Your compressor is the machine that intakes air and pushes it into a vacuum to pressurize it. You can typically hear your compressor running (the nicer ones are quieter than the others). If you don’t have any air pressure in your tank and you don’t hear your compressor kicking on, you probably want to check your compressor. The compressor is controlled by your pressure switch.
Your pressure switch senses the air pressure in your tank and tells your compressor when to turn on and turn off. Your pressure switch will have a pre-determined “on” and “off” pressure. For example, on most of our larger train horn kits, we run a 110-145 PSI pressure switch. The compressor will turn on when your pressure falls below 110 PSI. When the pressure reaches 145 PSI it will automatically turn off. You can actually change what pressure your system runs at just by switching out your pressure switch. Just make sure that if you are going to a higher pressure, your compressor and your tank is rated for that higher pressure.
If your pressure switch fails to shut off your compressor and your compressor continues to run, your safety blow-off valve will activate to release excess pressure from the tank. While you should definitely have an air gauge on your system to tell you just where your pressure is, if you don’t have a gauge your safety blow-off activating is a way to know that your pressure is too high. Getting the right safety blow-off valve is important. If your safety blow-off is rated too low for your pressure switch, it will activate prematurely and you won’t be able to build pressure. Make sure your safety blow-off is always rated for a higher PSI than your pressure switch. For example, in most of our train horn kits, you get a 110-150 PSI pressure switch and a 175 PSI safety blow-off valve.
Your air gauge may be the most under-rated component on your kit. While it is true that you don't need an air gauge in your system in order for it to work, your air gauge is absolutely an integral part of your onboard air system. The fact is, if you don’t have a way to check your air pressure, how will you know if your system is working correctly? Any troubleshooting with onboard air usually starts with seeing what your air pressure is doing. Without a gauge, you just have no way of knowing what's going on inside that tank!