Buying A New Replacement Pressure Switch

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My thanks to Tony M. who was having a real problem with buying a new pressure switch. So far, he’s purchased two, and neither seems to work. So he’s looking for one that is similar to the one that came off his compressor. Let’s see if we can help him out.

What’s critical about a pressure switch?

Same pressure range!

You definitely want one that’s got the same pressure range capabilities of the old one. For example, if your old pressure switch cut in at 90 PSI to start pumping air again, sure you can use one that cuts in at 110 PSI, or 80 PSI.

If you acquire one that cuts in at 110 PSI, and the old one cut in at 90 PSI, odds are pretty good that your compressor is going to cycle on and off a whole lot more often, and that isn’t good for them.

If you get one that cuts out at 80 PSI, that’s less of an issue, yet it does mean that any air tool you have that needs 90 PSI to run will have its operation compromised a bit. Not a deal breaker, but probably not best. Buy a switch that has the same cut in, cut out, or one that is adjustable.

Same terminal locations?

Nah, that doesn’t matter, as long as the new switch is wired for 120 VAC, it will have two sides. One is the supply side (power cord) and the other is the load side (motor).

Each side will have one hot wire and one return (usually black for power and white for return).

The switch will also have a place on it for the “earth” or “ground” wire. That’s typically a smaller wire, and is often green. It is important to connect the ground wire to “earth” or “ground” terminal on the switch. It’s often a different color screw where the ground attached. Look for that, or the “earth” symbol.

Comparable voltage!

If a compressor needs 120 VAC to run, you don’t want a switch that controls 24 VDC as the switch would fail immediately!

The good news is that most switches on the market that control 120 VAC are also good for 240 VAC.

If your air compressor plugs into a wall socket, it’s running on 120 VAC in North America. Make sure the new switch has this voltage included in its operating range. In other countries, it is possible that a wall socket will provide 240 VAC, so do be sure.

How the pressure switch is mounted!

There a many styles and shapes of pressure switches on the market. You want to emulate the mounting of the old compressor pressure switch.

Since Tony was kind enough to send in the question with photos. Let’s focus on his switch mounting style for the moment.

Note that there is a threaded hole in the bottom of the mount. This thread is normally threaded onto a nipple that sticks up out of the tank. This nipple ensures that the diaphragm in the switch is exposed to the tank pressure, in order to react to the tank pressure to turn the compressor on, or turn the compressor off.

Some pressure switches are not mounted on a cast manifold, or may not be gray as the one shown above is, but all will have a threaded port to screw onto the nipple from the compressor tank.

The “manifold” shown in the photo above is simply a device to allow a variety of compressor components to be attached under the switch rather than elsewhere on the compressor.

For example, one of the ports in manifold will be there to allow a gauge to be threaded in. That gauge will show the tank pressure. That’s the one on the bottom in the photo and the back of the tank gauge can be seen.

Here is another view of the compressor pressure switch manifold found on Tony’s compressor.

The nipple on the left of the photo above likely had the regulator threaded onto it, and then the other side of the regulator had the coupler into which the user inserted the connector from an air line.

Did you acquire a switch that didn’t have a manifold base like the one above. No worries. Make your own.

Visit a decent plumbing shop or hardware store, and buy enough tees, nipples and adapters and build your own.

The compressed air needs to get to the pressure switch, unimpeded. That’s important.

Weather the tank gauge is on one side or the other, as long as it gets air from the tank, it’s be fine.

The PRV too, has to have a direct feed from the tank. But it doesn’t have to be exactly the same location as the old switch.

There has to be a feed to the regulator too, so air can get out of the tank to be used.

Or, ensure that the switch you buy has a manifold base similar to the old pressure switch, and you are good to go.

Unloader valve?

What the unloader valve does is covered elsewhere on this site, so we won’t redo that page here.

Suffice to say that if the old switch had a unloader valve ( one shown on the side of the switch in the photo above) then the new switch MUST have an unloader valve too.

Some switches have an external unloader valve as shown in the photos above, and others have an internal unloader valve, where the line coming to the unloader as shown above is, instead, inserted into a fitting on the bottom of the pressure switch.

Whether or not the unloader valve is internal or external, the replacement switch must have an unloader.

If the old switch had an internal unloader and a switch with an external unloader was acquired, run the unloader line to the outside switch. It will work just fine. Or, if the reverse, the old switch had an external unloader (like those shown above) and the new switch has an internal unloader, simply plug the unloader line into the internal port, and all will be well.

On/Off switch?

Old pressure switch had an on/off switch, new one doesn’t? Who cares?

If you think about it, pulling the plug from the socket does the same thing as an on/off switch.

Sure, an on/off switch provides convenience, by allowing the operator to turn the compressor off at the switch. If you need an on/off switch and don’t have one, when work with the compressor is done, pull the plug from the socket! There’s your on/off switch.

Be aware!

You don’t need an on/off switch on a pressure switch. However, when the compressor cuts out when the tank is full, and if you leave the compressor plugged in when you are done work, if you have any leaks anywhere on the compressor or air line, the tank pressure will drop. When it drops far enough, the compressor will start.

That’s a waste of energy and, if the compressor is near where you are and it’s the middle of the night, nope, you don’t want that compressor starting up on its own.

So, if the old compressor switch had an on/off switch, and the new one doesn’t, make sure you remember to unplug it when you done working with compressed air.

Different base style of compressor pressure switch?

Unlike Tony’s, the switch in the photo above has no manifold base. Well, add a nipple in the bottom port, add a couple of tees, and you can create your own.

Questions or comments about buying a new replacement compressor pressures switch? Add them below.