Very good advice given so far. It seems from previous discussions on the air horns not working, it usually is a solenoid problem. Sometimes all it takes is a bit of cleaning as they are known to accumulate calcium deposits internally.
Don't make the same stupid misdiagnosis I did back in the days when I was new to diesel pushers ...as I was sitting in the driver's seat waiting for the air to build, I tried to blow the air horn and it didn't work so I just figured that they were broken and I'd have to add that to the list of stuff I had to eventually fix. Never did get around to it but several months later I was driving in a remote location and just for kicks I thought I'd try them. They worked fine! After thinking about it for awhile, I then realized that I must not have given enough time to build the air needed back when I thought they didn't work.
Here's a short troubleshooting guide from the Hadley website (link given below) if it will help:
Horn Won't Sound
- Is air coming out of horn?
- If yes, look down projector(s) to ensure there is no obstruction / restriction (examples: spider nest, leaf, moth, mud dauber nest, bee hive, etc.). If you find any type of obstruction / restriction remove and try horns again.
- If no obstruction / restriction found remove back cover and perform cleaning procedure to remove debris from diaphragm seats.
- If air is not coming out of the horn, check to see if there is air pressure in the tank.
- If yes:
- Verify there are no kinks in the nylon air tubing.
- Check that the solenoid has power (minimum of 10vdc) when the horn button is pushed.
- If power is available and you do not hear a clicking sound from the solenoid, the plunger is most likely stuck in the closed position and will need to be cleaned (550 Series only).
- First empty the air from the tank (Do Not attempt to remove solenoid with air pressure in tank as this could result in personal injury) and then remove solenoid in order to perform solenoid cleaning procedure (add link to solenoid cleaning procedure).
- If there is no air pressure in the tank, please refer to the compressor section.
- See more at: Technical Docs | Hadley