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Courtesy of Carl Hoseit
Listen for an audible relay click at the fuse box when you push a horn button on the steering wheel. With just the ignition on without the engine running you can hear it with your door open. If the relay is working you can test for voltage at the leads that connect to the horns. Odds are the problem is corrosion in the horn(s) itself. There are contacts similar to ignition points inside the horn. These become corroded over time and can cause horn failure.
To disassemble the horn you cannot pry the ring off. Use a small cutoff wheel with a Dremel tool and cut a split in the outer horn ring edge that holds the diaphram and cover to the case. A small sharp edged file or Zona saw could be used to substitute the cutoff wheel. Cut just far enough in to split the ring for removal. Pull the ring off from the front. Remove the front bolt. Note that the threaded nut post is adjustable but you should not need to tweak it. Pull the cover and diaphragm off. Keep track of the three washers that go between the two plates. There is an ultra thin paper gasket ring between the cover and the case. Try to keep this in one piece but it is not critical.
First take a close look at the coil windings. This is your basic electromagnet with wire wound on a bobbin. If it looks burnt /dark brown or black your horn is toast. Rewinding the coil would be possible but the terminal posts are also the rivets that hold things together inside, making a coil rewind much more trouble than it’s worth. Assuming the coated magnet wire coil still has a bright copper color, look for the contacts that resemble ignition points between the two silver plates. Give the entire works a good shot of electrical contact cleaner, WD 40 etc. Proceed like you would cleaning ignition points. Pry the contacts apart using a screw driver between the plates just enough to slip a small folded piece of 400 or similar fine sandpaper between the contacts. Let go of the gap you have with the screwdriver and pull the paper out. Use more contact cleaner on the paper and repeat several times. Clean any corrosion or rust from all the inner working parts.
Now is a good time to bench test if you can with any 12V source AC or DC. It does not matter as far as which terminal to make positive or negative on the terminals either. Just hold the cover and case together and test. If no buzz repeat contact cleaning. If still no buzz, go to the back of the case and pull off the black rubber botton. This covers a torx screw (T15?) that tunes the horn. Counting the turns, back the screw out and in several revolutions. Bench test again. If still no buzz ….well…..punt or check back in this FAQ for updates.
Reassemble everything and replace the retaining ring. There may be a slightly larger gap than the original cut which is okay. Just try and join the gap as much as possible by rolling and pushing the horn on its edge to seat the ring. Lay the edge of the horn on a vice and carefully peen the ring surface down. It does not take much.
Clean the back edge of the ring and apply liquid rubber or silicone sealant to the back edge of the ring and the cut. Put some sealant and the rubber cap back over the torx screw.
I will try and add pictures and updates asap
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