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.
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. A visitor wrote in and said “how do you monitor the unloader valve?” While that is covered in detail here: page describing what the unloader valve is which is a link earlier on this page, to recap the info on that detailed unloader valve page, monitoring the unloader means to listen near the unloader to ensure that when the compressor reaches the cut out pressure, the unloader dumps air. That means that there should be a very short venting noise, and if a hand is held near the unloader, a short blast of venting air should be felt.
If the unloader is not venting air when the compressor reaches cut out pressure, 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.
Related posts:
Air Compressor Electric Motor Won’t Start – Troubleshooting Tips & Tricks
Air Compressor is Always Leaking Air? Things To Check & Troubleshoot
Air Compressor Won’t Start Because Motor High Heat Thermal Cut Out Issue
I just replaced my pressure switch (120/150) and the pressure keeps going up – No shut off?
Air Compressor shuts off and all the air leaks out
i have a master elite air compressure model number mfac22 i replaced pressure switch and unloader after air drops when it trys to restart it kicks the reset buttonoff when i push the reset button back on then it will start only to do it all over again
Campbell hausfeld five horsepower 125 psi air goes all the way to cut out and then when restarted the motor goes and wants to start you can hear it hum and then out and out and out and then it it just hits the circuit out it try to restart
I have a 5 hp dual stage compressor. 80’gallon Sanborn. It was starting hard so I changed the pressures switch. Worked fir a while burned up another switch. Installed a magnetic starter and cleaned up switch and made sure contacts open and close. Can power it up but it will not cut out at top pressure or when draining it will not cut in. Acts like there is no pressure switch at all.
I have an Craftsman Model 3 919.176730 3 hp air compressor that I bought used. It had no belt, so after getting a belt online I though I was ready to go. It started right up, but when it reached about 30 to 35 pounds the lights began to flicker. It got worse and at 90 pounds I shut it off. I was not sure at what pressure it was suppose to shut off, but I was sure it was over 100 psi. I tried to start it again, but the motor could not get it going. I started dumping air, and finely at about 30 psi the motor was able to roll fast enough to start pumping air. I let it run and the lights were still flickering when it tripped the breaker at about 105 psi. The motor looks like it has 2 capacitors. I was wondering if maybe one of them are bad. Doug
Yup. It sounds as though they may be suspect, Douglas. Consider testing both. See this page and scroll down to the section about compressor electrical issues / problems for a page on how to check. Let us know, okay?
I have an Eagle EA-6500. The unloader valve seems to be working properly; it lets out a short blast when it gets to full pressure. At restart pressure, it just hums. Should I still try replacing the unloader or is it more likely the check valve?
More likely an electrical problem, assuming it’s not the unloader that’s humming, but the motor. Suggest you test the capacitor. See the page on this site. Let us know what you found, if you would.
Hello, my compressor is a NexKomp Nex A series (150A).
My problem is that, during the day, it works fine but after aprox. 7-8 hrs it stops with no alarm. It unloads and loads very smooth but after a while as I said, it stops…my guess is that it gets stuck on the unloading program and after that it shuts down.
If you can give me an advice it will pe super cool. 😄 thank you guys!
Posted for you, Dragos. I don’t have any experience with oil injected screw compressors, but hopefully someone else will and be able to help.
I was able to find that this compressor has built in protection against “Overload of main motor, Overload of fan motor, Prevention of phase reverse, Increase of internal pressure, Increase of discharged air pressure, Increase of discharged air temperature and etc” and wonder if perhaps, when it tries to start up, there is a condition that has tripped the safety so it won’t. Just a though.
Ok so I managed to solve the problem. Forgot to post it. I just turned the power off (“turkish reset”) and after 10 minutes I turned the power on again and it worked! I belive that it was something in the main console. So it’s all good. Thank you anyways! Good day!
I am having trouble getting my Craftsman 6HP compressor to start with pressure on it. I replaced the start up capacitor and it seemed to help for a short while then started having the same problem. Replaced the unloader valve and it didn’t help at all. It will start pretty well at very low pressure but will only make a few turns when it has pressure and then kicks the breaker.
I pulled and cleaned the check valve which seems to be ok, just looking at it.
I can’t decide if it is that which is bad or an actual electrical issue with the motor. Seems to run just fine when I start it with very low pressure.
Hey JC, I moved to this page as I thought the subject was more appropriate to your post.
That you’ve checked the things you did is good. Probably replacing some of the parts was unnecessary, but then, now you know the parts work, and if you kept the old, you now have spares.
That the compressor runs OK with little or no pressure in the tank, and you can stop it with that low pressure and it will start again easily, pretty much leaves out the start cap, the unloader, and likely the pump, as long as when it’s running it builds pressure normally.
