• Compressor
• Air Tank (optional)
• All the small hardware

The Compressor

The compressor I chose was a York compressor, which I got out of an AMC Eagle or Concord (or something like that) at a junkyard. York compressors are better for on-board air since they have their own oil reservoir, and don't let any of the oil get into the air you're trying to compress. They also put out more volume than other automobile compressors. The picture to the left shows the compressor, with the mounts still on it, as I pulled it from the car (and after I scraped away a small portion of the grime from the mount). As you can see, it was pretty dirty...

When you pull a compressor out of a donor car, it's important to try & get the brackets it mounted on, as well as the hoses (or at least the fittings with a foot or so of hose) for input and output. It's easy to get a barbed fitting to attach to whatever hose comes off the compressor -- it's much harder to find (after the fact) the proper fittings that will work with the compressor. You may also need an idler wheel for the belt, since the York compressor (at least on the bracket set I got) doesn't adjust by itself.

You will need to check two things with the compressor before you buy it:

• The magnetic clutch must work. This can be tested by grounding the compressor casing on your vehicle, and touching the wire coming off the compressor to the (+) terminal on your battery. The clutch should click, and the outer pulley should now turn the inner part, and the compressor shaft as well. When you remove power from this lead, the outer pulley should freewheel, without turning the compressor.
• The compressor must turn over, and pump air. You can check this by turning the inner portion of the pulley (or the outer portion if you have the pulley engaged as above), and seeing that the output hose blows air and the input hose sucks air.

I picked up my compressor at a place called "Pick-Your-Part", in Hayward, CA for $22. I paid a dollar to get in, and had to look around for about an hour before I found a donor AMC car with a York compressor

Once I got the system up and running, a "flaw" in my compressor became apparent: it would run for a few seconds, and then lock up hard. It wasn't seizing, since I could turn it backwards, but it wouldn't turn forwards. So, I went to a different junkyard (one much closer to home) and pulled a much newer looking York off a Volvo. Since I still had the brackets off the original compressor, it didn't much matter what car I got the compressor from. Total cost (with warrantee): $40.

The Tank

While you can probably get away with not using an air tank, your system will be much more functional if you have one. Plus, it doesn't add much to the cost.

I thought about a lot of options for tanks, including PVC pipe, copper pipe, scuba tanks, soft-drink CO2 tanks, air brake tanks from a truck, and fire extinguishers. What I ended up getting was the one that I found first, which was an air-brake tank off a Mac truck.

I got the tank from Specialty Truck Parts in San Jose for $15.00. They have a very wide selection of sizes to choose from, and most of them go for between $15 and $25. They are a truck wrecking yard (in case you hadn't figured that out).

The tank I got is cylindrical, 6" in diameter, and about 18" long. It has a small fitting on each end, a drain hole plug and a purge valve.

I had to assemble a complicated fitting, since the fitting on the tank was designed to use a compression fitting. If you can get the original fitting with the tank you buy, you'll be ahead of the game. Like the compressor, it's easy to buy a fitting that will adapt copper tubing to a standard air hose.

Mounting Locations
Where the tank is mounted is an interesting question. There are a number of good places on a Jeep to mount a small air tank. One obvious place would be under the hood, on top of the passenger-side fender, or below the battery, or wherever else you have room under the hood in your particular Jeep. Another good place is up inside the frame-rail, just beside the rear driveshaft.

The place I like best though (thanks, Joe) is up above the rear differential. There's a space up there that can easily take a tank of the above dimensions, and the differential will never articulate that high. I have my tank mounted up there, and although it was a pain to get it up there (working by myself), it's in a nice spot, and I don't have to worry about it. You can see the tank in that position above.

The Hardware

This is where most of the money in an on-board air system usually ends up going, and it's also where the hardest-to-find parts are. There are a number of basic things (besides the compressor & tank) that you need:

• pressure switch
• safety valve
• pressure gauge
• 12V switch, fuse, wiring
• lots of fittings & plumbing
  

I'll go over each one of these in detail...

The Pressure Switch
A pressure switch is simply an automatic on/off switch for your compressor that determines whether the compressor should be on or off based on the air pressure in the system. If you use a electric compressor, it controls the motor that runs the compressor. In the case of a York compressor (or most other automobile A/C compressors), it controls power to the magnetic clutch. When the switch is on, the pulley (which is always turning when the engine is running) turns the compressor. When the switch is off, the pulley free-wheels.

I bought my pressure switch from an electrical supply store called Consolidated Parts in San Jose. It is preset to turn on at 95 psi, and turn off at 125 psi. It cost about $21.00.

