Air Compressor Won’t Start Under Load will only Start With No pressure In Tank

Air Compressor won’t start with a full tank of air

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.

Reader’s Questions About Compressors That Won’t Restart With Pressure In Tank

Now on with our reader questions…

Compressor Will Only Start With Little Or No Pressure In Tank

Bill, my compressor will only start with little or no pressure in 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 in which it should turn on it struggles and eventually trips the breaker on the unit.

From reading all your post 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)

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Jay, from the sound of it, your compressor may have an unloader valve problem.

The unloader valves job is to open when the air compressor stops to allow any air trapped over the piston to escape. Why? Well, compressor motors runs 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 linked 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.

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Compressor will not cut back in with air in the tank by tony from (qld)

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

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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 getting a typical “unload”.

Tell you what. Drain the tank to zero, close the drain, plug it it, 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 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.

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Unloader valve operation issue  by Marty from Salt Lake City Utah.

Marty says hoping you can help me with a problem I’m having. I have a Campbell Hausfeld 5 hp 60 gallon 120 psi compressor and it seem that the unloader valve is releasing some air before the compressor kicks off at 120 psi.

I have replaced the check valve so my question is this. What activates the unloader? Is it that little brass pin that sticks out the end that goes up into the pressure switch and does the pressure switch push that pin down to release the air? I’m just trying to figure out if my unloader is bad or the pressure switch because it’s pushes on the unloader to soon.

Thanks for your help, Marty
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Marty, typically the unloader valve is closed, non-passing, when the pin on the unloader valve is released.

When the pressure switch trips to off it cuts the power to the motor and at the same time the mechanical part of the pressure switch depresses the pin (if that’s the type of pressure switch it is) and that opens the air path from over the piston to atmosphere. It is the tank check valve that keeps the air in the tank when the pump is opened to atmosphere so it’s good that you checked the tank check valve, too.

If the pressure switch is tripping to off and depressing the unloader valve pin before the pressure in the tank reaches the normal cut out pressure, I would think that it’s the pressure switch that’s pooched.

Please provide a few more details about the sequence of events when the unloader starts venting air.