They are electro-mechanical devices, and through repeated cycling they do fail, so it helps to know about wiring a compressor pressure switch if you ever have to replace yours.
It will take pages and pages to try to assist in the wiring of every brand of compressor switch, so here are some specifics about the process, and some generalities too.
Safety when wiring a compressor pressure switch
If you are well grounded the power in a 120 VAC circuit can kill you. If you are working on a switch with 240 VAC, the power there will kill you if you are not very careful.
In operation a pressure switch turns the power ON and OFF to an air compressor motor. When the air compressor is plugged in there is power at the switch. When you remove the switch cover, assuming that’s the type of pressure switch your air compressor has, liver terminals are exposed. So double check to be sure the compressor is unplugged – or the breaker to the compressor is off – before starting any work on the pressure switch.
Kill the power to the compressor completely when you are wiring a compressor pressure switch.
Basic pressure switch wiring
You have a power supply to the air compressor. For the typical DIY type home or small workshop air compressor, that power supply is a plug on a wire that connects to a wall socket.
Plug in the compressor plug and you have power flowing to the air compressor, through (perhaps) an ON/OFF switch and to the pressure switch, and if the pressure switch is closed, power will flow through it to the start capacitor on the air compressor motor.
The pressure switch will be closed (passing electricity) if the tank air pressure is below the pressure switch low pressure set point. If the air pressure in the compressor tank is higher than the pressure switch low pressure set point, the pressure switch will be open (not passing electricity) and the power stops there.
In the photo above you see two views of the same pressure switch. The image on the right has the cover on, and on the left, the cover off. Note that there are two wire-ways under the floor of the pressure switch. One of these is to bring the power cord from the wall plug through the wire-way and up to the terminals on the pressure switch, and the other is to allow the wires from the switch back out of the switch and up to the motor circuit.
The wire terminals themselves are usually marked with LINE – which is the terminals for the power supply from the wall plug, and the other side is noted as LOAD or MOTOR, and the motor wires are attached to these terminals.
On the right side of the left hand image above, you can see the screw to which the ground wire is attached.
A different style of pressure switch
Of course, if you have a pressure switch like the one below, the wiring is fairly simple.
One wire connects to the terminal on the compressor ON/OFF switch and the other to a terminal on the motor.
Wiring a pressure switch video
Master Tool Repair was kind enough to make the following video available for folks that are rewiring their own pressure switches.
With most electrical installations, it is important to remember that attaching the ground wire is critical if you want the circuit, and user, to be protected from shorts or faults in the system.
I’m facing a problem here,
I’ve installed a new switch and it gets up to pressure then stops. When it stops air is released from the switch till it gets low enough to start up again. Is this normal?.
I have same problem,
I have installed a new switch and it gets up to pressure then stops. When it stops air is released from the switch till it gets low enough to start up again. Is this normal?.
On my other compressor it fills up then stops and dosent kick in again till you have actually used some air. The other compressor is working non stop to keep it full of air. There is only one screw on the pressure switch, please advise if possIble.
I have insTalled a new switch and it gets up to pressure then stops. When it stops air is released from the switch till it gets low enough to start up again. Is tHis normal?. On my other compressor it fills up then stops and dosent kick in again till you have actually used some air. The other compressoR is working non stop to keep it full of air. There is only one scRew on the pressure switch, please advise if possIble.
I have a continuous run compressor and wanted to fit an off/off pressure switch; the type you show in your description. Wiring is not a problem. What is a problem is the plunbing. It has a 4 port valve; from which i presume i connect one to the tank, one to the line out, one to the pressure gauge and the last to a safety relief valve; all as previously connected to the tank. The problem I have is, that the pressure switch has another smaller outlet port; which after some reading I believe is a pressure release (unloader) for… Read more »
In the absence of a visible screw or bolt, they I would suspect that the cover simply snaps into place with tabs on the base around the bottom of the cover. I also think you will have to remove the button to removed the cover if there isn’t a clearance hole in the cover.
Thanks for your help. How do you get the cover off? I have an FBANG switch just like the one in your picture, but I can’t see anything holding it together. Do I need to remove the red switch button? Thanks for any help you can give.
OK, you’ve kind of nailed it, I think John. The switch may be wired correctly, yet if a full path the motor allows the compressor to cut in when the pressure is low, cut out when the pressure is at the stopping point, the compressor stops, and that’s repeatable without the switch in the circuit, and then when you add the switch there doesn’t appear to be enough power to drive the motor past the 50 HP point, then it’s got to be the switch that’s preventing enough power to feed the motor. If you have a meter, with the… Read more »
If we bye pass the compressor switch it works fine . And we got a new switch and and it wired correctly
OK, if the belt on the motor pulley is stopping the motor, first remove the belt and turn the PUMP pulley by hand a few turns. Make sure the pump isn’t seized. I doubt it is but check first. Then, assuming the new pressure switch is wired correctly – and if the motor starts when the pressure drops it likely is, then I think you may be developing a run capacitor issues. Here’s information on them and how to test them. Can you please check and let us know if that was / is the issue, John?
Motor stops turning
The belt is stopping you say John, but is the motor shaft stopping? If it’s not, is the belt actually stopping or slipping on either the motor or pump pulleys?
