F-3 horn

The horn runs on a 1.5 volt battery, so yes... 12 AC volts will damage the horn.

The relay (not a solenoid) in the loco ignores AC, but triggers/actuates with a small DC offset on the track. The relay's contacts connect the 1.5 volt battery to the horn.

Thank you for response, just how does the relay ignores the AC but allows The DC from The transformer to actuate the relay.  This  F-3 I have has the relay replaced, rollers replaced and now I suspect the horn, what is best way to test the horn and at what voltage. Where is AC horns used?  

Simply install the battery and manually articulate or bypass the whistle relay. If bypassing, be sure to only feed it 1.5VDC

Any 1.5 v battery (D, C, AA, AAA) and two pieces of wire will do.  Connect your wires to either end of the battery.  There is a single wire from the horn to the relay.  Touch one wire to this wire (you do not need to disconnect it) and the other wire to the horn frame.  Scrape the frame a little bit to be sure you have a good connection.

The horn relay is specially designed to pass AC voltage through it, but pick up with a small DC voltage. The key is the design of the relay core.

Larry

If you are going for nostalgia then by all means fix the PW horn.  If its not important to you, I would recommend replacing it with a Williams True Blast or Dallee electronic horn. They are so much more realistic sounding, require only a two wire hook up and don't use a battery.