By the early thirties the American motorcycle industry had switched from motor driven horns over to diaphragm horns. Diaphragm horns work on the electromagnetic principle. A spring steel diaphragm with a ringer disc is positioned over an iron core that is wrapped with wire. When electricity flows through the wire, the core becomes an electromagnet and attracts the diaphragm in. As the diaphragm is attracted in, a little tit on it causes a set of points to open, breaking the circuit and stopping the flow of electricity to the wire around the core. This causes the core to loose it's magnetism. With the core no longer magnetic, the diaphragm pops out allowing the points to close and the cycle is repeated (for as long as you keep the horn button depressed). As the diaphragm is pulled in it strikes the core causing the ringer disc to produce a tone. That's a very basic overview of what should be happening when you press your horn button.
There are basically two adjustments you can make. First is to the points. A small headless screw with a nut on the back of the horn adjusts the relationship between the points and the tit on the diaphragm, thus determining when the points open and close. The second is to the air gap between the core and diaphragm. This rarely needs adjustment (in fact the Harley service manual warns against changing this adjustment from the factory setting). For the Delco Remy horns this is the large screw and nut at the front of the horn in the center of the diaphragm (it actually holds the ringer disc onto the diaphragm). For the Autolite horn the air gap adjustment is on the back of the horn in the very center. A long headless screw that passes through the center of the core.
Now, lets say your horn isn't working. The most likely cause is dirty points. You'll need a multimeter (preferably one with alligator clips on the test probes) to do the following tests and adjustments (just as an aside, I fail to see how a household can function without a multimeter yet, it seems, the majority do). So ... take your multimeter and set it on ohms. Check each terminal post against the horn body. You should have infinite resistance. If not, either a terminal post insulator is missing (or rarely) the core winding is burnt out. Assuming everything is O.K. next verify you have virtually no resistance (less than half an ohm) across the two terminal posts. If you have resistance, you either have dirty points, the point adjustment screw is in too far (isn't allowing the points to close) or a wire is broken inside the horn. Resolve that (clean the points with crocus cloth, back out the point adjustment screw or fix the broken wire). Once you have continuity between the terminals squeeze the horn to flex the diaphragm inward (you'll need the horn face off to do this). This is simulating the magnetic core pulling the diaphragm in. The resistance should now go to infinity (because the points have opened). If not, adjust the point screw until it does (don't go wild turning the screw in or you may break something). If, after attempting to adjust the points, you can't get the points to open when the diaphragm is squeezed, either the diaphragm is installed wrong (tit not over the points), the tab the tit pushes on is broken or (rarely) the air gap adjustment is not allowing enough diaphragm movement. Once you've resolved that, the resistance should range from infinity when the horn is squeezed to zero when it is released.
At this point the horn should at least give a click when voltage is
applied. Then it's just a matter of fiddling with the point
adjustment screw until you're happy with the tone. If you back the point adjustment screw out too far, the points don't open and the core remains magnetic and the diaphragm is stuck in. If you move the point adjustment screw in too far, the points open before the diaphragm strikes the core and the ringer disc does not produce a tone. You want a happy medium where the points open just as the diaphragm strikes the core. If you can not get an effective tone by adjusting the points, then it's time to adjust the air gap. Of course changing the air gap necessitates readjusting the points.
The above steps should get most horns working. In rare cases the
points may be completely toasted or the winding around the core burnt out
(a quick test of the core winding is to disassemble the horn and apply
voltage, if the core becomes magnetic and attracts a screwdriver tip held
over it, the windings are O.K.. Don't leave the voltage applied for
more than 10 or 15 seconds). Another check is that the winding around
the core is electrically isolated from the core. You do this by touching
one probe of the multimeter against the core and the other probe against
one of the terminals. You should have infinite resistance.
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