Building an On Board Air set up from your AC compressor

By Jay Hart

I was looking into different options for my upcoming OBA set-up. I was shown a few different links about OBA. If you choose to purchase a system that incorporates a York or Sanden compressor, prepare to spend a little bit of money. I figured I could build a system which would suit my needs and cost less, leaving money left over to go toward my next big buy.

I took a few weeks to do my research. I looked at York, Sanden, and just about all the 12V compressors I could find. I even toyed with the idea of running a small 125V compressor off a power inverter. I was looking for the best most cost-effective method I could get.

Most, if not all the 12V compressors came up short in the Cubic Feet per Minute department. That means long wait time while airing back up. I decided none of the mid range 12V compressors would suit my needs. There are high-end 12v compressors out there, which basically combine a York compressor with a winch motor. These babies are expensive, and didn't meet my wallet criteria. None of the 125V compressors, that would work on a power inverter, were large enough to give me the CFM I was looking for. I decided a CO2 tank, although cheaper then the other systems, would require money to maintain (cylinder filling fee) and make me less likely to share my air with others.

I finally decided a converted a/c compressor was the way to go for me. I had a spare a/c compressor lying around, and decided it was time to fit it into place and fabricate mounts for it. It turned out to be too big to fit under the hood, so I scrounged the junkyards looking for an a/c compressor that would fit my make and model.

I have a TJ, and finally found a R-134 compressor from a Grand Cherokee that would bolt right in. $30.00 later; I bolted the compressor into place. The best part was that I didn't need to fabricate any mounts for the compressor, since the factory mount was already in place.

The next step for me, was to buy a longer serpentine belt, since the one I had, was not going to fit around the new pulley. I should have used a tape measure and wound it around all the pulleys, but instead decided to go to the auto parts store and ask for a belt for my year with a/c. That turned out to be a mistake, as the belt was just a smidge too short. Two returns later, I had the right size. Take it from me, unless you have a compressor already, and are abandoning the cooling capability, you should measure to be sure of your size. I now cart the stupid cardboard belt packaging around in my glove box, since I want to know what size to get as a replacement. I also carry the old serpentine belt since it is still good and will run everything but my OBA if an emergency situation arises.

While I took a few days to figure out what I wanted to do with the system exactly, I wired up the ac compressor.

The wiring of the compressor was relatively simple. I took an ignition switched source (turns off when the key is off) and went to a switch in my dash. From the switch, I went into the engine compartment, and left a long lead, since I didn't know exactly where I was going to have the pressure switch piped. All I needed to do to complete the circuit was to go from the pressure switch to the clutch wire of the a/c compressor.

After I drew out what I wanted to do, I made a rough list and went shopping. I needed to get the following into the system:

• Moisture separator (the design of the a/c compressor allows oil to go throughout the ac system. You don't want oily air. Get yourself a small moisture separator to trap the oil that leaves the compressor.
• Swing check valve (you do not need this. I found out after I had it installed, and just left it there. Originally, I was going to bleed off the pressure between the compressor and the check valve by drilling a very small hole in the piping, which would allow the compressor to start without being under a load. I never needed to drill a hole and found the compressor starts and runs just fine with pressure on the system. Save your money).
• Gauge
• Pressure relief valve. (For safety's sake, don't save you money here. I put my air tank in the people compartment. I would hate to find out what the burst pressure of the tank is with people inches from it).
• Pneumatic coupler (how else are you going to plug in your tire chuck to the system)? I didn't need a shut off valve, but have seen couplers go bad after a while. In harsh conditions, they need to be replaced from time to time. I figured I would throw in a valve for good measure.
• Pressure switch. (This is what tells the ac clutch to engage. More importantly, it also tells the clutch to disengage.
• Air tank. Many people run 1 or 1.5-gallon air tanks. I decided on a 5 gallon, because I had the room, and wanted a little reserve capacity to run pneumatic tools on the trail.

If you are able to get any of the items in 1/4" that I got in 1/2" size, you will be further ahead than I was. By only having 1/2" pieces on a 1/4" piping system, I needed to get extra bushings to make up the difference in piping size.

I changed my configuration based on motor vibration. Originally, I had everything hard piped right off the compressor discharge, which was working well, but was subject to vibration. I changed the piping around a little to what you see now in my diagram. I basically separated most of the piping from the compressor discharge with a length of hose.

The next step was to start assembling. I ended up tapping a ¼" pipe thread into the soft aluminum discharge housing. This was because I didn't have any hose attached from the junkyard. I might have used any existing hose to isolate vibration right off the compressor, had it had any.

During revision one (due to vibration of the piping on the compressor) I ended up making a neat little manifold that I attached to the firewall with rubber mounts. This worked out very well.

The manifold houses most of the gear for the system to function with the flip of the switch. It also has the pneumatic coupling for plugging in tools or air hose.

Straight through the manifold, I am feeding the air tank, which is behind the seat.

This gives me reserve capacity to run air tools, and also lets me air up very quickly. I aired up someone in the club who was used to a 12V OBA system he had, and he couldn't stop telling me how quick it was! That made all my efforts worth it.

The last revision I made was to remove the automatic oiler I had on the compressor inlet. I used a shut off valve and a little tubing to enable me to recirc any lost oil that got collected in the moisture separator, back into the inlet of the compressor.

Here is a piping diagram of what my system is comprised of. Feel free to use this as a guide in your set up. Remember, any changes you make may require different fittings.

Here are some helpful links to other OBA systems:
Kilby enterprises: http://www.onboardair.com/
Oasis 12V compressors: http://www.onboardair.com/trailhead.htm