The compressor pressure switch provides some key functions on the air compressor.
One purpose of the compressor pressure switch is to shut the compressor off when the air compressed into the tank has reached the correct air pressure.
Another function is to turn the air compressor back on on when the air pressure level in the compressor tank has dropped to the point where more air is needed.
One more key function of the compressor pressure switch is that it is the first defense to over pressurization of the tank and air lines on the air compressor.
Pressure switch appearance
On many of the smaller DIY type air compressors, there will be only a few manufacturers of pressure switches. Two common makes are shown in the photo below.
Compressor Pressure Switch for Smaller Compressors
There are a lot of air compressors sold today that we would classify as mini-air compressors. These are air compressors that can be operated by a 12 VDC power supply from a vehicle accessory port. Some of these mini-air compressors also work on 120 VAC. They all use small, fractional HP, electric motors to power the small compressor pumps. They do not use the air pressure switch shown above, but rather, theirs may be a tube shaped switch as shown in the next image, one from a DeWalt mini-compressor.
Compressor Pressure Switch Operation
A pressure switch is an ON/OFF device that reacts to pressure. They are used in many industries and in many media. We’re concerned here only about the air compressor pressure switch.
The pressure switch will have a device inside that reacts to pressure applied against it. In the Condor and Lefoo switches shown above, that internal device is often a diaphragm of some sort, one of which is shown in the photo.
When air pressure in the air compressor tank creates enough back pressure, that back-pressure deforms the switch actuator (diaphragm or other sort) inside the pressure switch and causes it to move. On the high pressure side, that actuator deformation typically opens contact inside the switch. With the contacts now being open, power stops flowing to the compressor motor circuit, and the compressor stops.
When the air pressure inside the compressor tank and lines drops, the compressor pressure switch internal operator begins to deform or move in the opposite direction. This movement is typically caused by an internal spring. When that deformation or diaphragm movement is far enough the contacts inside the pressure switch are made (come together) and power flows across them. The power flows to the motor circuit on the air compressor, the motor starts, and air starts pumping into the tank.
As the back pressure inside the tank increases, once again, the compressor switch internal actuator begins to deform or move the other way, eventually the contacts break, and the power stops.
The cycle repeats as long as compressed air is being used, or until external power to the air compressor is stopped.
Adjusting the compressor pressure switch
There may come a time when you have to adjust the compressor pressure switch. Typically that will only be when you have replaced the original switch that came with the air compressor, and the new switch cut in and cut out settings are different, and have to be adjusted to the levels of the replaced pressure switch.
As far as we know, you cannot field adjust the pressure settings of the tubular pressure switch similar to those shown in the photo of the DeWalt pressure switch. If your compressor uses this type of pressure switch and the switch fails, you are going to have to replace the failed switch with another of the same size and type. That will typically mean quite a search, as many of the brands of compressors on the market are mass marketed through big box stores which have a dismal record of stocking parts for the air compressors they sell.
Please follow this link for more on adjusting the compressor pressure switch.
Comments, advice or questions are welcome using the form below.
Can I use a different pressure switch for my 21 gallon harbor freight compressor in stead of the Lefoo one that it comes with should work right if I get one that fits?
Yes indeed. Once an air compressor is off warranty, basic parts like pressure switches, PRV’s, tank check valves, motor capacitors etc. can be sourced anywhere you can find them, in particular since so many store brand type compressors offer little or no after sale support. As long as the pressure switch you select fits on the compressor, as long as all parts connected to the pressure switch now are still connected after the new switch… Read more »
What causes the pressure switch to fail and not relay power to the motor side? My compressor stopped running and I found out using a multimeter that the power was not running to the motor and had failed. I purchased a replacement pressure switch (at $80!) and installed it. The compressor started right up. I thought my problem was taken care of. Now 6 months later the compressor is again not working. I went through… Read more »
Kevin, unless the compressor is really high cycle, the pressure switch should last for a long time, unless there is an issue with the power supply. It would be good to see inside the old switch to check the condition of the pressure switch points. Are they burnt, for example. While a 20 amp circuit may be ample for some HP compressor motors, it’s not enough for others. I don’t know the make or model… Read more »
Thank you for your reply. It is a craftsman 25 gal, 1.8hp model 919.195413 running on a 110V 20A line. I have it running continuously about 5 hours a day, 3-4 days a week. I wouldn’t think this work schedule would be too much for this machine. I’ve tried it out on other wall plugs around the house and nothing. No hum, motor noise, or any power going to the motor side of the switch.… Read more »
I, too, would think that a 20 amp circuit for a 1.8 HP motor would be ample… unless there are other things on the same circuit pulling power while the compressor is starting or running. I guess that you’ve no option but to replace the pressure switch… again! Cheers.
