Compressor Not Unloading

I have a Husky (Campbell Hausfeld) 80 gallon 2 stage compressor and it does not unload when it reaches capacity and when I drain air down it kicks reset switch.

Where is the unloader valve located on the compressor?

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Bill says...

Hello there.

For most DIY type air compressors, the unloader valve is adjacent to the pressure switch, and it's a toggle protruding from the pressure switch that toggles the unloader valve open and closed as the switch cycles.

Unloader valves can become plugged up, so when you find yours, after dumping pressure and unplugging the compressor, disassemble and clean it and see if that solves the problem.

Cheers,

Bill

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My portable air compressor will start but won't stay running

my portable air compressor will start if there is no air in the tank but when it is up to pressure and is slightly depleted and starts to recover it stops and humms until the thermal cutout works or i trip a breaker

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Mar 21, 2015
will start but won't stay running
by: Bill

Your compressor starts with no air in the tank, it runs up to cut out pressure and then stops OK.

Then you start to use air and the pressure in the tank drops to the cut in pressure level, the compressor tries to start, it hums, and then either the overload switch pops or the breaker pops... is that right?

It could be a couple of things, but since it starts in the absence of pressure, I'm leaning towards a motor overload when there is pressure there, and that suggests to me that it is the unloader valve that isn't working.

Check out unloader valve from the sitemap for more details.

Cheers,

Bill

Mar 21, 2015
Same question but I have a Campbell hausfeld
by: ez2mbr@gmail.com

I have a Campbell Hausfeld 3.5hp 11 gallon where would I find that unloaded valve what else could it be.
compressor starts with no air, bit will not start or restart after that or any time it has air in tank

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Sears 1 HP, 12 gal. compressor with head pressure issues

by Joe
(Virginia)

Sears pressure lines

Sears pressure lines

Hi,

Thanks for your site - full of valuable information! I have an elderly Sears compressor that more than 50% of the time can not overcome the residual head pressure when the pressure switch singals for more air. If I am there, I can pull the ring on the control valve to drop the pressure so the motor can overcome the pressure, if not, it blows the breaker.

Having reviewed your trouble-shooting tips I am stumped but have a few questions. Looking at the attached picture, I do not see a check valve between the head and the tank. I also do not see any other line from the head that would be an unloader valve. There is a plugged port on top of the head. Also, the pressure control valve does not have a second connection - just power in and out and the male adapter to the manifold/tank.

Is this unit designed to operate based on the toqure of the motor? Should it have a check valve, or does it and I just can't find it? Should I install a Square-D control valve that includes an unloader valve and plumb it to the head? If I do that, I bet I'll need a check valve, or a new comtressor...

Thanks!

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Bill says...

Nice photo Joe. Thanks. What I can't see is where the manifold is connected to.

In order for your compressor head to unload after the tank reaches cut out pressure, and if it doesn't unload then you will get the symptoms you describe, there MUST be a way for the air trapped over the piston to exhaust to atmosphere when the compressor stops. Have you taken the line off from the compressor head to the tank to be sure that there is no check valve, in particular, where the line enters the tank?

On your pressure switch, was there no "toggle" or "lever" (not the on off or run lever) that interfaced with an air line?

Since these eyes of mine don't work so good anymore, can you tell me what the "hook" looking device downstream from the empty pressure switch port is?

Maybe older compressors worked on some sort of torque situation as you suggest, but it's news to me.

If everything on the compressor works well and the cost to retrofit it with a new pressure switch, unloader valve and check valve at the tank makes economic sense, that makes sense to me.

Have a look at the tank size and the motor HP and see what a new one of the same specs would cost first though.

Bill

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Jun 10, 2010
My old Sears compressor
by: Anonymous

Hi Bill, thanks for getting back to me.

In the mean time, I found and tested the check valve. It is at the base of the fill tube in the tank.