Wire a new air compressors switch to a 220 v compressors but the belt stops moving when it hits 50 psi . Any body can help
While I would be delighted to help, now knowing the make or model of compressor you are working on makes this very difficult. What does ” compressor will not turn off after delay time” mean, exactly, please
Hi how are you.
I’m working in air compressor
I’m conect the pressure switch and the compressor load and un load automatically..but the problem is compressor not turn of after delay time? Why
From what I’ve read by NOT using one of the “messenger terminals”, or sets of terminals, you turn the 3 pole into a 2 pole. The installation instructions should show the flow paths and not using one of them makes the switch 2 pole. However, I am not an electrician, and I do not provide electrical advice. For advice that meets the electrical codes where you are, ask an electrician there. Good luck.
We have a 240v air compressor that had a bad contactor switch. The old one was a Square D brand 2pole switch. The guy at the electrical parts place gave me a new Eaton 3 POLE switch. How do I connect the new one correctly?
I have a new 240V air compressor. I wired the breaker using Romex 12/3 with ground. The power cord for the compressor 4/C 12 AWG 600V has 4 wires: Black, Red, White and Green. how do I wire the pressure switch using all wires. My electrician said leave the white unconnected?
I have two pressure switches ( one is written as base load & the other is written as peak load) and and two compressors.
Both compressors have it’s own contactors.
I have one auxiliary contactor (2 N/O +2 N/C) to switch between compressors whenever compressor needs to start.
I want to connect Coil of both contactors & both pressure switch to auxiliary contactor.
How to do the wiring for this ?
Should I use plc’s digital output to energize auxiliary contactor for switching compressors ?
I don’t understand
One wire will be coming from the power supply, the other wire going to the motor. I’d simply put a toggle switch, 2 way, non passing, into either line. Non passing meaning to me that power couldn’t flow until the switch was tripped to passing. Simply, breaking the power wire that’s going to the pressure switch, or from the switch to the motor would accomplish the same thing, stopping power to the motor. Make sure the terminals on the switch are guarded so someone couldn’t touch them, even with them only being 12V.
My compressor (12v on board with tank) has the small round pressure switch fitted to the tank with 2 black wires coming from it which lead inside a metal cover on the end of the tank.
There is no switch on the unit but I intend to install one in the car via a relay. Is there an easy way to determine which of the two wires goes to the motor, or does it not matter which one I ground via a switch.
To try to run two compressors with one switch will require some creative wiring and plumbing. Better to have two pressure switches, one for each compressor.
hi, i have a pressure switch like the second one pictured how do i connect it to two compressors?
Hi Chris. Sorry, I don’t have any idea what the numbers you refer to in your latest comment mean. I don’t have the manual you are looking at. If your switch is the same as the one with the red button, look at the view on the left. The power and return lines from the power supply connect to the two terminals on the left, wire going in the bottom of the strain relief on the left. The power and return lines from the motor connect to the terminals on the right, and when I do this, they match. By… Read more »
Hi I am installing into the back of a Land Rover . I have a Tank installed etc . I have connected heavy gauge wire from battery to Relay (30) wire from compressor to ( 87) ….. 86 is earthed .. so 85 goes to pressure switch ( does it go to the terminals marked P or N ?)…….. I have a switch ( mounted in dash) so a wire from this goes to pressure switch ? either P or N ( need to know which ) , and what terminal do I use for wire to the relay The… Read more »
The pressure switch in the photo on this page has two wires, one of which is connected to the compressor ON/OFF switch, and the other to the power in of the compressor motor. The other wire on the small fractional HP motor is the return, and typically bypasses the compressor ON / OFF switch and back to the power supply return. I’m still not quite sure of your use. Are you adding a tank for the switch to be inserted into? I suggest you visit a compressor store and have a look at the small, fractional HP compressors with air… Read more »
Hi . The Compressor is a 12v HD for a 4×4 . just has 2 wires red + and black – . I am hard wiring in . I am using switch with fuse etc . Relay . The Pressure switch is identical to the 1st picture ( showing bare and covered . I have wired up as follows .. Permanent live -fuse-87 to relay …30 out to compressor ( live) heavy gauge cable , so just need to know about terminals 86 and 85 and what wires would go to the pressure switch as the markings are P and… Read more »
Not knowing the compressor make and model makes answering this more difficult, Chris. Typically, the pressure switch will have terminals for two wires from the power supply, two wires for the motor supply, and one or two terminals or lugs for earth or ground. I cannot tell without seeing a photo of the actual connections of the switch, I’m not going to guess.
I have a pressure switch . its marked N and P ( not motor / load) . What wires would go into these ? I am using 12v via relay and switch. The large terminals on the relay are 87/30 so these go to the compressor . So basically I am just using positive and negative wires , Any help would be appreciated as searched and not found anything .
Kind regards chris
Here’s how I would do it. Make sure the power of off, Phil. Bring the power supply line into the pressure switch through one of the strain relief ports on the bottom. Attach the green wire to the ground screw, and the black and white wires, one each, to the two screws on one side of the switch. Bring the motor wires into the switch through the other wire portal, green to the ground, and put the white and black wires on the matching screws on the other side of the switch. That should do it. Flip the breaker on,… Read more »
Hi Can you tell me how the wiring goes please for the Red knobbed pressure switch you have shown above as i have one and there are no markings on it at all. Thanks