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Air compressors can sometimes struggle to restart after reaching the cut-off pressure and switching off. The question is, why won’t the air compressor restart under load? This article will provide all the possible reasons as to why your air compressor won’t restart under load, along with existing reader questions & responses.
Table of Contents
- Reasons Air Compressor Will Not Restart
- Air Compressor Won’t Start Under Load
- FAQs (Frequently Asked Questions)
- Reader Questions & Responses
Reasons Air Compressor Will Not Restart
When the air compressor will not restart, the compressor troubleshooting process is similar to that of when your air compressor won’t start in the first place.
Except, that you know for sure your air compressor will run as it just did that. But now, that the pressure in the tank has reached cut-out and the air compressor stopped, the compressor will not restart when air is used.
If the compressor won’t start, there are a number of conditions that will cause this. These include:
- Power supply
- Overheating mot
- Faulty unloader valve
- Faulty capacitor
Power Supply
Since the compressor was just running, you can typically pass on checking the power supply. The compressor is trying to restart, which means that power is still getting to the motor circuit. If it was running before, the compressor tank pressure reached cut out and the compressor stopped normally, it’s likely that the power supply is okay.
Overheating Motor
If you’ve run the air compressor for a long time in a single session, totally ignoring the fact that many air compressors have a duty cycle and need time to cool down every ten minutes or so, it may be that the motor has overheated.
Compressor motors often have an air compressor thermal overload switch in them which trips open and stops the power to the compressor motor if it gets too hot. If you’ve been abusing your compressor a bit in terms of how long you’ve had it running, see the page on this site about duty cycle for some tips. Visit our air compressor overheating for more information!
Faulty Unloader Valve
One of the easiest diagnoses is to determine if the compressor unloader valve is working. Check out our air compressor unloader valve guide if you don’t know what the unloader valve is. If the unloader isn’t working your compressor may not restart, though it wants to.
If you pull the power cord, open the tank drain, and let the tank empty completely, you are emulating what the unloader valve is designed to do. Once the tank is empty, make sure to close the drain and plug the compressor back in.
If it starts easily now, then you can be pretty sure that the compressor unloader valve isn’t working properly, and you will need to correct that.
Faulty Capacitor
In the simplest of terms, a start capacitor boosts the compressor motor to help get it going. If the start cap is weak or failing, though it tries to, your air compressor motor may not re-start, even though it had been working fine just a short time before.
Checking the start capacitor will require a bit of fiddly work, including removing it from the motor side without shocking yourself, and using a multi-meter to check it. Visit
Is your air compressor still not re-starting? Is it because the air tank is full? Please read on!
Air Compressor Won’t Start Under Load
When an air compressor won’t restart after the air tank is full, it suggests that the check valve is stuck in the open position. This will cause a reverse pressure through the system from the tank into the compressor pump itself, meaning the compressor motor struggles to restart the compressor overcoming the tank pressure.
Air compressors have unloader valves for this same reason, to remove compressed air from between the tank and the compressor pump to enable restarting. Therefore, this could also be an unloader valve problem.
Testing if Check Valve is Preventing Compressor Restarting With Pressure
To test if the check valve is stuck open, release pressure from the tank until you reach and go below the cut-in pressure for the compressor motor. You should hear the pressure switch click to indicate the motor is being provided power. If the compressor doesn’t pump, it’s likely you have a check valve issue.
Diagnosing if Unloader Valve is Stopping Air Compressor Restart Under Pressure
To diagnose if the unloader valve is working, simply listen to the compressor closely as it reaches the cut-off pressure. Just after the compressor stops running you should hear a short sharp discharge of air from the unloader valve usually located on the pressure switch itself). If you don’t hear the unloader valve releasing pressure, this could be the cause of your air compressor not restarting when there’s pressure in the tank.
FAQs (Frequently Asked Questions)
The first thing you should do when your compressor won’t start is ensuring that the unit is receiving power. It is likely that a fuse could have been blown or that a breaker has been tripped. Check the power supply, the unloader valve, the check valve, motor, and capacitor to diagnose your problem.
Reader Questions & Responses
Compressor Will Only Start With Little Or No Pressure In Tank
Question
Bill, my compressor will only start with little or no pressure in the tank. The compressor is a Northern Industrial Tool 2 HP 8 Gallon.
The compressor will only start with no or very little pressure in the system and will fully charge starting from there. When it runs and reaches the pressure at which it should turn on it struggles and eventually trips the breaker on the unit.
Photo: manualzz.com
From reading all your posts it seems it’s either the start capacitor or the start relief valve? Is there any way to tell the difference or any other recommendations? Thanks, by Jay (Enfield, CT)
Question
Jay, from the sound of it, your compressor may have an unloader valve problem.
The unloader valve’s job is to open when the air compressor stops to allow any air trapped over the piston to escape. Why? Well, compressor motors run so close to the line in terms of their ability to start that any added load may be enough to prevent the compressor from starting unless all the air is let out of the tank.
There is a page detailing what they are and what they do link from the troubleshooting page. Check out the UNLOADER VALVE page and take corrective action as noted on that page. If this doesn’t resolve your issue, please post to this thread.
Air Compressor Will Not Start Under Load – Air Compressor Troubleshooting
Question
When the compressor is first turned on it will fill with air (because it doesn’t hold air), but when using it after it is full it does not want to start again and the motor cuts out.
Response
This sounds like the same issue that Jay was experiencing with his compressor in the post just above. I think that since it runs to cut out pressure when the tank is empty, and then shuts off, you may not get a typical “unload”.
Tell you what. Drain the tank to zero, close the drain, plug it in, and listen to the unloader valve as the pressure in the tank nears and then reaches cut-out pressure. When the compressor stops, does a blast of air come out of the unloader valve?
If not, then there is likely air pressure trapped over the piston, and when the compressor tries to start, it cannot overcome the additional load, and the motor cuts out on overload.
If you have any questions regarding an air compressor not restarting underload, please leave a comment below, with a photo if applicable, so that someone can help you!
I have a 5hp 2 stage compressor with an 80 gal tank.starts and fills fine from empty to 180lbs shuts off. I use air down to 135 lbs when it attempts to start. Runs slow for 3-10 seconds and trips the main breaker. I replaced the unloader and the start capacitor. Does the same. What should I try next? Thank you.
Can you give a model number, or at least a make and maybe a few pix of it?
Meanwhile, are you sure what you replaced was the start cap? And whether or not you changed the right capacitor, there’s a set of contacts on the back side of the motor near the armature. Those should be closed when the motor is stopped. If they don’t close, the start capacitor can’t do its job.
Glad it’s sorted.
Thanks for letting us know.
I have the exact same issue except I’ve replaced the motor and the pressure regulator switch, both brand new and it still won’t refill with any pressure in the tank. Only thing left is to tear down the compressor pump and have a look ??
So I’ve got the pump open and the pistons have some dirt and metal shavings on top. Could be from the shop grinding metals I can’t tell for sure. Pistons move ok and not much wear on the side walls. All air passages are clear. I’m changing the oil again and it looks fine no shavings in the oil. I took the back pressure valve off the tank and took it apart. Seems to have been working properly nothing sticking. The tank has been drained of condensation. Air intake filter is clean I’m adding a magnet to hopefully catch metal… Read more »
If you still have trouble after reassembly, please give your make and model, and/or some pix of it.
thank you both for the help – turns out it was the capacitor! .. .I got it changed out and all is well again. Thanks a million!