A year ago, I added onboard air to our Jeep. It had always been something I wanted to do (since the last Jeep) and I can say it’s definitely one of the best things I’ve done to it. Not only do I air down more often, knowing that airing up will be quicker and easier, but I ended up with a much better compressor than the cheesy portable one I used to have.
The compressor is an ARB twin, and it’s mounted under the hood of our JK right above the brake booster. It’s wired to the second battery, which means I can run it without the engine running if I want. Especially on the Jeep, it’s fairly easy to open the hood, hook up a hose and go to town.
Even with the high output of the twin-piston compressor, airing up four 35″ tires from about 12psi to 38psi, as well as two 31″ tires from 20psi to 50psi does take quite a while. I have a little device I built that automates the process of airing up a tire to completion (which works great), but in order to do its work it has to open a valve, wait, close a valve, check the pressure, and decide whether or not to open the valve again to keep adding air. This causes the compressor to cycle on and off as the closed valve quickly causes the compressor’s pressure switch to trigger. Thus, while the valve is closed, the compressor is doing no useful work. This same thing happens when I’m switching the hose to the next tire.
So, I wanted to add an air tank to the system. This would allow the compressor to run continuously (it’s rated for 100% duty cycle), whether it was filling a tire, or just filling the tank. Synergy makes a bracket for the JK, which allows mounting a Viar 2-gallon tank above the rear axle in some dead space. This means that I needed to run an air line from the compressor in the front to the tank in the back. It also meant that I had an opportunity to plumb in an air outlet at the back of the Jeep, which would be more convenient to access than opening the hood.
But, what kind of air line do you use for on board air? I’ve used the plastic air line to run air in the garage, and plenty of hoses and fitting for tools, but I didn’t really find many clear writeups of what to use for on a vehicle and how to do it. Thus, I decided to write this post just to document what I ended up with.
It turns out, the best option for air line on a vehicle is … air line made for a vehicle. Specifically DOT air brake line. This stuff is rated for some pretty high pressures, and temperatures up to 200C. It’s not super flexible, but it’s not too bad. It seems like a lot of low-flow-high-pressure situations use 1/4″ OD line, but I wanted more volume than that, so I opted for the 3/8″ OD stuff (which is about 1/4″ ID, or about the same as a standard quick connect fitting.
I ordered a few specific parts from a 4×4 vendor:
- The Viair tank (VIA-91022)
- The Synergy bracket for 2012+ JK (PPM-4022)
- The Viair package of 1/4″ MPT pre-sealed plugs for the tank (PPM-4022)
- A 1/4″ MPT drain cock (VIA-92835)
- The ARB quick connect (ARB-0740112) and dust cover (ARB-0740113)
I have no idea how well the dust-covered quick connect will really hold up, but it has a nice large rubber ring on the sleeve that seems obviously better for cold/dirty/gloved hands than a regular one. If it doesn’t hold up, I’ll put a regular one on there and figure out some sort of cover.
The rest of the line and fittings came from the usual gettin’ spot. Specifically:
- 30m of 3/8″ OD DOT air brake line (which was more than enough)
- Two right-angle-and-swivel 3/8″ to 1/4″ MPT push-to-connect
- Two straight 3/8″ to 1/4″ MPT push-to-connect fittings
- Two 90 degree 1/4″ NPT street elbows
- A 1/4″ FPT bulkhead coupler
- 5ft of 1/2″ fiberglass heat shield braid
I first decided where the drain cock was going to go, and where my lines were going to interface with the tank. I installed the drain and the plugs in the appropriate spots and then mounted the tank and bracket before installing the push-to-connect fittings. I covered the holes for those in the tank with tape while doing the install to avoid getting anything in the tank (and installed the fittings afterwards to avoid damaging those). Synergy tells you to jack up the vehicle by the frame to let the suspension droop and I can say that this is definitely worth the time and makes the process much easier.
Next, I taped one free end of the DOT air line and started scouting my route from the engine bay to the rear. Since the JK has a V6, there are two hot exhaust headers and cats on either side of the engine, and the driver’s side one is pretty much exactly where I would have wanted to go straight down from the compressor. Instead, I ran over to and down the transmission tunnel behind the engine. The air hose is rated for 200C, but the cats could definitely get hot enough to cause problems. There are other wires and things in the tunnel area, so that seemed like a better plan.
I put probably 4ft of the fiberglass heat tube over the line for the trip down the tunnel and over the first part of the transmission and secured the ends with heat shrink. This stuff fits pretty loosely and in addition to blocking a lot of radiant heat, also makes me feel good about abrasion resistance and anything else in this sensitive area.
Over the top of the transmission and the transfer case, I was able to keep the line pretty much right down the middle of the chassis, going over the frame supports to keep it zip-tied down snugly and away from moving or heating parts. Over the evap canister and bracket and right into the air tank via straight fitting it went.
Once I had this run done, I was able to cut the line to length in the engine bay and used a straight push-to-connect fitting and an elbow to interface with the compressor.
After that, I used another section of the air line to go from the tank (via right-angle fitting) over to the rear passenger corner. The ARB bumper has a cover here, which is removed if you install their tire carrier. Since I don’t have that, it seemed like an obviously non-structural place that I could drill through for the line, which was also easily replaceable if I needed. The metal the bumper (and thus this cover) is made out of of is super hard, and it took a lot of drilling on my press to get a suitable hole through it.
Once I did, I was able to mount the bulkhead connector on the cover plate, with a swivel push-to-connect elbow fitting on the bottom to accept the DOT air line, and a fixed elbow on the top to accept the coupler. I used another small section of the fiberglass wrap to protect the line as it sits just below the bracket for the Gobi rack. This provides is some resistance to abrasion.
Now I have a quick-connect located on the outside of the vehicle, where it doesn’t interfere with the operation of the tailgate or the roof rack ladder, nor is it facing front or sticking out the side to be caught on anything. It’s also in the middle of the jeep-trailer system, which means making the hose reach all six tires is no longer a challenge.