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.

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.

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.