Compressor Won’t Re-start When Air Is Used

The compressor won’t re-start when air is used. Not an unusual air compressor problem and often relating to the malfunctioning of the unloader valve.

How does the unloader valve affect the compressor re-start? If you take a moment to read this page describing what the unloader valve is and how it works. you will be well on your way to understanding that.

compressor pressure switch with unloader

The unloader valve on this pressure switch is the brass colored item mounted on the side of the switch, as can be seen in this image. Look close to the bottom of the unloader and you can see the pin that is alternatively depressed and released as the pressure switch cycles in response to the changing pressure in the compressor tank.

We won’t get into the details of the unloader valve operation since it’s well covered on the page linked above.

Air is trapped over piston

When the unloader valve fails, however, air will be trapped over the piston or pistons on the air compressor.

That adds load to the drive train. Smaller air compressors operate with a very thin line between motor size and the power needed to drive the compressor pump. Adding even a little bit of extra load to the compressor startup process means that sometimes the motor cannot overcome that added load, and cannot re-start.

Compressor Won’t Re-start When Air Is Used

You use some air working with your air tool, the pressure in the tank drops, and eventually the pressure reaches the cut in setting, the pressure switch trips, and the compressor motor gets power through the pressure switch to start running, and pump more air into the tank. But the motor cannot start due to the extra load from the air trapped over the piston, since the unloader valve did not do its job.

When the air compressor motor cannot start it pulls more amps.

You may hear humming coming from the air compressor motor if it tries to start and cannot.

The lights in the house – at least those on the same circuit – may dim as the compressor motor tries to start.

If the motor tries to start for long enough and still cannot get going, the motor will overheat and shut down on a thermal cut out switch, if it comes equipped with one.

What to do when the compressor will not restart

Test the unloader. How?

Pull the power cord or switch off the compressor. Drain all the air from the compressor tank.

Identify your compressor unloader valve.

Power up the compressor. If the unloader valve was the cause of it not starting, then the compressor should now start.

As it runs up to the cut out pressure setting and stops, monitor the unloader valve to ensure that it is working.

If it is not, remove and clean or replace it.

Replacing it may mean that you have to replace the pressure switch if the unloader valve is an integral part of the switch.

Other reason why the air compressor will not start when air is used

There are a number of other issues that can cause the compressor not to restart when air is used. Most of these are covered in the pages linked from the list on the troubleshooting page on this site.

If you cannot determine the reason from the existing pages, leave a comment here providing compressor make and model, and complete details of the problem.


  1. Mark Neretlis says:

    old westair compressor where the motor cuts out and i have to push reset button on motor. cuts out at all sorts of times and under different loads. has a fairly new pressure switch.

    • Needing to push the reset suggests that the compressor motor is going off on thermal overload, Mark. Repeatedly running it when this is happening may cause compressor motor damage. See the page on this site about how to check your motor capacitors. Do that, and add a comment here telling us what you found, if you would.

  2. Perry Brunet says:

    I have a Craftsman Model 919.167242 air compressor. It would run and build pressure but never shut off. I replaced the pressure switch and regulator. Still had the problem. I checked the cylinder head and found one of the bolts loose. I tightened all of the bolts.
    Per the instruction manual I turned the regulator to the closed position (all the way counter clockwise). Started the pump and it runs continuously. I turned the regulator up a little and found that after the pump runs for a while,(but does not shut off) I can manually turn the switch off. The compressor does not start. I bleed some of the air and the pump will begin but again will not shut off automatically. I have noticed that with the regulator closed the tank pressure gauge will not register pressure. When I open the regulator the gauge does register pressure. How do I get the unit to start and stop as designed?

    • Sounds like a couple of compressor issues, Perry. What is the normal cut out pressure of the Craftsman Model 919.167242? If, when the compressor runs, the air pressure does not reach that normal cut out pressure, then the compressor motor cannot stop running, as the pressure switch isn’t “seeing” the tank pressure it needs to shut off power to the motor.

