On Board Air - 7/15/04

This project is undoubtedly one of the best modifications I've done so far, but it is also probably the most involved and time-consuming one as well. That is because a lot of what I've done I had never actually seen anyone do before the exact way I did, although I'm sure someone has done it somewhere I just couldn't find any online write-ups to show me how to do it. I did glean valuable information from several other OBA installation articles even though they were designed differently than what I've done. Unfortunately, when you try something new you always end up, through trial and error, spending more money than you should have. Hopefully this will help someone out there with their OBA ideas, especially if they plan to use a similar setup.

photo #1
What type of OBA system to use?
The first thing to decide on is what type of system you want and there are three basic types to consider: belt driven, 12 volt electric, and portable (CO2). I won't describe each one in detail but each has its advantages and disadvantages. Which one you choose will also depend on what you want to use your OBA for. I wanted something primarily to use for ARB lockers but also something powerfull enough to fill tires. I decided on using a 12 volt electric compressor and here's why. I don't want to lose my A/C in order to convert that to OBA and there's just not enough room in my engine compartment for installing an additional belt driven compressor. Relocating the battery and everything else to make room would be too much right now. I also wanted something permanently mounted and out of the way. And thinking about where I wanted to mount the compressor, I wanted to try and use the little "cubby-hole" that is on the passenger rear corner across from the stock spare tire location. (see photo #1)

photo #2
Choosing the compressor.
Since I was going to have ARB lockers, using an ARB compressor would probably have been the easiest both to wire and to fit in that cubby-hole. However, I read that while the ARB compressor works great for the lockers, it really isn't powerful enough to fill big tires in a reasonable amount of time. Not to mention the ARB compressor costs over $200 - I knew that I could find something about the same price or cheaper that would pump more cubic feet of air per minute (cfm). I also had to consider the size (length, width and height) to make sure I could mount it in the cubby-hole. After quite a bit of research, I decided on the Viair 400C compressor(see photo #2). Here are some comparisons that helped me in choosing this compressor:

Viair 200 Viair 400 Quick Air 2 Extreme Air
max working pressure 150psi 150psi ? 150psi
duty cycle 20% 33% 33% 100%
length 7" 11.25" 8" 14"
CFM @ 0psi 1.94 2.54 2.18 4.0
CFM @ 20psi 1.27 2.12 1.76 ?
33x12.50 tire15-30psi fill time (advertised) 3 min. 5 sec. 2 min. ? 1 min. 30 sec.
price range $160 - $170 $195 - $240 $230 - $240 $360 - $375

I could not find any numbers on the ARB compressor, but I believe any one of the above compressors would be better suited for filling tires. While the Extreme Air compressor is clearly the best, it is simply too long to use in the cubby-hole even with the interior panel removed, and its price was a little too high for my budget. I ended up buying my Viair 400C compressor from www.central4wd.com since they had a good price and have given me good customer service in the past.

What is Duty Cycle?
Duty Cycle is the amount of time a compressor can run continuously before needing to be shut off for a cooling period. It is calculated as a percentage of 10 minutes that it can run continuously. For example, a duty cycle of 40% would mean the compressor could run for 4 minutes before needing a cool-down period of 6 minutes. However, the amount of time a compressor can be run also depends on the conditions it is operating in, such as ambient temperature, working air pressure, and mounting location. For example, a compressor mounted in a hot engine compartment, will have a decreased duty cycle versus the same compressor mounted in a cool location with adequate ventilation. In theory, you could turn a 33% duty cycle compressors into a 100% duty cycle if you had the means of keeping it cool enough. Most compressors include an internal thermal overload switch that will shut the compressor off if it becomes too hot (duty cycle exceeded).

