going into it, which I haven't traced yet, but I'm guessing it is this yellow wire on the fuse box. The black wire from the transformer connects to the positive of the air horn , which, presumably, is an after market modification.


Next step is to trace the yellow cable to confirm, confirm connectivity on all three wires ( yellow, black and purple/black ) and if it's still not working, I assume I either have to replace the transformer or, if that's not possible, get a new horn entirely - recommendations welcome.

Perhaps this is all standard fare for the more experienced Morgan owners - I'm still climbing that hill, probably in the climbing over the stile to get on the footpath stage

Umm, probably a relay rather than a transformer.
Air horns can take a quite hefty current which would quickly destroy a horn switch.

Yeah, as Graham says, it looks like an old relay that's lost its cover.

I imagine it will be purple/black to the switch, black to the horns, yellow to 12V live and then an earth somewhere.

New relays are pretty cheap, so I'd just stick a new one in.

As the horn doesn't work I would replace that relay.

Yes agreed any 4 or 5 (ignore 87a) pin 30 amp relay will do the job. I would rewire so the horn push provides a ground connection to relay pin 85. Pins 86 and 30 can then be looped together with permanent fused +ve feed. Finally pin 87 can feed air horn positive with other side of air horns to ground as they probably are already.