The symptoms do suggest a power problem. Starting against load is hard for a compressor, and even though a 6 HP motor should have lots of power supply from the supply line from the panel, has anything changed there? Relocated the compressor farther away from the panel, for example?
If not, then I’m thinking you’ve got a motor problem, and if the cap is good, it’s coming down to the ability of the motor to convert electrical into mechanical energy to drive the shaft, pump and build pressure. If this were my Craftsman compressor I’d get the motor checked.
Willyr, actually if I let the compressor build up to 60 psi or so it will not start up again. It struggles to get started then after 5 seconds or so trips the breaker. The compressor is at its original location where it has been for years.
Replacing the parts were quite cheap and most of the troubleshooting tips were, capacitor, check valve, unloader valve, motor. So I don’t have much invested in it yet compared to a new air compressor.
Yeah, if it starts OK at low or no pressure in the tank, and then starts to slow and trips the breaker as the pressure builds, that’s increasing load on the motor. As long as the compressor builds air properly, if it slows down as the pressure builds, it’s typically motor related.
I have a Craftsman 919.156611 I have the classic motor-struggles-and-trips-circuit-breaker upon restart. The motor starts fine on an empty tank and pressurizes fine with no leaks. I replaced both Run and Start capacitors.
I replaced Unload Valve and still had the problem. When motor would stop, I can hear the pressure exhaust to the atmosphere as it should. The Pressure Switch works fine and would cut off at set pressure and restart when it should Except upon restart, the motor would struggle then trip the circuit breaker.
I emptied the tank and disconnected the tube from the Unload Valve. I held my finger over the end of the tube and started the compressor. The motor started fine and filled the tank, then stopped normally. I removed my finger from the tube and the head pressure relieved, just like the Unload Valve would do. I put my finger over the end of the tube again (simulating the Unload Valve being closed ready for a restart) and started the compressor. The motor struggled and tripped the breaker. Hmmm.
So, I removed my finger from the end of the tube, started the motor and it ran fine but of course air was flying out of the tube. I covered the end of the tube while the motor was running and it continued to run fine. It seems that once the tank get pressurized and the head pressure is relieved, something is still causing a back pressure. If the Check Valve was bad, I would hear a leak but I don’t. If it was slightly bad, would it add pressure back into the Head causing the motor to struggle upon restart?
The symptoms suggest it’s either a start capacitor that’s failing / failed, or it’s the motor. Please see the page on this site about testing caps, do so, and then add a comment with what you found. Thanks.
The starting capacitor was replaced. I would have hated to think that a brand new cap was bad! I even checked the old and new starting capacitors and they were both good. I am beginning to think that the motor has gone weak since it’s the restarting that it struggles with and it is not the Unload Valve. The starting capacitor is already about 500 uF. If I put both the old and new caps in parallel, that would double the capacitance and add more starting energy. I’ll try that.
Nope, higher capacitance Starter cap didn’t work. Motor winding problem for sure. Will cost me almost as much as a new compressor. Maybe I’ll rig it up as an external tank.
I have a California Air Tools 10020C that does not kick back on when the pressure drops bellow 90psi. I originally assumed it was the pressure switch, and replaced the whole Condor MRD 11 unit, thinking that should solve the issue. Unfortunately it did not and now I am lost as to what my it be. Touching the electric motor by hand it does not seem like it is overheating.
Any suggestions what to look at next and how to test?
Thanks!
Does is try to kick back on, or when the pressure drops below cut in, nothing happens. If the latter, drain the tank entirely and try again please. Let us know what resulted.
Even if I drain it it does not kick back on. Checking voltage at the pressure regulator switch, it seems like it kicks it back to 120v, so that works. Only after about 30 min resting, I plug it back in and it works. Can it be the temp switch not allowing power to the motor? I am getting like v15 at the capacitor when this happens, so I think power is not reaching it too. Should I just open the motor? Find the temp switch if there is one?
So the motor isn’t hot, but one the air is used the compressor won’t restart? That being the case it could be a failing thermal cut out switch, often found inside the non-shaft end of the motor under the cap. It might be that or it might be the capacitor itself. Might want to check the cap before ordering a replacement thermal cut out.
If you aren’t sure how, this page may help: http://fix-my-compressor.com/finding-a-new-compressor-capacitor/
Thanks Willyr, I was thinking about changing the capacitor, but assumed that is should be reading 120v on the wires coming into it once the pressure switch is trying to pump up the compressor back up? Am I wrong with the assumption?
Thanks!
If you aren’t getting 120 in the wire from the switch to the cap, then it’s possible that a “point” in the pressure switch isn’t allowing full contact. If you have 120 volts going into the pressure switch, and not out the other on the way to the cap, with the tank well below cut in pressure, then that sure points to the switch. Are you sure you’re getting 120 on the line or power supply side of the switch?
