compressor will not cycle

by Ed
(Pleasant Hope, Missouri USA)

Compressor will not cycle - hums tries to turn over but cannot - blows a new 20 amp circuit breaker - I've had it to the compressor repair guys twice - first they replaced a pressure switch and sent it home as OK - bringing it home it pumped up fine (like always) then when I used some air and it tried to cycle, it just hummed and blew the breaker like it did with the original pressure switch. I disconnected the tubing to the check valve and found the check valve working fine - the new pressure switch includes a new unloader - I loosened the line from the the compressor to the unloader and the compressor runs fine untill I reconnect the unloader line. Any ideas? Could the unloader be out of adjustment? Am I lucky enough to have had a bad unloader replaced with a defective unloader? Any help is appreciated. Thank you.

Empty the tank. Will the compressor start? If so, let it run up to cut out pressure, and monitor the unloader valve to be sure it works.

Did it?


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Jul 31, 2012
Unloader valve
by: Ed

I located the unloader - it was not working - I removed the unloader (a needle valve) - with pliers I pulled the needle past its normal range of travel, jamming the valve open. The compressor hisses and wheezes now while pumping but cycles fine and does not trip the breaker. Thanks guys, Ed
Except, now that you have locked the unloader open, while the air compressor is running, compressed air will be bleeding out of the unloader all the time, meaning that your compressor will take longer to reach cut out.

Best to replace the unloader with one that works, but that's just my .02 worth.


Jul 30, 2012
by: Ed

I don't think so. I heard nothing - the compressor guys tell me it takes awhile to unload. Still if I use no air and leave the compressor alone, it will pump, set for a half hour then try to start and blow the breaker. If I let it pump up, use some air, unplug it when it tries to start / stalls, disconnect the air line to the unloader, plug it back in it starts fine. I bought this compressor used. It seemed to have a new pressure switch then. Since that time I have had the pressure switch / unloader combination replaced with no change. I could try a third switch but I am hoping to be able to adjust the unloader or somehow modify it so that it works. Thanks, Ed
Ed, find the unloader, empty the tank, run it up to cut out, and make sure the unloader works. If you have done that, and the unloader valve is not unloading, then I believe that is your problem. If the unloader does not work, most DIY type air compressors will blow a breaker or blow a fuse on restart.


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wont restart after use

by chris fletcher
(miamisburg oh)

it wii fill up, but when I use it it will not start back up to keep air in tank,I had a problem with it before,it would filland then as i used it it would try to start but couldnt,and then blew reset, so I put a new preeure switch on it and it wont even try to restart
Chris, I suspect that it's not the pressure switch at all, but the unloader valve that isn't opening when the compressor reaches the cut out pressure level.

See the unloader valve page for more info. Then, empty the tank, hopefully the compressor will start, let it run up to cut out, and monitor the unloader to be sure it is working.



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Aug 23, 2014
Can't maintain pressure
by: Bill

Just to be sure, when you say the compressor cannot maintain the pressure, are you saying that it will not restart after you draw down the tank pressure to compressor cut in, or are you saying that air is leaking out of the tank and you are losing pressure that way?

If the former, and the compressor will not cut in at low pressure, and you're sure that the unloader is working, and the power supply to your compressor hasn't changed since it was working, then sorry, you may be looking at a start capacitor problem. You can check that yourself. See that page under troubleshooting.

Given that this is a model specific question, why not start again on the Campbell Hausfeld page, add the question, add a photo of your compressor too, and then other Campbell Hausfeld compressor owners can see your question and contribute?


Aug 22, 2014
Unloader Valve
by: Russ

I have a Campbell Hausefeld Compressor. 26 gal. 5HP. I had a piston problem. Replaced Piston, Cylinder, and fan. Also new valve and gaskets. Compressor came on and filled to cutoff pressure. Used down to restart and it tried to pump but couldnt get going, kicked breaker. Decided after two or three attempts the unloader wasnt working. Bought a new pressure switch and unloader combo. It did same thing, pump to cutoff and then can't maintain pressure. Help please. I could buy a new compressor for what i have invested.

