There are many types of air compressors in use around North America, and though many of them look different than other air compressors, and they certainly vary in size and other areas, there are similarities that will allow you to check and see why “an air compressor will not start, by yourself and hopefully not taking it to a shop which is both expensive and time-consuming. Follow the guide below to figure out why an air compressor will not start.
Air Compressor Will Not Start – Primary Troubleshooting Checks
Is There Power to Your Air Compressor?
I know, I know, it’s a dumb question, but needs to be asked.
If you are using a power bar or power strip… don’t! Plug your air compressor directly into a wall socket. The power strip may have failed.
If you are using an extension cord… don’t! You are putting a choke on the power to your compressor unless the extension cord is 25′ or less, and at least 10 gauge.
Move the air compressor to get it close to the wall socket, make sure there is power at the wall socket, and plug your compressor in there.
Is the Air Compressor Switch On?
If your air compressor is one of those that come with an ON/OFF switch, either separate from or part of the pressure switch, so please make sure that it is switched on.
Of course, if you have been using the compressor and it won’t start, the power switch will likely be on already. But just in case, please check it anyway.
What is the Air Pressure in Your Compressor Tank?
When my air compressor will not start I always check the tank gauge to see what the pressure reading is in the tank.
The reason for that is if the pressure switch is set with a cut-in pressure (the setting at which the air compressor will start) that is below the air pressure currently in the compressor tank, then your compressor pressure switch has not tripped to on, no power is flowing to the air compressor motor, so the motor cannot start.
Even if the air pressure in the tank is below the air compressor cut-in pressure setting, part of the diagnosis as to why it won’t start is to empty all of the air out of the compressor tank.
Try Emptying the Compressor Tank
Unplug or shut off the compressor.
PRV and Tank Drain
Drain the air tank by attaching an air tool or blowgun to the hose and blowing off all the air, you can drain the compressor tank by opening the tank drank valve and allowing all air (and any trapped water) to blow out the tank drain which you should do regularly anyway, or you can empty the compressor tank and test your air compressor pressure relief valve at the same time, by pulling on the PRV ring (wearing gloves is a good idea). This will allow all the air to blow out of the tank as well as make sure that the PRV internal workings actually do work. After all the tank air is gone, push the PRV valve back in and your PR Valve is reset.
By draining the tank you are emulating the function of the unloader valve just in case the one on your air compressor is not working properly. A complete description of what the unloader valve is and does is on the unloader valve page.
Emptying all of the air from the air tank allows any air trapped over the compressor piston – or compressor pistons – to escape out through the tank as well. If there was trapped air, the additional load on the air compressor motor may have prevented the compressor from starting.
OK. Plug your compressor back in or turn it on… any luck with it starting? No? OK then, let’s carry on.
Air Compressor Still Will Not Start – Secondary Troubleshooting Checks
We know we have power from the wall socket to the compressor, we also 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? Let’s take a look at some secondary troubleshooting checks.
Is Power Getting to the Air 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 Air 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 above shows you – a bunch of wires attached to screws.
NOTE: 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 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 multimeter 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 Air 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 website 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.
If you have any questions about your air compressor not starting, please comment below!