Train horn compressor wiring help

I was given an omega train horn kit when I purchased my truck and i’m having some problems with the compressor. I wired my compressor to an accessories jack under the dash so it would only power on when my truck was on. I notice after a few attempts to use the horn that the compressor is not working properly. It powers up and runs for a few seconds then it slowly starts to die until it just has enough power to whine. I did a few tests and wired it directly to the battery and it runs powerful and strong with no issues. ( I used an inline 15amp fuse woth ways without blowing anything) I put a meter on the accessorie jack and it shows 12v but when i connect the compressor and turn it on I can test it and it gets all the way down to about 8v when the compressor stops working. What in the world would be killing the power if its still a 12v power supply? If both are 12v power sources why is one able to be drained down like this? Sorry I’m not very familiar with this.

Thanks for any help with this problem!

I’m still stumped as to why the 12v source in the fuse box cant maintain 12v but after digging around I think what I need to do is wire in a relay between the compressor and battery but my questions is can the switch for the relay be the accessory jack in the fuse box that is only powered when the ignition is on?

Welcome to the forum. I think HB has a Viair pressure switch/relay designed for that purpose.

Ok I will have to check into it. Any clue why the power drops so low when using the ignition power source?

No source, other than the battery or alternator, can carry enough current to run most compressors.

Thank you for the reply I did not know that. So do most people use a main on off power switch for this set up? Just thought for convenience it would be easier to use with the ignition switch so you don’t have to remember to power up the compressor.

Some use a remote switch and others use a horn relay that gets its power from the ignition. There should be a schematic for the relay in one the install threads. The horn relay is used in conjunction with the pressure switch/ relay.

Yes you have the right idea. For the main power (load side of relay), use heavy wire straight to the battery. For the trigger side (coil side of relay), you can use any ignition source such as your accessory plug. Lighter wire is ok since there is not much current draw. The pressure switch should also be connected inline with the trigger side of the relay.

Thanks for the reply. In my head it seems to make since but have never done anything with a relay. From what I’ve seen it has four side. One for battery source, one for pressure switch, one to the ignition wire, and is the last a Ground?

In this high-level/rough diagram, the ‘switched power’ would be your pressure switch (which you SHOULD have a switch to turn off/on) and the accessory would be your compressor.

As for why you were seeing issues:

  • The current draw from your compressor was too large for the wire carrying it, resulting in loss (heat)
  • The wire you were using could have been a lengthy run, resulting in voltage drop across the distance
  • The ground could have been poor (it usually is with IGN and accessory wiring, which is why good car audio people never use OEM/stock grounds)
  • The run back to ground could also have been a lengthy run (remember, the entire circuit is both the positive AND negative runs!)

Most likely it was several of the aforementioned issues…

Thanks for all the information. My compressor and pressure switch are built in together. Would a diagram like this work? Found it earlier and it looks like the accessories power would switch the relay on and off.

The latest diagram you supplied would also work, just keep in mind that the pressure switch must now carry the higher amperage associated with the compressor’s electrical demand … whereas in the diagram I supplied, it doesn’t have to do that.

Also, ideally you should have a toggle switch at the +12v ACC end of that diagram … so that you have some manual control over whether the compressor will run. (Example – in the event of a major line leak, tank leak, etc. you wouldn’t want the compressor to keep going; you’d want to shut it off until you could travel somewhere and address the problem.)