My take on air horn failures
After doing a bit more research, I have a bit more information to share.
It appears that air horns ARE less reliable than electric horns, largely due to increased complexity, and increased vulnerability to temperatures and debris. However, a good installation can work well for years.
I first thought that most of the problems were due to incompetent installers. From
www.crazyred.com (who sells air horn kits for Miatas):
"I looked under the hood of some of my friends' Miatas to see what they had done. Oh boy, what a mess! One person had the horns sticking out of the mouth like buck teeth. Another had a messy morass of electrical relays and wires drilled into the firewall of the car. Another guy kept blowing his horn fuse. There wasn't a one with a truly clean installation."
But I don't believe that all the failures are due to incompetency. I feel that theogre is probably more competent than I am, but he lives in DE which gets much worse weather than GA.
They are more susceptable to cold. I give mine a blast about once a week to verify that it still works, but I don't do it if the temps are below freezing. On pampered exotic cars that rarely venture out of a garage, this is not likely a problem. The fact that I live in the South has probably helped me. I did try it ONCE in freezing temps to verify that it would work.
They are more susceptable to clogging from dirt, bugs, debris & water. Some of the Miata folks have had their's clog up in 6 months. I STRONGLY suspect that this is due to poor location and orientation. The trumpets should be angled downward, and not exposed to direct air flow from the road. The factory horn location seems to work well for this, and the wiring for the horn is already there.
Air horn compressors draw a ton of current. 15 to 20 amps. This doesn't seem to be a problem in my Fiero. I've layed on the horn for a good 5 seconds once at some idiot who ran a red light. No problem. My research has indicated that the included relay is a good idea if you are adding an air horn to your existing horns. If you're replacing them, the relay is most likely not necessary. It's not necessary in my Formula which has a 20 amp fuse. I believe that this fuse is also used for the stereo, so if your Fiero has a powerful stereo, you may need to up your fuse rating. I think the horn relay is rated for 30 amps, so if you need more than that, you'll need to use the extra relay, and if you power it directly from the battery, be sure to include an inline fuse.
Air horns need more maintenance. Most air horn compressors have an oil hole that needs a drop every so often, I'm guessing every year or so. At that time the hoses should also be inspected. I've used the same type hoses for fuel lines on motorcycles. After a couple of years, these hoses get very hard & brittle, but then again, that could be due to the fuel. I don't have enough data yet.
One item I never see addressed is making the electrical connections more robust. I've never seen any installation writeup mention anything about solder, heat shrink, or dielectric grease. These things are your friends, and should be used to protect any exposed wire or contact. Use them. They keep your connections from oxidizing.
Another item I never see addressed is threadlocker. I can't imagine NOT using threadlocker (Loctitc blue) on all the fasteners on a horn. The purpose of a horn is to create high amplitude sound waves... which create vibration. That cool-sounding air horn wouldn't sound nearly as cool as it bounces between the road and your undercarriage on its way out the back.
So, there's my take on it. I still like my air horn, and will install one again if I get a car with a poor oem horn, but I think I now recognize the limitations of such a horn.