Here are the Duty Cycles of the compressors listed above based on running them at 100 psi and 72* F:
20% = 8 min. On / 32 min. Off
33% = 15 min. On / 30 min. Off
100% = continuous duty

photo #3
Other needed parts (see photo #3).
There's a lot of other parts needed for an On Board Air system than just the compressor. The parts needed will vary depending on how elaborate your system will be. The parts you will most likely need are:
on/off switch
pressure switch (hint: ARB recommends a minimum 85psi and max of 105psi for air lockers)
electrical wire
electrical connectors
electrical fuse / fuse holder
air tank
air hose
pressure guage
various air hose connectors / brass fittings
safety valve
drain cock
moisture trap
teflon tape

Not knowing exactly all the parts I would need or how the compressor should be wired, I ordered the Viair Onboard Air Hookup Kit from www.tellico4x4.com. This kit was helpful to see how everything "could" be hooked up and was a lot easier than trying to buy all the little parts separately, but it can be done. I also used www.onboardair.com for a lot of the fittings. They also have some excellent information and diagrams about OBA systems that were quite useful in showing me what I needed.

photo #4

photo #5

photo #6
Mounting the compressor.
After I had all the parts ready to go I could get started on mounting the compressor. The first thing to do was to remove the interior panel (see photo #4). Inside you can see some factory wiring and I also had my CB coax running through that area as well.

In order for the compressor to be securely mounted, I had to make some brackets for the compressor to mount to (see photo #5). This was not all that difficult but it was rather time consuming. The far right corner of the bracket (in the picture) was the hardest because there was nothing to support it. I ran a metal strip up to the body panel just beneath the window, put a 90* bend at the top, drilled a hole and supported it with a nut and bolt. Also because the compressor will become hot when in use, I wrapped up those factory wires in some plastic wire wrap to help protect them. It also helped get them out of the way.

Photo #6 shows the compressor mounted to the brackets.

diagram #1

photo #7

photo #8

photo #9
Wiring the compressor.
See Diagram #1 for a schematic on how I wired the compressor.

I ran the wires from the compressor along with the 1/4" vinyl air tubing (for dash gauge) up and tucked them inside the headliner along the passenger side of the jeep. I tore almost the entire dash apart trying to find a suitable place to go through the firewall (on the passenger side) to the battery (see photo #7). Notice the airbag module on the seat... I'm sure there is an easier way to do it (like from the engine compartment) but I gave up and ran the 10 guage wire through a body seam near the door. For the switch, I used vacant fuse slot #24 which is switched power, which means the key must at least be in the "on" position in the ignition before the compressor can be turned on. The compressor draws up to 30 amps so it should only be used with the engine running.

For the dash guage (see photo #8), I used a 2" hole saw and cut a hole in that space on the center console right in front of the 4x4 shifter. Its the perfect spot since when you are in 4wd, the shift lever is pulled back and you have a clear view of the gauge. The brass fittings attached to that air line is a drain cock. I figured why not install that here so I can drain air from the system from the drivers seat. Not a mandatory item for my system, but a convenient feature nonetheless.

Photo #9 shows the dash and center console put back together. The dash gauge is illuminated which is very nice. The on/off switch is next to the factory rear wiper switch. Its the rocker switch with the blue light (bought this cheap switch at AutoZone). My jeep didn't have the optional fog light switch so I used that location for the on/off switch. You can also see part of the drain cock mounted up out of the way behind the dash gauge.

photo #10

photo #11
Gettin everything hooked up.
A picture is worth 1,000 words... (see photo #10).

