I think I want to politely disagree, though tentatively since it has been
over 10 years since I rebuilt one of my horns. The other is still working in
unrestored condition. Lucas reliability!
I believe Ed has an internal short in the horn. This was the problem with
the Windtone horn I rebuilt.
I assume you are talking about the Lucas New Alto horn, though I confess I
thought these were on pre-war cars only.
The horn works by a set of contact points opening and closing as a solenoid
pulls the vibrating diaphragm in and out. Its a little bit like the SU fuel
pump, though not quite the same.
Disconnect both wires, wrap a bit of tape on them to insulate them from
ground (earth), and see if the other horn toots or the fuse blows. If the
fuse blows your problem is elsewhere. If the other horn toots, connect a 12v
bulb across the two horn wires and see if tooting the horn lights the bulb.
If it doesn’t, you have another bad connection somewhere.
One wire is the power from the relay to the points, the other just goes out
of the case and directly to ground. Yes, they could have made that wire
internally grounded, but they didn’t.
This description is for a Windtone, on the assumption that a New Alto is the
Rob Reilly - 679187