On-Board Air

Article by Ed Holnagel


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Since we first built our OBA system back around 2002 a lot has changed. Although the components we used worked at the time they have inevitably been rendered obsolete by time, technology and experience.

With this updated write up of an OBA system we will streamline the process just a bit of installing an OBA system. For the purposes of this writeup we install OBA on a 1998 Jeep TJ Wrangler. Installation on other vehicles even other years of Jeep will vary and thus the way we install ours may not work if you have a different year or engine configuration.

First of all lets discuss some of the common misconceptions about installing OBA which you may find on other writeups on the internet:

  • "York compressors won't put oil out in the air." ALL of the air conditioning compressors will put oil out with the air, the York just puts out less. You have to be sure to mount the York the correct way for this to be true. The reason the York passes less oil is that it has an internal oil return gallery to the oil sump on the output side of the compressor. 
  • "I can just use a simple filter from Home Depot to get the oil out." Wrong!  You must use an oil removal or "coalescing" filter to remove oil from the air supply.  No the cheap filters at Home Depot are NOT coalescing.   If they don't say "Oil Removal" or "Coalescing" on them then they don't remove oil. A good coalescing filter will start at around $60 and they go up from there. 
  • "I can just coat the inside of my compressor with Prolong or Slick 50 and I won't have to put any more oil in for a couple of months." Again, trust us, we have done this and it DOESN'T WORK!!!!  We have also seen where some have completely torn down the compressor pump and packed it with grease.  Again we don't really see the point as it seems like a lot of work as there are other modifications that have to be done for this to work and we still would not run a compressor this way without filters to ensure clean air. 
  •  "I can use an air tool oiler to keep my compressor lubricated." WRONG!  An air tool oiler does not have the capacity to provide enough oil to your compressor to keep it working, again, trust us, we've tried it!   You must use a mist lubricator. These are about the same size and shape as the coalescing filter but they have an adjustable drip rate that you can see through a bubble in the top of them that provides oil directly into the intake on the compressor.
  • "I just use 10W-40 to lubricate my compressor" I hear that some people do this and maybe it works but regular motor oil will over time build up a varnish inside your compressor that isn't good. Use air compressor oil available from a compressor supply store, Home Depot or the auto parts store.   This oil is more like a mineral oil and will be clear and almost water like in viscosity.

Components in a compressed air system

First of all, all of these components are available through local parts and hardware stores or through many online retailers. There is NOTHING special about them. Most online sites specializing in OBA setups for 4x4 vehicles tend to drastically overcharge for these items. When you decide what you need, shop around!

In order to correctly set up your compressed air system you will need some basic and essential items to insure continued operation of the compressor and oil/water free air for your tools, air lockers and tires. Below is a list of the required components in the order they should be installed following the compressor.

Mist Lubricator.  The mist lubricator is installed on the intake of the air compressor. Looking almost identical to the Coalescing filter pictured below the Mist Lubricator is vital to keeping your air compressor running.  The bubble on the top of the lubricator allows you to see the drips of oil as they enter the air stream.  There is an adjuster on the top that allows you to vary the amount of oil that is fed into the air entering the compressor.  Be sure to fill the bowl of the lubricator with compressor oil, available from many local hardware shops as well as Home Depot.

Coalescing (oil removal) filter.   This is a filter that is specifically designed to remove oil from the air.  It should be the first thing down stream from the compressor to ensure that the rest of the air system stays oil free.  Better coalescing filters will have a gauge bubble on top that will indicate when the filter is saturated and in need of replacement.  I cannot stress enough how important this filter is to your air system, not to mention your lockers, tires etc.!  You can find these filters with either manual or automatic drain valves. With the automatic drain the filter will drain off the trapped oil when the catch bowl fills to about half full. Manual drain filters have a petcock at the bottom that must be opened periodically to drain off trapped oil. I have seen where some will run a return line from the bottom of the filter back to the intake of the compressor. I would advise against this because the coalescing filter will also remove moisture from the compressed air. I do not want to be putting watered down oil back into my compressor and I don't think you do either.

Check valve.  The check valve should be located after the coalescing filter and prior to the pressure switch.  This valve only allows air to flow in one direction.  This is essential to keep air from your system from back flowing through the compressor and being lost. It also allows you to change out a bad compressor without loosing the air stored in the system. We have found and started using some neat little inline check valves made for the nylon air tubing we use. These valves require no tools and are installed simply by inserting the tubing into them and giving a quick tug to make sure they are seated. However other types of check valves are available and they all serve the same purpose.

Pressure Switch  The pressure switch senses the pressure of the system and shuts off or turns on the compressor at the preset pressures. Usually it turns on at around 90psi and turns off at 120psi.  These valves are pre-set and cannot be adjusted. There are many online sources for them but we would recommend not buying from a "jeep" related site as many of them tend to greatly overcharge for these items. Price should be somewhere between $10-$20 for one of these switches.

Pressure safety (pop off) valveThese valves come in varying pressure calibrations and open at the preset pressure to keep the system safe in the event that the pressure switch does not shut off the compressor.  If you have a 125psi system it is common to use a valve no higher than 150psi.  Most components in a standard system are safe to about 175-250psi, so a 150 psi valve will work just fine.  The safety valve can be located anywhere in the system after the check valve.  We recommend that the valve be located in the engine compartment so it can be conveniently used to bleed the pressure off the system in case of maintenance. It should also be in an area that is not susceptible to lots of mud or debris as dirt or sand in the valve seat can keep it open just enough to cause a system leak down.

TankFor storing air, the tank is important in that it helps to keep the duty cycle of the compressor pump down.  For most short wheelbase Jeeps about the best you can hope for is about a 2.5gal tank.  Some people do use their rock slider bars or bumpers to store air, but be warned that the tank does collect condensed moisture from the air and therefore must have a way to be drained or your slider bars will rust from the inside out! Like the pressure valve, tanks listed on "jeep" sites or 4x4 OBA sites tend to be very expensive. There are many sites that sell tanks for air suspension and other uses that are much cheaper.  We have actually used old surplus stainless steel refillable fire extinguishers in the past for tanks but we usually just make our own.

Filter/Regulator.  These are a particulate filter and regulator in one unit. We prefer these as they do 2 functions in one package and thus take up less room in your already crowded engine compartment. Filter regulators usually include extra ports to allow the installation of an air gauge so that you can see what the regulated pressure is but typically do not come with the gauge, those are purchased separately. The Filter Regulator should be the last thing in the system right before the air distribution block. Regulated air should be set at around 90-95psi for ARB air lockers and air tools.  Be sure to use a filter with a metal catch bowl as the polycarbonate bowls will deteriorate in the under hood temperatures that the engine produces. As I have stated before, clean air is of the utmost importance.  When possible try and find filters that are rated to the higher 175 degree ratings. Do not skimp on filters.   Air leaving your air tank at a high pressure will condense and carry a large amount of water with it.

Note: ARB air lockers cannot be run off of an OBA system such as this without voiding their warranty. ARB requires the use of a clean air source such as their electric compressors. If your lockers are out of warranty then this isn't an issue for you. However in many many years of running our OBA setups we have never had an issue with contamination inside our ARB lockers due to the air supply. This is due largely because of the use of both the coalescing and standard particulate filters in our systems.

Air distribution blockThe distribution block is where you install air disconnects for your air hose and solenoids for air lockers and air horns if so equipped.  It is also a handy place to mount the filter/regulator.



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