Photo #11 shows how it looks finished with the interior panel put back on. The back part and a good portion of the sides of the cubby hole have been trimmed away.

photo #12

photo #13

photo #14

photo #14a

photo #15
Air Tank.
Initially, I used the smallest tank I could find which was a 1/4 gallon tank that I had mounted above the compressor. The reason I wanted to use a small tank was because I was afraid that with a 12 volt compressor that doesn't generate as much CFM as a belt driven one, that a big tank would take longer filling tires because not only does the compressor fill the tire but it has to fill the tank at the same time (after the initial burst of air). This is true to some extent but I found that I didn't like using the small tank and using a larger tank wasn't as bad as I had thought it would be. Plus there are other advantages to using a larger tank and having more volume of air in the system. The tank I ended up using was an old 2.5 gallon fire extinguisher that was designed to hold water that I had sitting in the garage. There are a couple online articles about converting these into air tanks, so I knew that this tank would work well with my system. They are used as portable tanks in the articles I've read but can be used as part of a system as well.
See http://www.off-road.com/jeep/cherokee/2001/06_jun/airtank/ for reference.

Since my jeep is lifted pretty high (6.5" actual lift over stock) I had plenty of room to mount the tank above the rear differential. It isn't exactly above, but a little forward more above the pinion of the differential. (see photos #12 and #13). With the amount of lift I have, extended rear bumpstops, and carefull placement of the tank, it will not hit anything when flexing or if both sides of the suspension compress at the same time. To mount the tank I simply used a ratchet strap which has secured it very well but I will re-check it every so often along with my other regular maintenance. In the time I've had it on, it hasn't come loose and it doesn't look like it will on its own.

The air hose that connects the air tank to the manifold exits the jeep through the factory plastic plug that is near the passenger taillight (see photo #14). Also seen is the blue ARB air line which exits in the same place. I also installed two quick connect couplings under both taillights to hook up my air hose to, but because the rear quarters and tail lights are a very vulnerable spot on XJs, I later removed the couplings from this location and mounted one directly on the manifold instead.(see photo #14a). Then I ran an air line to the front of the Jeep (underneath and along where the gas lines run) to the front bumper where I mounted another coupling. (see photo #15) This gives me an air hose hook-up at both ends of the Jeep.

diagram #2

photo #16

photo #17
Wiring the ARB switch and accessory Fuse Block
Refer to Diagram #2 for a schematic on wiring the ARB switch and fuse block.

I decided to put the ARB switches on the bezel for the transmission shifter (see photo #16). That will take up just about the entire space of the bezel with these two switches so if you are using the ARB compressor switch also, another location might be better suited for all 3 switches to fit. Underneath the bezel, there is just enough room for the wiring. In fact I had to cut off a useless peice of metal underneath for it to fit but that was no big deal. The thing you need to be careful of is when placing the bezel back in that the wires are not hitting or obstructing the shifter. I found this out the hard way as the first time I shifted down into the 1-2 spot, the shifter pinched one of the wires inside, causing a short and blowing the fuse. Once I figured out what happened, it was a quick fix and I made sure the wires were running away from the shifter.

Also before installing the switches in this location, I wondered if the shifter would be in the way and the switch would be hard to use. I have found that is NOT the case. No matter where the shifter was shifted to, I never once thought that it was in the way. Even in the lower D, 3, and 1-2 positions the switch is easily accessible and visible.

I have started to accumulate a few electrical gadgets (CB, IPF lights, compressor...) so now I was running out of places in the fuse panel to tap into and I really don't like the idea of tapping into other factory circuits possibly overloading them. So I decided it was time to install an accessory fuse block that I could use to run the ARB switch and other items in the future. In fact, if I had done this first I probably would have ran the compressor off this fuse block as well. I set my fuse block up very similar to the one found in this link http://www.patricksnorton.com/cherokee/tech/Batcable2.php. However for the ARB switch, I wanted an additional safety feature to ensure that the ARB switch couldn't be accidentally activated when I didn't want it to. (If using the ARB wiring harness, there is a safety feature built in because you must first turn on the compressor switch, then rear ARB, then front ARB.) Refering to the wiring diagram, the relay is controlled by switched power (tapped into cigarette lighter wire) so the fuse block cannot have power without the key being at least in the "On" position. I also installed another on/off switch (see photo #17) so that even with the engine running, this switch needs to be in the "On" position for the fuse block to have power. This is the safety feature because now I simply leave that switch "Off" for daily driving and there is no way that someone can engage the ARB by accidentally flipping the ARB switch. By the way, that little switch was the switch that was included in the Viair Onboard Air Hookup Kit but I felt it would look good in the location I put it instead!

