Versatile Hardmount Onboard Air Installation
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By: Jim Paget - January 2006

The steps for installing an electric On-Board Air System (OBA) are straightforward and universal:

There were three goals for the installation. The first was to create a system that looked as “stock” as possible from the inside of the vehicle. Second, I wanted to create a system that would initially inflate tires while being able to quickly install one or two ARB lockers by simply adding wiring for the solenoids and piping air to the differentials. Third, the entire system needed to be easy to maintain and upgrade.

As usual, the devil is in the details and the differences between vehicles. I chose to install a Viair 200 compressor with an external tank in a 1991 Isuzu Trooper. I made the process more complex and custom by using Isuzu switches in the dash and also installing the pressure gauge in the dash, rather than using the small control panel that came with the installation kit for the Viair 200.

The Components of the system are:

Switch and Pressure Gauge

The three switches with the new harness. The pressure gauge custom fit in the dash.

On the dash of 1985-1991 Isuzu Troopers, there is a blank panel to the right of the rear window wiper switch that conceals three empty switch bays. I chose rear window defogger switches for this application as they fit the switch bays and provided a lighted indicator that the switch was on.

I created a wiring harness that provided illumination to three switches (one for the compressor and two, hopefully, for future locker activation). It was necessary to remove the stock connectors on the switches, as I was unable to locate a compatible pigtail to match the stock Isuzu switches. I cut off the stock connectors and replaced them with 6-pin connectors from the local electronics supply house. The rear wiper switch provided power for dash lighting.

I fit the Viair pressure gauge to the left of the instrument cluster and to the right of the vent outlet. There was just enough room for the gauge, which required a 2 1/8” hole. Installation of the gauge turned out to be the most difficult portion of the entire project. I removed the dash cluster, the vent tubing under the dash and the large dash trim piece found on LS model Troopers. This allowed just enough room to reach into the dash to secure the gauge.

Compressor and Air Manifold

The air manifold installed on the inside of the right fender. The compressor fits nicely between the battery and headlight.

The compressor is installed between the battery and the headlight bucket. This was possible because my Optima battery has a smaller footprint than the factory battery. Because the compressor mounts needed to span the edge of the battery tray, I put two pieces of high density, ¼” rubber pad under the compressor. The compressor now sits level and the rubber pad provides some vibration isolation from the body.

A manifold created from brass fittings proved key to creating a system that supports current use and provides for future use by connecting to existing fittings.

I used 3/8” brass fittings from the local hardware store to create the manifold. It took some (a lot) of figuring to determine the right length of pipe nipple for each section. As shown in the picture, there are a total of seven connections in the manifold – one quick disconnect which leads to the compressor, 2 plugged connectors for future locker installation, one connection to the air tank, one connector for the pressure switch, tire inflation quick disconnect, and one pressure line to the gauge in the dash.

I used joint compound rather than Teflon tape to seal the joints. I find the joint compound easier to use and a better sealant than the tape. The downside is that it makes the joints much harder to take apart, so I didn’t use it until I was satisfied with the final layout.

Storage Tank and Air Line

One of the two angle iron mounts. The weld really isn’t that bad. This is the back of the weld. The tank is shown here upside down prior to installation. Here's the tank mounted to the vehicle.

A four-gallon trailer air tank fit neatly under the vehicle on the right side, opposite the muffler. The mounts were made of 2X2 angle iron, which I notched, bent, and welded, and then welded the assembly to 3/8” by 2” by 12” plates which sandwich the floor of the truck. The 3/8” plate was obviously overkill, but it was available and free.

In the following view, the tank is shown upside down. Also, after the picture was taken, a collar was welded around the tank drain to protect it from trail damage.

The tank was sprayed with a hammered finish after the labels were removed.

While the tank was out of the vehicle, I installed a pressure relief valve and a tank drain.

Once all the components are installed, the entire installation is nearly done. All that remained was to run the wiring and to run the air lines.

This called for a heavy-duty shop air line which is ¼~T ID and 3/8~T OD. Only two issues presented themselves at this step. The first is the need to install a union in the line so that line can be tightened. In my local area the local plumbing distributors did not carry a bronze 3/8~T union so I used a ½~T union with a bushing to 3/8~T.

The second issue is ensuring that you route and secure the line in such a way that it cannot come in contact with exhaust system components. The line on the Trooper was secured with wire ties.

Wiring and Pressure Switch

Final location of the dash switches for this onboard air setup.

Just two wires needed to be run to the dash switch. I ran my wiring through the existing grommet behind the glove box. There should be a similarly located grommet on nearly every type of vehicle. Just as with a stock ARB switch, the switch internally turns on the indicator light when activated. The pressure switch is located in the manifold. The compressor is wired directly to the battery and is protected by a 30 amp, in-line fuse. While operating, the compressor draws a maximum of 23 amps.

I ran the supplied ¼” OD tubing through the grommet behind the instrument cluster on the driver’s side of the vehicle. Again, you should be able to find a grommet in a similar location on nearly all vehicles.


All the installation goals were achieved:

In retrospect, I would consider using a 2-½ gallon tank rather than the 4-gallon tank. While the larger tank fits, a smaller tank would be more protected from trail damage. The only advantage of the larger tank is a small reduction in the time to fill the first tire. After that, the output of the compressor is the constraining factor.

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