So when the tank gets bellow 90 psi, at the pressure switch, I get 120 at the line connection and the motor connection. However I don’t get 120 at the capacitor. And there is only the motor between the pressure switch and the cap (see attached). So would that mean that somewhere in the motor (potentially temp switch) is shutting of the current to the capacitor? Or would a faulty capacitor not send back 120 somehow?
Thanks!
Nice schematic. It seems odd to me that the power supply goes to the motor first and not the cap. I would expect, given that the cap is necessary to get the motor started, that the cap would come first and then the cap to the motor. Check the cap is what I’d do next.
I have changed the pressure switch, unloaded valve and pressure relief valve. Still have the same problem. What next?
The pressure switches from China are s**t. They do unload but on restart the pump opens the back pressure valve. the unloader needs to unload a little on startup. Or the china motors don’t have enough power to to push the pump.
Just wondering if you have used a multi-meter to trace thepower to ensure that it’s actually getting to the motor. If so, have you tested the start capacitor?
It will pump up to 120 lbs. The first time, but will not restart unless the tank is empty. I have the capacitor out. May be able to get it tested later today.
5hp capacitor start motor single phase 230V motor hooked up to 208V with a 2P 40A breaker will start normally under no load and at 175psi it will shut off and unload. When it tries to start up again at 150psi the motor really labors to get up to speed and will sometimes trip the breaker. What is the reason for this? The motor nameplate says 22A but at 208V it draws 29A, hence the larger breaker.
Matt, if it used to work fine, and now is lugging at 150 PSI and struggling to continue, and you have not changed location or power supply, then a logical check is to determine if the start and run capacitors are still sound. Please see the troubleshooting page on this site for links to pages about how to do that. Please also let us know if that was the problem with your compressor, K?
Yes, I checked the capacitors with a multimeter and they are fine. It’s almost as if when it starts up at 150 the line from the pump to the tank fills too fast and it can’t get up to speed fast enough. Would slowing that inrush of air somehow fix the problem?
Am I correct in understanding that the cut in pressure for this compressor is 150 PSI, and the cut out is 175 PSI? Or, is the motor lugging when the tank pressure reaches 150 PSI, and the cut in is lower? That’s just for my info.
There is a “pressure” valve in the head of the pump. If that valve is damaged, that may account for the overloading. Or, if the back pressure in the tank begins to grow, if there is a problem with the tank check valve not operating properly, that could cause it too.
With the tank empty of air and the compressor shut down, maybe pull and clean the tank check valve. That being all good, test again, and if the problem persists, then it’s either got to be the pump back-loading the motor, suggesting a pump mechanical issue, or, the motor itself is not functioning properly.
I always check the easy stuff first, then work my way into the more complex, and ultimately, use a motor shop to check the motor under load.
Air compressor; CP make 20 hp;when motor stops for no air use and then when air is used restart not taking place.
So, when you read the page and saw what the possible reasons for this, what have you checked, please?
I have to agree with some other commentors…This line on this page isn’t helpful at all..
“As it runs up to the cut out pressure setting and stops, monitor the unloader valve to ensure that it is working. <<< What does that even mean? Monitor it how? By doing what and noticing what? What are we supposed to be looking for? That's like saying, if your car doesn't start…turn the key and monitor the engine
If it is not, remove and clean or replace it."
Replied to on the page and here, Christopher: “A visitor wrote in and said “how do you monitor the unloader valve?” While that is covered in detail on the page that is reached by the link to the working of the unloader valve found at the top of this page, monitoring the unloader means to listen near the unloader to ensure that when the compressor reaches the cut out pressure, the unloader dumps air. That means that there should be a very short venting noise, and if a hand is held near the unloader, a short blast of venting air should be felt.
If the unloader is not venting air when the compressor reaches cut out pressure, remove and clean or replace it.”
Compressor will build good pressure but while using air motor hum until pressure drops below 70psi then it restarts
Check to make sure that the unloader valve is actually unloading when the compressor reaches cut out and stops, and if it is, then it’s time to check the motor capacitor.
Hello. I have a Husky WL650702aj. I turn the compressor on and it will fill up to about 80lbs then cuts out. I have to turn the compressor off for 5 min or so and then turn it back on to get it going again. It will fill another 20lbs and we are doing the dance again. Any idea on where I can start looking? It’s plugged directly into the wall which is a 20 amp circuit. Thank you for any help.
Shane, that’s symptomatic of a failing run capacitor, or a problem with the motor itself. Take a look at this page for help in testing your compressor motor caps.
I have a Sears Model # 919.177850 and it will not restart and trips the breaker after it fills the tank and then tries to restart. I have replace the check valve and pressure switch.
HELP!!
