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C1Nelson,
The field winding are different on a 12 volt horn than a 6 volt horn. On a 6 volt horn the field windings will be 20 gauge, which is 0.032 inch in diameter and on a 12 volt horn the field windings are 24 gauge which is 0.020 inch in diameter. The field windings are the stationary windings on the side of the motor. You can use calipers or a micrometer or a sparkplug gauge to measure the size.
Take the cover off and measure the windings. Then examine the connections of the wires, the condition of the commutator, and the rotate the armature to make sure it rotates easily. The contacts should be clean and bright and some electrical grease helps to keep the corrosion down. The commutator is the rotating part where the bushes rub and they it should also be bright and shinny. Clean with some solvent and polish with some 400 grit sandpaper. Oil the front and back bearings where the felt pads are. Just a few drops of oil.
Now loosen the adjustment screw a few clicks and see if the motor turns. If not, try to find the fault with a volt meter. Once you are getting the motor to turn, tighten up the adjustment screw one click at a time until you get the sound you want. Do that again with the engine running so that the battery is charging.
Button things up and you are ready to go.
To convert a 6 volt horn to 12 volts follow Tom Eddy's instructions at
http://www.santaanitaas.org/wp-conte...orn-to-12v.pdf