I bought a used, working motor driven horn. There are no markings on it, is there a specific + and - post or does it not matter.
Unless it's a permanent magnet motor, which is highly unlikely, polarity won't matter. It will rotate in the same direction regardless.
Just take care that your terminals are well insulated from the horn body.
I would have sworn being a DC motor that hooking it up backwards would make the motor turn the other way (makes sense to me anyhow), but I am certainly no electronics expert.
It seemed to work either way when tested so I just wanted to be sure before I proceeded with wiring it up.
It's entirely possible that the motor will turn opposite directions depending on polarity, but so what? The bumpy wheel on the motor that hits a bolt in the diaphragm probably won't care.
No, it won't rotate backwards on reverse polarity unless it uses permanent magnets in place of field coils. As strong permanent magnets did not exist at the time these horns were made, they invariably use field coils.
A brush motor with field coils does not care about polarity. If it did, things like vacuum cleaners and electric drills would not work on AC.
If you want a field coil motor to run in reverse, you have to reverse either the field coil connections, or the armature connections, but not both.
Usually a aahoogggaa horn sounds better wired one way or the other but doesn't matter this I'm guessing is from use ?.