So, your air compressor still will not start? This is page two of help when trying to troubleshoot why an air compressor will not start. In case you have landed on this page first, page one is right here.
We were talking about power supply to the air compressor. Based on the previous page we have checked that we have power from the wall socket to the compressor, we now know that a malfunctioning unloader valve isn’t the problem, we are going to assume that if your air compressor has an ON/OFF switch that it is turned to on, so now what? What is it that the air compressor still will not start?
Is power getting to the compressor motor?
Knowing that power is getting to the compressor does not necessarily mean that power is getting to the motor.
There may be an ON/OFF switch in the power supply line between the wall plug and the compressor motor. Your air compressor will also have the pressure switch in the line from the wall plug to the compressor, either of which may be preventing electrical power from getting to the compressor motor. If the compressor pressure switch has not tripped to allow power to flow through it to the motor, the compressor cannot start.
Unfortunately, if you don’t have a poke probe or a multi-meter you will have a difficult time doing the next check.
Remove the compressors pressure switch cover
We need to try and find out if the pressure switch has tripped to the on position, and if power is flowing through it to the motor circuit.
Carefully remove the pressure switch cover. You will see something along the lines of what the photo left shows you – a bunch of wires attached to screws. Exciting, what?
Be very careful. If your compressor is plugged into a wall socket, the screws (terminals) will be powered, and if you touch them and you are grounded, you might be electrocuted! If your compressor is a 220 volt compressor I recommend that you do not mess with this at all!
Generally speaking (and this is anecdotal as I am not an electrician – don’t pretend to be – and do not dispense electrical advice) the wires of a 120 volt circuit are going to be white, black and green. Typically the green wire is the ground wire.
Also, typically, the black wires are the hot in and the white wires are the hot return.
Pressure switches often have terminals where the wires are attached. Some compressor switches have these terminals labeled. You may see the terminals labeled as line (the power supply in from the plug), load (the power supply to the motor), the terminals might be marked as power supply, motor… a variety of labels, if your compressor power switch has labels at all.
Using your multi-meter or poke probe, you want to see if the power is flowing from the line side to the motor side, so you are going to test the motor side of the switch. Is there power on the motor side of the switch?
If your tank pressure is zero PSI, then the pressure switch should have tripped to on, and power should be flowing across the switch to the motor side. If not, your pressure switch itself may have failed, assuming it tripped to on when the pressure in the tank fell below the normal cut in pressure setting.
What else stops an air compressor from starting?
After you have emptied the tank and plugged the air compressor in and/or turned it on again, is there any sound from the air compressor at all? For example, is the motor trying to start and cannot?
If that is the case, and the tank is empty of air, the issue may be that the start capacitor has failed.
What does the compressor start capacitor do?
Since an electric motor needs more power to start than may be available in a 120 volt power supply, in order for the motor to get going, it needs a boost. The compressor motor start capacitor does this.
Essentially it’s a quick discharge battery that recharges after use and is pressed into service when the compressor motor tries to start.
It charges and discharges throughout its life, and it does get used up. If the start capacitor has completely failed, then power cannot get through it to the start circuit of the motor, and nothing happens when you try and start the compressor.
More compressor start problems?
Yes, there could be other things preventing your compressor from starting. The compressor motor itself may have failed.
Diagnosing a failed compressor motor is getting pretty complicated for a web site help page.
If you have done all the checks noted here and you have not been able to get your air compressor going, and repairing compressor electric motors is not what you do, then I would suspect that it may be time to take your air compressor to the shop.