Non-functional horn, strange symptom

[drj434343]

I've been troubleshooting a non-functional horn on my 1983 Westy, including reading past Samba threads on the issue.

After tracing the ground side of the circuit through the horn button and not finding obvious issues, I pulled the horn itself and worked directly with the harness at the horn.

I can confirm battery voltage at the black/yellow wire at the horn connector with the ignition on. This is with a voltmeter.

Now things get interesting. As soon as I put any load on that wire, the voltage drops to zero, but no current appears to be flowing. For instance, I grounded the original horn directly to the chassis to take the horn ground out of the equation, and when I connect the black/yellow wire to horn, nothing happens and my voltmeter reads 0V across the horn contacts.

I bought a new horn which I was going to install as a part of this project. This new horn does the same thing, as soon as it is connected to the black/yellow wire and grounded right to the chassis locally, voltage goes to 0 and nothing happens.

The best test is when I take the black/yellow wire and touch it directly to the chassis. Voltage drops to zero, there is no spark on contact, and the fuse doesn't blow. I can hold the black/yellow wire shorted to the chassis as long as I want, nothing gets hot, the fuse never blows, and when I disconnect the black/yellow wire, the voltage goes right back to 12+ battery voltage.

I've swapped in a new fuse with no change.

A voltage drop to 0 when a high load is requested suggests some kind of current dependent break in the circuit, but I can't figure out what that would be.

The wiring diagram indicates the black/yellow wire comes directly from the #12 fuse which is powered with the ignition on.

What am I missing here?

[tencentlife]

There's some massive amount of resistance somewhere upstream on the hot side.
_________________
Shop for unique Vanagon accessories at the Vanistan shop:
https://intrepidoverland.com/vanistan/

Please don't PM here, I will not reply.

Experience is kryptonite to doctrine.