May 15, 2013
Commpressor restart failure after bleed down!
by: Dean

When first plugged in it builds up fine as soon as the cutout trips and bleeds down it tries to restart and begins and then stops as if the piston pressure is to great!

Tried dumping tank pressure to zero and this continues to occur I can play with the on of switch and it tries to start but cuts in and out, and I can her the pop of a switch shortly after it stops even when I toggle the on of switch in an attempt to restart the motor?

am I correct in thinking it is the pressure switch or the pop of switch that actuates after it stops !!
So, your compressor starts easy after you empty the tank. Compressor fills. You use some air. Pressure drops to cut in, and the compressor won't start, but you say it will start with an empty tank?

Read the page of information on the unloader valve so you know what it is and what it does. Please make sure that when your air compressor reaches cut out pressure, that the unloader works.

If it does work, and the problem continues, start a new thread with complete details.


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Unloader Valve may not be working

by Bill
(Marshall, MI)

A friend of mine has a compressor he uses to run various tools and paint equipment. The compressor builds pressure Ok all the time most but every so often (at random) the compressor will bring the tank to prescribed pressure, shut off (with a hissing sound), and then when tank pressure decreases and switch calls for more air pressure the comprerssor tries to start but cannot.

Reading the unloader valve trouble shooting article it sounds like the unloader valve is possibly bad and only works sometimes. My friend states that he does not dare leave the compressor on for fear it will not re-start properly and burn up the motor (or worse yet start a fire) so he brings the tank to pressure, shuts down the compressor, and uses air until running out of pressure and starts the whole process over again.

Do you think the issue is with the unloader valve? Could an unloader valve fail intermitantly? or is there someting else we should check? Please let us know your thoughts.

Joe and Bill

Bill says...

Bill, I would dump the air, unplug the compressor, remove the unloader valve, clean it, replace it, and test it to see if it dumped the air over the piston on shut off for, say, 5 consecutive times. If it worked each time, I would suggest that the problem may be solved.

Or, the unloader valve was not the problem, I would be thinking along the lines of a start capacitor that is failing.

I would also check to see what else is trying to start or run at the same time as the air compressor, appliances that are on the same power circuit as the compressor. The compressor must have clean power for reliable starting and operation.



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Unloader valve

by Kevin

I recently installed a u.s. General brand compressor and I'm having issues.

It runs great from an empty tank but can't start under pressure. The trapped pressure is never released from the compressor.

Manufacturer had me replace the check valve which did nothing. There is a small relief valve with a ring on the tank and the same relief valve coming right off the compressor. Not sure how this standard relief valve is supposed to act as an unloader valve?

Any help would be appreciated.
Well Kevin, I don't believe the check valve has anything to do with your compressor starting problem.

If you wander over to the troubleshooting section on the sitemap page, read the pages that describe what the unloader valve does, and then, check out to see if yours is working, will you?

Post a response here as a comment.


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Aug 30, 2015
Unloader valve location
by: Bill

I don't know the model and I cannot find an image of your compressor.

However, if you look at the line from the pump head where it enters the tank, typically there will a smaller air line running over to the pressure switch. The end of that line on or near the pressure switch is where the unloader valve normally is located.

See the pages on this site about unloader valves for more information about them.

Aug 30, 2015
Kicks out
by: kenny

Where is the unloader valve located on a 5.5hp Devilbiss 20 gal compressor?

May 01, 2015
13 gallon Campbell won't start
by: Bill

Empty the tank, start it, run it up to cut out pressure, and when it stops at the cut out pressure, check to be sure the unloader valve works.

If you are not sure what that is, see the Unloader Valve information pages on this site.

Apr 30, 2015
13 gallon Campbell
by: Anonymous

I have a 13 gal Campbell and it starts empty but will not restart with any air in tank. I replaced the start control with the same problem! What gives?

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