The Safety Valve

Compressed air is one of those things that is relatively benign up to a certain point, but can be dangerous when it gets too compressed. The York compressor is quite capable of pumping air to 300-400 psi. Many of the components used in a budget on-board air system can only withstand 150 or 200 psi. If your pressure switch fails for whatever reason, and doesn't switch off the compressor, you want a backup fail-safe in place. That is the purpose of the safety valve. It bleeds air out of the system when the pressure gets past a certain point. In my case, the safety valve I got is rated at 145 psi, which is fine for me, since my plumbing hose and pressure gauge are both rated for 200 psi. I got it at OSH for $12.99.


The Pressure Gauge

You could probably get away with not using a pressure gauge, however, for the seven to ten bucks it costs you, it's a worthwhile investment to have. I picked up a 0-200 psi gauge with a 1/4" fitting for $10 while I was in Canada over Christmas (which is about $7.50 in US dollars). You can mount the gauge under the hood, or plumb an air hose through and put it in your dash, next to the switch & light mentioned below. Mine will be under the hood, and I may add another one in the dash some time later.

The Wiring
In order for the compressor to work, you need to supply +12 volts to the wire that controls the magnetic clutch. This +12 should either come from a fused source in the fuse box (preferably one that is keyed to the ignition switch being on), or direct from the battery (with an inline fuse). The problem with running from the battery is that if you forget to shut the main switch off sometime, and you have any kind of leak in the air system, your magnetic clutch will be on while your Jeep is sitting in your driveway, and will eventually drain the battery.

You need a switch somewhere (I mounted mine right on the pressure switch housing) that provides power to the pressure switch. This is basically a manual override to keep the compressor from coming on when you don't need it. Since it takes about 15 seconds to fill my tank (1.8 gallons) to 100 psi from empty, keeping the system empty of air (at least while driving on the street) seems like a reasonable thing to do.

The Fittings & Plumbing
This is the most complicated part of the entire system, both from the complexity of the system itself, and the cost and difficulty of finding the parts and pieces for a reasonable price.

For plumbing, I bought a 25 foot long 3/8" rubber air hose at OSH (Orchard Supply Hardware) for $8.99. It has a standard 1/4" male fitting at each end, and is rated to 200 psi. This hose was cut to length to provide the connection between the manifold and the tank. I used the fitting end to attach to the manifold, and put a barbed connection on the tank. I've been told that the nylon brake hoses that big trucks use make a better tank supply hose, and they are easy to adapt from a compression fitting to 1/4" pipe fittings. They are more expensive than the hose I chose, but will last much longer.

I also bought a 25 foot long air hose coil, which is my main air supply hose from the system to whatever tool I am using (e.g., an air chuck or impact wrench or drill or die grinder or...). This was also from OSH, and cost $6.99.

The manifold is the piece (or pieces) that split the output from the compressor to the various pieces mentions above (pressure switch, safety valve, gauge, tank, and output). You basically need an input (from the compressor's output) and five outputs, to go to each of the above.

OSH and Home Depot both sell a cool-looking cast aluminum 1 -> 3 manifold. At OSH, the price was over $8.00, and at Home Depot, the price was $4.22. Guess where I bought mine... :-)

I bought two of them, for a total of eight holes. Two of the eight holes were used to join the two manifolds together. That left six holes, which coincidentally is exactly what I needed! You can get these manifolds in two sizes, 3/8" and 1/4". Since all of my plumbing (except the hoses) are 1/4", I got that size. Even on the 3/8" hose, the end connectors are 1/4".

So, in addition to the manifolds, I bought:

• two 3/8" barbed->1/4" threaded fittings (OSH, $0.99)
  
• two small hose clamps for above barbed fittings (OSH - $0.99 each)
• one brass 3" straight extender (since the pressure switch I bought is so big, you want to mount it above all the other stuff sticking out of the manifolds - OSH, $1.35)
  
• one brass 2" straight extender (to provide a male fitting for the quick-disconnect - OSH, $0.99)
  
• one brass 1" straight extender (to connect the two manifolds together - OSH, $0.99)
  
• two quick-disconnect plug & socket pairs (OSH - $2.72 each)
  
• one male/male joiner (a joiner that joins two male fittings -- OSH - $0.99)
  
• depending on what the hardware on your tank looks like, you may need a couple other fittings to connect the supply hose to the tank, and possibly plug one or more other fittings on the tank.
• one air chuck (OSH - $2.25)
• one roll Teflon tape to make air-tight connections (OSH, $0.99)
  

All in all, the hardware cost about $95.00, bringing the total to about $150.