While all of the systems and equipment on your truck are important, your tires are literally where the rubber meets the road—or off-road. They are what get you into, and out of, a bind. The ability to vary tire pressure increases the terrain your vehicle can safely traverse. Maintaining optimum pressure prevents premature wear and improves fuel economy. Having some manner of on-board air compressor means you won’t need to take a slow, bumpy ride to the nearest gas station. With a beefy enough unit, you can also run air tools to perform vehicle repairs, or a pneumatic nailer to build that off-grid greenhouse on the Back 40.
Air Compressor Types
You can get portable units that plug into your vehicle’s cigarette lighter, or options that are driven by the engine’s belt system. These systems vary widely in the amount of air they produce. I’ll go over them based on their power source: engine driven and electric.
Engine-driven systems use an AC compressor mounted to the motor to provide air. These units move an incredible amount of air, quickly. You can disconnect the factory AC and build a manifold to distribute air. Or, you can use an additional compressor mounted to your engine to provide air for your tires and retain cool air in the summer. However, there are some downsides to these systems. While they deliver the largest volume of air, they take up a considerable amount of real estate under the hood. This is fine in an older vehicle, but more modern models tend to have an already cluttered engine compartment. These systems are fairly complex, and you may need to fabricate some of the components. If you have an older pick-up and are a skilled do-it-yourselfer, this may be a good option for you.
Electric models are more universally adaptable. There are two types: hard-mounted and portable. You can find portable units at just about any auto parts store, but most of these are junk. The cigarette lighter units are fine for occasional use, but lack the power to fill up tires quickly. The better portable units will have larger gauge wires with alligator clips that go directly to the battery, thus supplying them with greater power.
These systems are typically rated by the duty cycle, which is the measure of how long they can continuously work. As an example, a 30-percent unit will run for three out of 10 minutes, using the other seven to cool down. Portable units are a good choice for someone who may use multiple vehicles. Look into their ratings, and get the best one you can afford, an extra three minutes a tire adds up when there are four involved.
The hard-mounted electric units typically deliver more air than the portable ones, and are easy to install compared to belt driven systems. These systems incorporate a tank and a compressor, both mounted to the vehicle. They are a great way to have air whenever you want, and can run other systems such as air lockers and suspension systems. They are also rated by the duty cycle; get the highest one you can afford. You may not always need the added run time, but you don’t want to spend any more time filling your tires than needed.
I chose to go with a hard-mounted system from Viair for my rig. It is a constant duty system capable of putting out 150 psi with 100-percent duty cycle, delivering up to an impressive 1.66-cubic-feet per minute. It fills the included 2.5-gallon tank to 145 psi in only three minutes, and a 35-inch tire from 15 to 30 psi in about 3.5 minutes. A complete fill up of all four tires should take less than 15 minutes. That’s more than enough air to fill my tires, as well as run air tools such as impact guns. The system also has enough power to inflate my float tube and blow the dirt out of my cab and bed. The unit comes as a complete set, with just about everything needed for installation and use, including a gauge and switch in a powder-coated metal panel.
To install the unit, the first step is to find a suitable location to mount the compressor. If you are using an oil-less unit, you can mount it at almost any angle. However, mounting the unit in any position other than upright can lead to premature failure of the seals and gaskets. If you have road salt to contend with, under the chassis may not be the best bet. Provided you have room under the hood, this can be a good place, but you must also take engine heat into consideration. You can choose a location inside the vehicle or in a tool box, bearing in mind that you want to keep the unit as close to the battery as possible. If you do make a long run to the battery, be sure to use a minimum of 8-gauge wire.
Position the Air Tank
Next, you’ll want to find a good place to put the air tank. Because this isn’t a complex piece of machinery, you can go ahead and place it under the vehicle. Tucked up alongside a frame rail, it should be protected from road hazards. Just make sure the drain valve is facing down. If the kit you purchased didn’t come with any mounting hardware, you can pick up some shelf brackets from the local hardware store. Remember, most manufacturers suggest you drain the tank periodically, so keep it in a relatively easy-to-reach place.
Once you have the compressor and tank securely mounted, it’s time to begin the plumbing and electrical portion of the install. The heart of the electrical system is a relay, a pressure switch, and a switched power source that’s only on when the key is on. You could connect the system directly to the battery, but an unlikely event, such as a rupture of the tank, could cause the system to stay on and drain your battery.
Instead, disconnect your vehicle’s battery; you don’t want to cause an unintended short. Next, find a switched power source. You can tap into an existing line, or use an unused bay in the fuse box. You’ll need to take a trip to the local auto parts store and pick up a test light and an add-a-fuse. Using the test light, locate a fuse holder that’s only on when the key is turned to “run” or “ACC,” depending on the vehicle. Simply ground one end of the light to the chassis and use the probe to test one of the openings. After you’ve found one, turn the key to “off” and verify that the light goes out. Put the add-a-fuse into that spot.
Power from the Trailer?
Alternatively, you may use your vehicle’s trailer charging line, if your vehicle is so equipped. Run this line to the switch, and then back out to the relay/pressure switch. You are also going to need to run a line from your battery directly to the relay. The one that came with my kit has a fuse holder in the line already; you just add a ring terminal to attach it to the battery. There are wires that go from the relay to the compressor itself; just match the color-coded wires and attach the spade connectors together. Finally, you’ll need a ground; I chose a spot between the relay and compressor and grounded both using a self-tapping screw. I like to cover all exposed wires with plastic wire loom as it adds protection and gives the installation a professional look.
Now we’ll need to plumb the system. The kit contains a pressure relief valve, the pressure switch/relay, a petcock, several compression fittings and a quick release chuck. The majority of these go right into the tank itself, after you’ve applied some pipe dope to the threads. Position the pressure relief valve at the top, and the compressor hose and pressure switch in adjacent bungs—if they are directly across from each other, they can give false readings. The petcock should be placed at the bottom so gravity can do its thing and allow you to remove water from the system. Put the compression fittings in the remaining openings.
Take the included hose out and let it uncoil in the sun for a bit. It’s fairly rigid tubing, so it can be a little tough to deal with. Take time to find a suitable location for the chuck; I opted for a spot in the rear bumper so I can reach my trailer tires when needed. Now you can run these hard lines to each fitting, starting at the tank and working your way out.
One needs to go from tank to the gauge, and the other to the chuck. Make sure you cut the ends square; a pipe cutter is best for this, but a razor blade and a flat surface will also work well. Loosen the collar nut on the fitting and slide in the hose. Gently tighten the nut until you feel firm resistance. The last step is to mount the remote air filter. Simply slide the tubing on and route it to a spot that won’t receive any splashes.
Gauge and Switch
You’ll need to mount the gauge and switch somewhere that’s visible inside the cab. The bottom of the dash is the best place for it, but you can place it anywhere you wish. You could also cut a hole in the dash and recess it for a custom look. Hook up the wire and tubing; it’s almost time to test your handy work. The kit comes with the option of tying into the factory dash lights to illuminate the gauge; I chose not to go with this option. It will not affect the function of the kit.
And, now it’s the moment you’ve been waiting for. Reconnect the battery. Install fuses in the holders—I put a 5-amp in the add-a-fuse; the 20 included in my kit come off the battery. Start the vehicle; flip the switch and marvel at the hum of success. As the tank is filling, take some soapy water and pour it over the airline fittings. You’re looking for bubbles that indicate a leak somewhere. If you do see some, turn off the compressor and bleed the pressure from the system by pulling on the relief valve. Simply recut the line and reconnect.
Enjoy your newfound freedom from filling stations by testing it out on a backcountry excursion.
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