On-Board Air

Page 2


Selection of an air compressor

There are basically two different types of compressors used for on-board air.  One is the York and the other Sanden (Sankyo).  Both compressors are of a piston design.  The York has a piston with a connecting rod much like an engine, the Sanden usually has 5 to 7 double ended pistons that are actuated with a swash plate or wobble plate on the center of the drive shaft.

There are some pros and cons to both types but they both will work.

The York compressors tend to be physically bigger than the Sanden and therefore a little harder to mount in an already crowded engine compartment and they tend to be really heavy.  The York compressors are internally oiled which helps keep them running.  However, it is hard to find a York in the wrecking yard with a stock serpentine pulley arrangement and due to their size the York can be hard to fit into a TJ without some modifications to your inner fender wells, etc.  The York compressors do tend to have a higher CFM output than Sanden compressors. However, to keep the oil in your output air to a minimum they must be installed upright as they have an internal oil reservoir.

Sanden compressors are much smaller and more compact.  They come in several different mounting configurations one of which is the type used in most of the early TJs that use 4 bolts passed through bolt tunnels on each corner of the compressor and attach to a plate that the compressor sits on.  The other type had 8 "ears" 4 at rear and 4 at the front and the bolts insert parallel to the compressor to attach it to mounting bosses on the side of the engine.   Both types will fit but we tend to prefer the latter with the "ears" as they tend to be easier to fabricate mounts for.

Stock compressors from most Jeep Cherokees work really well and usually have the correct serpentine pulley on them. Sanden compressors have an oil fill plug on the top which is used to add oil directly to the compressor.  Several sites on the net refer to making a dipstick to check the oil level in the Sanden type of compressors but believe me, don't waste your time, it isn't necessary.  This is because there never is a measurable "level" of oil in these compressors.  They were meant to be lubricated by the oil in the closed system since it is always circulating.  It is advisable to periodically inject a little oil directly into the fill plug but if you forget it really isn't much of an issue since most of our compressors have been running for over 10 years now without having done this.

Tips for finding the right compressor:

  1. Take a 12 volt battery and some test clips with you to test the clutch (a motorcycle battery works well). If the compressor has two wires then place the clutch wire on the positive and the other one on the negative terminal of the battery, you should hear a "click" as the clutch closes.  Turn the pulley and see that it turns the compressor. If the compressor you choose has only one wire, then place it on the positive side of the battery, connect the negative post of the battery with a test clip to the side of the case of the compressor.
  2. Hold your finger over the high pressure side of the compressor (should be the smaller of the two hoses) and turn the compressor, it should get hard to turn after just a few rotations as the pressure builds, remove your finger and listen to make sure you get a good puff of air. I have found that some compressors will not make much air by just turning them by hand, so you can have an assistant take a serpentine belt and use it to turn the compressor faster by using a sawing motion with the belt around the pulley. Make sure you have a 12v battery connected to the clutch to engage it or the pulley will not turn the compressor.
  3. Pay close attention to the pulley! Even on the same model year of car I have seen them use 4, 5 and 6 groove pulleys.  For installation in the Jeep TJ you need a 6 groove pulley.
  4. Make sure you retain the stock hose and fittings that are attached to the compressor. The threaded connections on AC compressors are special and can be hard to find unless you go to a shop specializing in automotive AC systems. We usually use a knife to cut the rubber hose a few inches past the metal tubes to ease in removal.

Example of a compressor we picked from a Jeep Cherokee.


Note on compressors: Although all Jeep Wranglers use a serpentine belt set up, Jeep changed the location of other components (alternator, power steering, AC) on the engines from year to year. '97 and '98 model year TJs can use the serpentine belt to drive the OBA, however, '99 TJs require the installation of a special serpentine/V-groove pulley on the alternator as an OBA compressor cannot be mounted directly inline with the path of the serpentine belt. For this reason when you select a compressor you need to look for an older Cherokee with a V-groove pulley on the clutch. The compressor is driven off of the alternator using the V-groove pulley. These dual belt pulleys can be found through Kilby Enterprises but aren't cheap. Be sure of where you will mount the compressor and how you intend to drive it before you go to the wrecking yard!

Click here for a diagram of a typical OBA system.

Compressor Install

Installation of your compressor will vary due to the type of compressor you choose and the model year of your vehicle.  Later Jeep TJ/LJs can be a bit tougher as the different engine accessories are located in different places around the engine than the earlier TJs.  Each vehicle and engine design presents a different challenge in mounting a compressor.  The main thing to keep in mind is that the belt must come in contact with at least 1/4 of the circumference of the pulley and the compressor has to be mounted so that the belt runs smoothly around the pulley (cannot be out of alignment with the path of the belt in any way). For the purpose of this write up we are referring to '97 and '98 Jeep TJs.


Removing the stock air box

As our '98 TJ has air conditioning we need to remove the stock air filter box to make room. This may not be necessary if you don't have factory AC or if the model year of your Jeep requires you to mount the compressor in a different location.

With a hacksaw  cut the air tube just behind the existing air conditioning compressor over the engine. The stock air box can then be removed.

You can then install a K & N air filter number RU-0960 available from many outlets online.  Slip the new filter over the end of the air tube and tighten in place with the supplied hose clamp.

Intake noise is increased a bit with initial engine start up, but it does subside when the engine warms up.  As you can see from the pictures, removing the air box frees up a lot of room for a second compressor to be mounted next to the air conditioning compressor.




The air compressor master switch was installed just forward of the 4x4 lever in the center console. We like to use a stock ARB compressor switch, which has a small light that is wired to your dash lights and a larger light that illuminates when the switch is turned on.  Other switches are also available online with a compressor symbol on them.



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