I rebuilt my original 65 horns over the weekend with the kit from Zip Corvette.
When voltage is provided to the horn, a small coil is energized which pulls in the plunger on the diaphragm. This movement also causes the switch to open and the plunger to reverse due to a small shoulder on the plunger that operates the switch as it is pulled in/out. It is this switch cycling and plunger movement that makes the diaphragm vibrate and create the sound produced by the horn. I found that the plunger on the diaphragms provided in the kit was a little bit longer than the original and when assembled caused the contact switch to always stay in the open position, thus rendering the horn inoperative.
For one horn, I created a duplicate gasket out of cork, which with the additional thickness took the preload off the switch and let the switch be in the normally closed position thus allowing the horn to produce sound. For the second horn, I thought the original diaphragm sounded better/louder than the replacement from the kit, so I reused the original and did not have to make the shim.
The main reason my horns did not work was that the contact switch was heavily corroded. Both horns read open circuit prior to disassembly with a Fluke 87 DVM. I did not like the file that came with the kit and instead media blasted the contacts. After blasting, I was able to obtain a reading of 0.8 to 1.0 ohms, which I concluded was the resistance of the small coil.
I did not do any media blasting on the original diaphragm that I reused. I did not want to warp it or do anything to alter the tone it would produce. I did do some light cleaning with a scotch brite hand pad. Reusing original diaphragms might not always be possible though depending on how corroded the diaphragms are in the cores being rebuilt.
If you want to get really picky, the rivets provided in the kit were a bit too long. For the hardcore concours restorer, one would want to trim the rivets a bit and create a waffle pattern die for crimping. I had about 3 hours in this job and to go that next step would have doubled or tripled the time required. I was just happy to have original horns that are riveted together instead of the screwed together reproductions. I was able to tune the horns to C and F by using the adjustment screw on the horn and a guitar tuner, so when activated together the horns actually sound good.
Here is a picture where you can see the corroded switch (green colored area). When you take the screw out of the horn to spray WD40 or some type of contact cleaner, the switch is spring loaded closed. For best results, I would try to stick a very small pin in the screw hole and force the contacts open, then spray the cleaner. This way the cleaner will have the best chance of getting on the switch contact surfaces and actually doing something.
