Q89 Compressor Modification

One of the first things you need when you start into off-roading is a good compressor.  This is because you air down your tires each time before you go, and have to air them back up before you drive home.  A while back, I got a dual-cylinder compressor called the “Q89”.  It’s made in China, of course, and is actually rebadged by several other companies like Smittybuilt.  When I got it, it had a cheesy coiled air hose with an even more cheesy schrader valve chuck on it.  The stock hose would leak if you didn’t hold it perfectly straight, the integrated gauge was useless, and the chuck didn’t really fit into my wheels with their recessed valves.

Last summer, I replaced the hose by getting an appropriately-sized barbed fitting, which had a 1/4″ NPT fitting which I mated with a standard quick-fit adapter.  This allowed me to use a normal air hose and normal accessories from that point on.  The problem with that setup was that it still included the use of the funky (read: flimsy) large diameter air hose exiting the cylinders.  This was so flimsy that it would kink and cut off the air supply really easily when pulling against it to reach a distant tire.  The other problem was that standard air fittings (like a tire chuck) expect to only allow air flow when necessary and not all the time.  This meant that while moving from one tire to the next, the compressor would lug down trying to pressurize the hose, which it’s not designed to do.  Thus, while moving from tire to tire I either had to turn off the compressor or hold my thumb on the valve core part of the tire chuck to allow the pressure to escape.

After getting fairly frustrated with this on my last trip, I decided to get a little more intimate with the thing and fix it for good.  The first thing I did was order a 90-120PSI cut-off switch that is normally closed, and breaks the circuit when the pressure climbs past a certain level.  This needs to be fit into the air line and the electrical feed for the pump.  It’s only rated at 5A, but the inline circuit breaker box actually has a relay already.  I wired it to that relay along with the switch so that it would cut the pump off properly.

If you take off the top housing that runs between the two cylinders, you can see that the output port is actually just a 1/8″ NPT female port of a manifold joining the two cylinders, the same as the cutoff switch.  I got a matching long nipple from the local auto parts store, as well as a tee and the adapters necessary to mate with my 1/4″ NPT female quick fit.

Now, you can turn the pump on normally with no hose attached and the pump will quickly turn off.  When you attach a hose, it will run again to pressurize the hose and then stop.  As you fill your tire, it cycles with you, turning off when the air flow stops.  It’s a much more useful tool now, and I might even add a tank at some point to help it cycle even less and give me the opportunity to power some pneumatic tools.

Category(s): Hardware

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