Portable Onboard Air Installation


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By: Scott Winter - January 2006

This article describes an onboard air setup that is inexpensive, versatile, and portable. My truck is a daily driver, so I did not want to sacrifice the stock air conditioning compressor, nor did I want the expense of a CO2 tank system or the hassle of getting it refilled. Therefore, based on how I use my truck and its cargo space, I chose to build a portable one-box system using an affordable 12V electric compressor.


Parts and Relative Cost

Editor's Note: The Author sourced parts for this system through many common online and local outlets. 4x4Wire does not ordinarily publish costs in articles. In this case, however, we thought it important to include the Author's costs at time of writing to give our readers some context on how much this setup could potentially cost you. Your costs, depending on your sources, are likely to differ!


Compressor

The affordable MV-50 compressor.

The heart of this setup is an inexpensive compressor that has been available at Pep Boys, Checker/Schucks/Kragen, Auto Zone, and 4 Wheel Parts, among others. This is the Superflow model MV50. Depending where you shop, you may find a MasterFlow Tsunami, model MF-1050; they are the same. They could be had for as little as $24.99, but are more commonly priced in the $50-70 range. This compressor has more than held its own in head-to-head tests against more expensive compressors from more well known manufacturers.

The compressor comes with a carry bag and a number of accessories including an air hose with a fitting that screws onto your tire valve stems. It draws too much current (specs say 30A max) to power from a cigarette lighter socket, so it comes with battery clamps. As purchased, it has everything you need for basic tire inflation.




Air Storage and Distribution

New, standardized output air fitting on compressor.

To give the system a little extra capacity, I chose to mate the compressor to a 2-gallon air tank. First I decided to ditch the oddball original air coupler, and replaced it with one of standard design. This will make it more universally compatible with existing pneumatic attachments. The compressor itself is threaded for 1/8" pipe thread so I used a 90* elbow and a 1/8" to 1/4" adapter to connect the new air coupler. If it causes any interference problems, the carry handle can easily be removed.



Tank and compressor in the storage bin.

Here are the compressor and tank sitting side by side in their plastic carrying and storage bin. I used a short piece of air hose (not shown) to run from the compressors coupler to the air tanks inlet, which is barely visible on the extreme left side of the pic.



Click on the picture for details of the air manifold input and outputs.

The next pic shows how I have configured the air tanks output. An elbow adapter piece was used to attach a manifold that splits the tanks output among 5 ports. Output ports came tapped for 1/4 NPT. The first port is used for the air pressure switch. This particular switch is configured to close (turn ON) when air pressure falls below 85 PSI, and opens the circuit (turns OFF) when pressure reaches 105 PSI. This switch babysits your compressor, so your system will automatically cycle on and off as necessary to maintain 85-105 PSI in the tank. I used a safety valve that will pop open at 115 PSI, in case the pressure switch ever fails. The main output (center of manifold) has an adjustable regulator, and there are two unused outputs that I have capped off for now. Those could be used to supply ARB air lockers, for example.


Further notes on air pressure switches: The switches are available for a variety of pressure ranges. I chose the 85-105 PSI range so as not to stress the compressor too much; it is rated for a maximum working pressure of 120 PSI. Switches are also available that will handle different amounts of electric current flowing through them. The particular switch I have is from Viair and has a 1/8 NPT fitting, which I adapted to 1/4 NPT to fit my manifold. The switch is about $20 through Viair distributors and can also be found on eBay. Importantly, the particular switch I have will NOT handle the maximum current drawn by the compressor. It therefore MUST be connected to the compressors power supply through a relay. If you choose, you can purchase a heavy-duty pressure switch rated for 30A that will handle the full load of the compressor and not require a relay. Heavy-duty Viair switches are available for $25.95 at Wheelers Off-Road and elsewhere. Because I wanted to control my setup using an existing low-current switch in my dash, I was already committed to using a relay and therefore opted for the less expensive pressure switch. Choose whichever switch will best suit your needs.


Electrical Wiring

WARNING: Dont forget to disconnect the vehicles battery before you attempt any wiring!

Quick disconnects for sub or compressor, and the dash-mounted switch which controls power.

I already had wiring in the back of my 4Runner for my subwoofer box, consisting of power leads and a turn-on lead, with quick disconnects. The power leads are connected directly to the battery (and fused) and the turn-on lead is controlled by a dash switch that I had previously installed.

I cut the battery clamps off the compressors power leads and wired the compressor to a relay that I mounted in the storage container. The relay then connects to the existing wiring, providing the compressor with power from the trucks battery. I used a Bosch-type automotive relay rated for 30A, and it is switched on and off as needed by the air pressure switch in the manifold. Wiring this part is simple: the blue turn-on lead (controlled by the dash switch) goes to one side of the air pressure switch, and the other side of the pressure switch goes to the control terminal of the relay. Refer to the schematic that came with your relay when determining how to connect the wires from the battery and your compressor.


Pressure switch wiring detail. Battery clamp leads reattached via the quick disconnect.

Using the existing wiring worked well for me because I can quickly swap in the subs or the compressor. I dont need the compressor around town, and I dont need the subs on the trail.

Finally, I attached one of my standard electrical quick disconnects to the original battery clamp power leads that I had cut off. This allows me to use the battery clamps to power this one-box system from any vehicle that has a 12V electrical system.

Further notes on wiring:
The compressors specifications list a maximum current of 30A, and good engineering practice dictates that you use 10 AWG wire for a 30A load. Therefore, it is worth noting that my existing wiring, and the wires supplied with the battery clamp power cable, are only 12 AWG. With my truck running and the compressor on, I got out the multimeter and measured a slight voltage drop from the battery to the compressor. In theory, too much voltage drop would decrease the compressors performance, but Im still reading well over 13V at the compressor, so in practice my 12 AWG wire doesnt seem to cause any trouble. It is certainly no worse than using the supplied 12 AWG battery clamp cable! Bottom line 12 AWG seems fine, but if youre doing a new install from scratch, go with 10 AWG.




End Result

The compact and portable compressor and tank installation. Quick-connect air chuck for airing up tires.

Everything fits inside the plastic storage bin. It does not take up too much cargo space, and it can be moved around easily. Tire filling is accomplished using a clip-on tire chuck attached via standard air couplers to a 50-foot coiled plastic air hose. If youve ever used this type of air hose, you know that it takes a good bit of pull to stretch it out to anywhere close to 40 feet, but it is plenty to reach the four tires on my truck. The air coupler on the end allows me to attach another extension hose if I need to air up the truck parked next to me.


Summary

The MV50 is a great value and makes a good starting point for a basic onboard air system. I designed my system to be portable, but you can hardmount the compressor and air tank if you choose. Because the MV50 is so inexpensive, some folks may opt to run dual compressors with a larger air tank to gain more flow and capacity. However you choose to build your system, I hope this write-up has given you some good starting ideas.


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