  3. My IR 2475 7.5 hp compressor has problem when restarting. Smoke comes out from the motor and breaker trips. If pressure is first drained from the tank , it will start no problem. When it fills there is a hiss sound at the end which means the unloader is working properly. What else could be the problem? We changed 2 of the capacitors because they were hot to the touch but the problem is still there. It’s connected to 60amp double pole with 2 rated wire

    • willyr says:

      I would have checked the caps first too. You say you replaced 2 of them. Are there more?
      The symptoms suggest either a motor problem or, as pressure builds, the backpressure is overloading the motor.
      Are you getting full power to the pressure switch, and also on the motor side of the switch, both voltage and amps?
      A plugged tank check valve could cause back pressure, as could a failed valve in the pump.
      All mechanical issues good, then it’s back to the motor. Does this motor have a centrifugal switch? If it’s failing, then, under load, that might be the cause.

  4. Compressor wont restart when try to build air i’m not sure is this unloader valve??

    • let me send you other picture

    • Does the compressor start and run up to cut out after you empty the tank and plug it back in?

      • The air compressor It starts up just fine when I first turn it on and it will run flawlessly to fill the tank to full capacity, but when I use up air and the compressor tries to turn back on it just strains and eventually trips the breaker.

        • Don’t know what make or model compressor you have, so I don’t know what kind of unloader valve it has. I’d certainly empty the tank, start the compressor, let it fill to cutout pressure, and ensure that the unloader valve works when the compressor cuts off.

          • I have this problem here and runs good and when is time to cycle becuase it is in use motor just do a hummm sound for a little bit and try again and do the same hummm sound but fail to start. If I empty the tank it will start immediately. Now I have seen a lot “let it fill to cutout pressure, and ensure that the unloader valve works when compressor cuts off” but no body explain what I am looking for to ensure that the unloader valve is working good”. What am I looking for to know that is working fine? a short explanation will help a lot.

          • Richmont, I’m surprised that you’ve not been able to find info about the unloader valve as lots of it is right here: //

            Also, mmore information about your compressor problem can be found here: // . It certainly sounds as though your start capacitor is weak.

  5. Umar Majid says:

    Hi sir, the problem with my compressor is that at start it works fine but when it trips it does not restart and the a.c ends up flashing and the compreasor heats up. Then after almost 1 to 2 hours, when the compressor cools down it starts again but the same problem everytime.

    • It sounds like the motor is overheating and the power is being cut by the thermal cut out to protect the motor from damage. Once the motor cools, the thermal cutout resets, and the motor can start. All of the typical things you need to check are found on this page: // . Please do the checks and add a comment here with your findings.

  6. This is a really great site – very helpful.

  7. Kenneth Morgan says:

    Compressor wont restart (electrically) as air is used down. It won’t restart until the unit is unplugged or stop-start switch is turned off for about 3 to 5 minutes, when restarted it goes all the way to the max pressure setting. has a Condor MDR 11/11 pressure switch with 4 ports and 160-200 psi. There is no air in the compressor supply line, it is an electrical issue that won’t allow the motor to start when tank pressure is bled off.
    I tried adjustment procedures for Cut-in pressure and Pressure differential adjustment.
    This is a Morgan Series Factor Recondition model # 1950 with 15 gal tank and 200 psi

    • Morgan Air Compressor -
      It would appear as though Northern Tool is the source for the Morgan air compressor. It would also appear as though they purchase job lots of returned air compressors, have them dolled up or fixed, painted, and branded as Morgan air compressors, but that we are not sure of as we could not find the actual manufacturer.

      In any case, to confirm that this is, indeed, an electrically related compressor problem, it would be good to know if, when you are trying to start the compressor, that power is flowing past the pressure switch to the motor circuit? In other words, with no air in the tank, and power to the compressor, the pressure switch points should be closed, and power flowing to the motor.

      Given that you apparently have to stop periodically and wait time for the compressor to start, that suggests that the motor is going off on thermal overload, meaning it is getting too hot and shuts itself down until it cools.

      So, power to the motor even if the compressor motor will not start, and waiting for it to cool down if that is what is happening, suggests to us that you have a failing start / run capacitor. You may want to check that next.

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