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.