Where do I hook up My quarter inch line From pressure switch go to the tank?
Hello Ray. Since you don’t indicate the make or model of your compressor, I can only provide a generic answer. I believe the line you refer to normally connects the unloader valve on the pressure switch to the the fitting inserted in the tank with the tank check valve. Since the air line from the pump head also connects to that fitting, when the compressor stops, and the unloader valve trips to open, the line… Read more »
The pressure switch contacts fuse together at startup due to sparks do you think I might have a bad breaker in the electric panel?
I am not an electrician so this is just my opinion, Stephane. I would think it more likely that you have a short in the compressor motor.
One thing that would most helpful is a diagram showing the internal mechanism of a pressure switch. I have a home compressor that has a Chinese made pressure switch…really just an off/on type with a red button on top. However it needed some work recently and while doing this a small spring came out from the bottom. Trying to figure out where that goes and how to get it all back together has proved very… Read more »
A bit difficult to do Paul, as not all pressure switches are made the same way. We do know that reassembling a compressor pressure switch that has “sprung” apart is a real witch of a thing, so we don’t do it. Don’t need the aggro. We replace one that has failed instead.
I just purchased a Lefoo LF10-4H pressure switch. I have everything wired correctly, but it won’t auto fill. There is this little black button in the middle of all the wiring connections, that if I press it , the compressor kicks on. What am I missing?
Ken, a couple of things come to mind. This switch has an ON/OFF lever. Make sure it’s on. Is there air in the tank now? If the pressure in the tank exceeds the cut in pressure of the switch, then the switch will not trip on to allow power to the motor. If it’s wired correctly, and if it is installed correctly, and if you let all the air out of the tank and the… Read more »
Compresser runs but will not shut off at a 120 psi, I have manualy pushed the shut off down and it has locked and when I pull the air relase valve the compresser comes back on itself at a 115 psi, but it will not click the compresser off, theres no name on the compresser i dont know what age it is but i have some pictures of the cut off valve, i dont know… Read more »
Not seeing the compressor is a problem, Kieran. If you have a multimeter, pull the cover off the pressure switch, and probe the motor side of the pressure switch after the tank pressure bypasses the normal cut out pressure setting. If the pressure switch does not trip to OFF when the pressure reaches the normal cut out, we think it may be that the pressure switch itself has failed. Please check and advise as a… Read more »
I have an older 60g/, 5hp, Campbell Hausfield, compressor. I have needed to replace the pump, so now I have a “V”, cylinder Omega pump, to replace the older vertical cylinder pump.The New Omega, has a definite orifice for the main line. But unlike the original, that had a fitting that went to the manifold for the pressure switch, I can find no such fitting on the new pump. Can I tap into the line… Read more »
The images of Omega compressor pumps we were able to find did not clearly demonstrate the pressure switch plumbing. In general, as long as the pressure switch has access to the pressure level inside the compressor tank, the switch will allow power to flow to the compressor motor when the tank pressure is below cut in, and shut the power supply off when the tank pressure has reached cut out. If you are still in… Read more »
I just picked up an old Campbell hausfeld compressor. I’m trying to replace the pressure switch, and I’ve noticed there’s no unloader valve on the switch. Similarly there’s no line or port coming off the two fill connection. Anyone familiar with this type of set up? This machine is from the mid 70s, but it runs great and is quiet, so I’d like to try and keep it working. The current switch is a Furnas… Read more »
We couldn’t find an image of the pressure switch using the part number you provided, Mike. Are you sure there is no unloader valve? Sometimes the unloader valve is inside the pressure switch and the line to it enters via the bottom of the switch. Normally there is a line into the tank from the top of the pump. In your case you refer to two lines going, presumably, into the tank. Or, is one… Read more »
A lot of old air compressors got an check valve at the output of the head. This way they don’t need an onloader because the compressed air dont go back to he head (giving a hard time to the motor to restart at high pressure).
This kind of compressor head will sometimes have a little butterfly air drain valve near the air output for servicing.
Keep it that way and it will last forever.