I also bought a new pressure switch with an unloader valve that I plumbed to the port in the head ($20 including shipping). I am sad to report that though the reduced pressure helps the motor on startup, it quickly gets bogged down and stops. Only way I can get it to work is to back the low pressure cut in down to ~50 psi. Me thinks the motor may be getting tired.

The hook you asked about is just the wire to the motor. The manifold connects to the tank by a tube between the pressure switch and the safety valve.

Thanks Bill!

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Compressor wont start unless tank is empty??

by Rodney
(Houston, Texas, USA)

I have a 80gal 220v Delta compressor and if there is any pressure in the tank it will try to start and struggle and then trip the breaker after about 6 seconds.

I replaced the pressure relief valve that unloads the pressure from the pump and I even put on a start capacitor and run capacitor (I hope I have all the terms right since it has been over a year since I did all this). The only way it will fill is if it is 10psi or less. Any pressure above 10psi and it will trip the breaker. I really need to get this working correctly since I now need to start using it more than I have been.

Any ideas with what could be the problem?

Much appreciated.

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Bill says...

One reason the compressor motor won't start - and blows fuses or breakers - is because the motor draws too many amps on startup, exceeding the capacity of the circuit breaker, and it pops.

A common reason for excessive motor load on startup is that the compressor hasn't unloaded properly when it reached cut out pressure and stopped.

To be sure that this isn't your compressor problem, bleed the tank to zero pressure, start the compressor, and let it fill to cut out pressure.

Observe and listen near your unloader valve to be sure it activates when the compressor stops. If it doesn't that's your problem.

Since you appear to have corrected any potential problems with the motor, please also check and see what else is on the supply circuit to the compressor. If another device is starting or running and drawing power from the compressor circuit when the compressor tries to start, that too could be causing a problem.

Please comment on this thread to let me know if the unloader valve is not, for sure, the problem.

Cheers,

Bill

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Compressor Will Not Start until pressure drops.

by Frank
(Burbank, CA)

Motor just hums and will not turn over until pressure in tank is reduced. It is a Sear craftsman

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Bill says...

Frank, don't keep trying to start it until you remove, clean, and replace the unloader valve which is likely part of your pressure switch assembly.

Your symptoms are those of too great a load for the motor to overcome, and that too great a load is often caused by the unloader valve failing to clear the compressed air over the piston.

I think that this is your compressor's problem.

Cheers,

Bill

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Jun 10, 2010
I think I fixed it.
by: Frank

I replaced the start capacitor and she starts everytime now.

I will check the release valve and make sure that is also functioning correctly. Thanks for the quick response. Frank

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Where is unloadvalve located on compressor

by Luesengvang
(Sanger,ca)

Where is unloadvalve located at on the compressor or how does it look like
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Hello Luesengvang;

Have a look at the Pressure Switch page. On many makes of DIY type air compressors, the unloader valve is located near or attached to the pressure switch.

In the photo on this page, look at the copper tube behind the switch. It's leading down to the unloader valve on this switch, and it's the pressure switch tripping that opens and closes the unloader.

You can also see a photo of the unloader valve on the unloader valve page.

Cheers,

Bill

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Matco 25 gal has hard time refilling

by RAFAEL
(IL)

It starts ok will build pressure to 120. When i start using the air it drops to about 80 and the compressor trys to restart. At this time the compressure motor has a hard time turning over the pump and goes into a stall amp so high it blows the breaker. If i lower the pressure in the tank to lower the 20psi it will restart and run normal with no lag. Will automatic shut off at 12-120 psi. Any ideas?

I though it was the one way check valve that was pushing back at the compressor and not let it start bit i have checked it at 125 psi doesnt leak back to the compressor. Also if i loosen the feed line to let the compressor start and then connect the line it will work normally. Compressor when running has enough pressure to blow my hand off the exit. is it a week compressor or motor or do i have a problem with a valve.

______________________

Bill says...

Rafael, your problem is consistent with an unloader valve problem. Have a read of this page and see if it makes sense as it applies to your compressor issue.

Cheers,

Bill

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