Vintage BMW Motorcycle Owners

Hoping someone can help me diagnose the Klaxon horn on my '65 R69S. I purchased the bike with a non-functional horn, previous owner didn't have state inspections and so never bothered with it. Here in NY I need it to pass, so started looking into it. Quickly realized the horn button on the headlight switch wasn't even wired.

I installed a set of Hella bar-end turn signals and decided to wire the horn up there. After I finished, turn signals worked, horn didn't. After spending an hour tracing my wires I realized it was the horn itself. Wouldn't work when directly wired to the batt, had no continuity across the contacts.

I took the horn apart and cleaned up the contacts. Inside had a lot of corrosion and the membrane was torn. Put it back together (I think the right way...) and now had continuity. Wired it up and now it sounds like this:
https://www.youtube.com/watch?v=qEmEVsd ... e=youtu.be

It's basically a little ping whenever I release the button, which I presume is the electromagnet releasing. But, no actual horn sound.

Any idea what might be up? Would the torn membrane be the issue? I figured it would at least buzz. Is there a diagram of this horn available anywhere? I searched around but couldn't find much.

Thanks in advance!

Does the horn work at all when the terminals are directly connected to the battery?

It's really a simple circuit. When one side of the horn is ground (via the horn button), current flows and causes the electro magnet to energize. That pulls on a connection to the diaphragm and then the points open, stopping current flow. The spring in the system pushes it back so the points close. The process repeats over and over. It is the pull and release of the diaphragm that causes the sound. You should chase this circuit.

The points need to be clean. Also, isn't there an adjustment screw on the back? This should be experimented to find the best position for sound.

Carefully take the horn apart. Inside you will find (if it is like mine) two sets of “breaker” points. Clean the contacts on those points, then use the screw on the body of the horn to adjust the amount the diagram moves as the points make and break. Too much and the horn won’t sound, not enough and the points will stick closed.

Icarus

There’s a small adjustment screw on the back side of the horn. Maybe a 10mm? Quite possible all you need to do now is turn this in or out until you get the desired tone. Worked for me after a proper cleaning. (Thanks Dave!)

Thanks, all, I did fiddle with the adjustment screw but perhaps not enough. Will go back at it!

@Kurt no, there's still no sound when connected directly to the battery. Does polarity matter? The terminals aren't labeled, that I saw.

Polarity shouldn't matter.

Polarity doesn’t matter on the bench, but it does on the bike, since the button grounds the circuit. Wiring it backwards on the bike will make it go all the time iirc. Icarus

The polarity does not matter
It does not matter what side gets power as long as the other side gets grounded by the horn button
Tommy

I have same horn and it did the same thing. Andi at Barrington fixed. Very reasonable.

With the horn on the bench connect an Ohmmeter across the terminals; anything other than "open" is good. When the horn button is pushed current goes through the coils in the horn and pulls the diaphragm down. When it comes down it opens the points, breaking the circuit. The diaphragm bounces back up and closes the circuit and the process repeats very rapidly.

If there are two screws on the case, one holds one end of the point bar and the other spring loaded screw goes to the other end of the points bar. There is a pusher from the diaphragm to the contact point below the point bar.

For the horn to sound the points need to be closed at rest. After the points are cleaned the resistance at the contacts should be small. If the circuit is open the pusher is down too far. Screw out the sprung screw until the resistance comes back. Check continuity between the inside of the horn and the outside connections. Rig a battery with leads and a switch. Polarity doesn't matter. Hook up the leads. If you get a toot adjust the sprung screw for loudest sound. Put the horn back together. Check it again for the loudest sound. Play with the sprung screw to get a feel for how the adjustments work. When the horn is mounted the adjustment may be different.