photo #18
Testing / Impressions
Finally I got the whole thing done, and was able to give it a good "field test" recently when I went on a trail ride that took about 6 hours which included some rock crawling so it was a good test for the ARB as well. Everything worked flawlessly and I was very pleased. With the larger 2.5 gallon air tank, the system takes a little over 2 minutes to pressure up from 0 psi to 105 psi. After it pressured up I was able to engage the air locker several times throughout the day and I think the compressor only had to kick on once when the pressure finally dropped down to 85 psi. It sure was nice after the trail ride to have OBA to fill up my tires (and my friends tires as well). I was running 15 psi in my 33x12.50's and I wanted to see how long it would take to air up compared to the data I listed in the chart above so I timed it (see photo #18). It took 9 min. 30 sec. to bring all four tires back up to 30 psi. That is roughly 2 min. 22 sec. per tire. Thats a little slower than the advertised 2 minutes per tire and even though its not blazingly fast, its not that bad in my opinion and might be due to the larger air tank I am using. Previously I was using one of those cheap "plug into the cigarette lighter" compressors which took about 10 minutes to air up EACH of my old 31" tires. So compared to that, this IS blazingly fast!

Mistakes made and downsides to this system
Compressor is noisy. Mounting the compressor inside like I did is pretty noisy when its running - louder than I thought it would be. Luckily, once pressured up, the air lockers can be run for quite a long time before the compressor needs to come on again. But if I had to do it all over again, I would still install the compressor in the same location.
Buying the small 1/4 gallon air tank. Like I mentioned above, the 1/4 gallon tank just wasn't enough volume of air that I wanted. It set me back about $35 or so.
Gauges read 115 psi. For some reason both my gauges say the system is pressurizing up to about 115 psi instead of 105 psi which is what my pressure switch should be shutting off at. Viair doesn't offer a 115 psi switch that I know of so maybe the one I have is just slightly off. Either way it has caused no ill effects with the ARB and the installer didn't think it would be a problem at all.
Using a regulator. I tried using a small manual regulator to try and keep the air flow to the ARBs at a constant rate like 90 or 100 psi (middle of the 85-105 range). This doesn't work because when airflow stops, the pressure will build up to 105 psi past the regulator regardless. Out about $8 and time messing with it...
Using a water trap or air dryer. I don't really have one at all, but since I live in Nevada where humidity is extremely low, my ARB installer was surprised that I was concerned about it. I installed a small dessicant dryer before the solenoid to prevent moisture from passing through anyway. I am not concerned with other water in the system because it will likely end up in the air tank which used to hold water when it was a fire extinguisher. So no big deal really.
Bad placement of the air tank. Initially I had the tank mounted in the tunnel above the rear differential like this. As it turns out, I didn't have enough rear bumpstop and the tank came into contact with the ARB line sticking out of the diff housing. This happened on the Rubicon Trail when the rear of my jeep dropped off a ledge unexpectedly causing the whole rear suspension to compress. This caused a hole in the ARB line but I used some JB Weld to perform a trail fix which got me through the rest of the trip. After returning home I repaired the ARB line properly. To make sure this doesn't happen again, I had to either add more rear bumpstop or move the tank to a different location. I simply rotated the tank to its current location (photos 12 and 13) which gives a lot more clearance and there is no way the tank can hit the ARB line coming out of the diff again.
No more air tank. The tank worked okay in the above position but in 2008 when I replaced my rockers with 2x6 rectangular tube I made the driver side one into an air tank. This allowed me to get rid of the air tank I had under the jeep. For more info on the 2x6 rocker replacements see the "Beefing It Up" write-up.

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