I couldn’t find a photo of this model to check to suggest a couple of things. First off, if you dump all the air and try to start, will it? If so, so then check the unloader valve to be sure it’s working. There’s a page about that here: http://fix-my-compressor.com/the-air-compressor-unloader-valve/
Please read my post again. I have a brand new check valve and pressure switch installed. If I reset the breaker and turn the lever on the pressure switch to on.auto, it will start and run up to about 125 # and then stop. When the pressure drops after I am using it, it will not turn over and it trips the breaker. I have read tons of simple trouble shooting. They have not helped me. To put it more concisely, if one replaces the check valve and pressure switch and the compressor still won’t restart, what else could cause this???
Does your pressure switch have an unloader valve and is it working? When you turn the switch to on.auto, and it runs up to normal cut out, will it start with pressure in the tank or not, David.
How do I check the unloader on the pressure switch? It is brass and has a pin on the bottom and is mounted to the side of the pressure switch. It will not start with pressure in the tank, To start it, I have to manually bleed the pressure off.
Dump all the air from the compressor. Start it. As the tank pressure nears the pressure at which it shuts off, watch the lever on the unloader, and make sure it moves when the pressure switch trips. You should also be able to hear a brief expulsion of air from it.
Checked unloader this AM. The lever on the unloader moves when the pressure switch trips. I did not hear any expulsion of air, The compressor made a sound crossed between a fart and a duck quack but that was the only sound I heard. I turned it back off as it will trip the breaker upon restart.
I guess that the sound you heard was the unload, as the lever moved, and that opens the unloader valve to atmosphere. There typically is only a split second or two of air exhausting. OK, next step is the motor capacitor, as it seems clear that the motor cannot get going under light load, then it pretty much has to be the motor, and before replacing that, the logical next step is to check the start cap to see if it’s failed or failing, as that will account for a blown fuse / reset upon restart. More info on that here: http://fix-my-compressor.com/is-the-compressor-start-capacitor-working/
Thanks for stayin’ with me! I do not have a meter to test capacitors so I ordered one today on Amazon. Should be here tomorrow. I will test both capacitors as soon as I get it and let you know the results. The capacitors show no sign of heat or shorts but I understand that visual observations do not indicate the health of a capacitor.
Checked capacitors. Run capacitor tests OK. Start capacitor bad. Having hard time finding a start capacitor for Harbor Freight 3 HP motor. Any suggestions on where I can find one that fits and meets the specs on the old start capacitor?
I have a Kobalt model 03800841. It will build pressure and shut off as it’s supposed to. When pressure drops and should cut on it tries to start but wont. I have replaced unloader, and pressure valve and both capacitors. Have tested at lower pressure, and any pressure in the tank will cause it not to start up. As long as there is no pressure everything runs fine.
I have the same compressor and it’s having the same problem.
Hi Joshua. What have you tried as per the suggestions on the page, please? Thanks.
Empty the tank and remove the tank check valve. Make sure it is operating. If it is, and not preventing air getting into the tank, and you are not using any extension cord or power bar, I would suspect either the pressure valve on the valve plate or the motor itself being at issue.
I pulled the unloaded cable no air came out the drain cable is open but the gauge still reads 35 psi and the switch has power.
Pretty hard to even start figuring it out when I have no idea what the make and model of the compressor is you are asking about. Please provide more info.
I have a Belle Aire 7.5hp 80 gallon tank. This week I’ve changed the entire pressure switch, the motor AND the mag starter! It runs fine when I manual reset but it won’t start automatically. Any ideas??
Hey Brandon, is it Belle Aire or Belaire, your compressor make as I know the Belaire brand, but not the other. Anyway, can you tell me what the settings on the new switch are? In other words, is it a direct replacement for the old one? Also, at what point is the compressor supposed to auto-start, and is there any indication of action when you bleed the tank pressure down below that point?
When the pressure drops below the normal start setting, the pressure switch should trip, allowing power to flow to the motor circuit. On manual, obviously the pressure switch has tripped to allow power to flow, but when it’s on automatic, is power actually getting through the switch when the pressure drops below cut in? Let the pressure drop to zero as a full test, then use a meter to check for power on the motor side of the switch. Is power getting across?
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.
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.
Cut out is 150 psi
Is the tank pressure getting to the 150 PSI cut out, Perry? If not, I’d suspect a pump issue, maybe valves, maybe gaskets or piston rings.
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
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.
Compressor wont restart when try to build air i’m not sure is this unloader valve??
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: http://fix-my-compressor.com/the-air-compressor-unloader-valve/.
Also, mmore information about your compressor problem can be found here: http://fix-my-compressor.com/air-compressor-just-hums/ . It certainly sounds as though your start capacitor is weak.
let me send you other picture
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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: http://fix-my-compressor.com/air-compressor-just-hums/ . Please do the checks and add a comment here with your findings.
This is a really great site – very helpful.
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
limit.
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.