Automatic cycle hack for a small compressor

This is a project I did quite a while ago, which has been working well for me ever since. I get a lot of looks when airing up my tires and so I thought I’d do a bit of a historical how-to of my setup in case other people are interested. The exact parts used here may no longer be available, or may have been revised so some improvisation will likely be required.

If you take your vehicle off-road, you probably air down your tires before you get started. That means you have to air them back up when you’re done. This can involve fancy things like on-board air, or un-fancy things like driving slowly to the nearest gas station to use their pump. It’s pretty common, however, to have a little 12v air compressor that you can use to re-inflate your tires. You typically plug it into power, turn it on, and it runs until it overheats or you have inflated all your tires.

The one I’ve got is a pretty cheap dual-cylinder one, apparently from “Q Industries”, model “Q89”. This compressor is re-badged under many names, including SmittyBilt. I got this because it was reasonably quick and pretty cheap. Surprisingly, it hasn’t let me down yet. One of the biggest downsides when I received it was that it came with non-standard fittings. I wanted to be able to plug the thing in (and maybe even mount it) and reach all four tires with the air hose without having to reposition between each one. So, the first thing I did was replace the proprietary fitting with a standard one. Luckily, after removing the original fitting on the manifold between the two cylinders, I found a 1/8″ female NPT fitting and was able to use an elbow and a couple nipples to fit a standard M-style quick connect:

This let me use standard air hoses and chucks to reach as far as I want. However, there was a problem. It was actually an opportunity and it led to a much better hack.

By default, these kinds of compressors come with air chucks that are free-flowing. When not connected to a valve, they just blow waste air out as the compressor runs. When you clamp it down on your valve stem, it creates a seal and the air is forced into the tire. The above quick-connect, however, is meant to operate differently. In a big shop, your compressor runs as needed to fill a tank, and then shuts off. If your fittings (intentionally) leaked air all the time then your compressor would have to run constantly. Thus, fitting this cheesy compressor that expects an “normally open” fitting with a “normally closed” one, you’re setting the stage for it to explode or destroy itself as it tries to compress the small volume of air in the feed lines to infinity (and beyond).

Thus, the awesome part of this hack is actually a pressure cut-off switch. This causes the compressor to turn on when the pressure in the lines drops below some number, and then cut off once it has built up pressure past a specific point. Just like a shop compressor. Turns out, this is pretty easy to accomplish. Here is a (bad) diagram of how this has to go:

In the center, you’ve got the compressor, which takes power from your battery (red/black coming in on the left) and pumps air out the grey outlet on the top. The air is fed into a manifold with both the quick connect fitting and the pressure switch attached. The pressure switch is normally closed, which means it allows the compressor to be powered until the pressure in the manifold rises above about 100PSI. When it does, it interrupts the power to the compressor. When the pressure drops again (as you start to inflate your tires), the switch allows the compressor to be powered again.

Here’s the rest of the first image above, with the pressure switch attached:

The pressure switch is the black cylindrical device on the left-hand side of the manifold, with white wire leaving the terminals and headed for the main feed.

This compressor has a¬†“small” junction box in the feed wires just before they enter the body of the compressor. This houses a circuit breaker (and a lot of air), but provides a perfect place to interrupt the +12V line and re-route it through the pressure switch. I used 16 gauge wire for this task, which is probably a little light, although the run is short and I’ve never noticed it heating up even after extended use. There doesn’t seem to be any noticeable voltage drop such that performance of the compressor is affected.

Here’s a view of the routing of the wire. Imagine the +12V line entering the box, taking a detour out through the pressure switch and back via the white wire, and then resuming its path into the compressor itself.

When choosing a pressure switch, you’ll want one with a fairly low cut-off limit. This is the amount of pressure you’ll need to develop in order to stop the compressor from running. A very small/cheap¬†compressor probably can’t really make 150PSI so if you have a switch rated for that, it’ll never cut off and likely burn itself up trying. It has been years since I bought mine, but as I recall it is a 90/110PSI switch. That means it turns off when the pressure reaches 110PSI and turns back on when it drops below 90PSI. Anything around this should be fine, and although it’s not exactly what I have, I think this one from Amazon is likely just fine.

So, for probably less than $25 you can make these changes to your cheap compressor and have it behave like the expensive one you have in your shop. Here’s a video of it cycling as I release the pressure with a tire inflator:

Obviously it goes without saying, but it’s your responsibility to make sure that you don’t blow up yourself or your friends using the instructions provided here. But, I hope it helps make your use of this kind of compressor a little less of a hassle.

Category(s): Miscellaneous
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