Compressor will only start with little to no pressure in the tank

by Jay
(Enfield, CT)


The compressor is a Norther Industial 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 differecne or any other recomendations? Thanks

Bill says...

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

There is a page detailing what they are and what they do linked from the site map. Check out the UNLOADER VALVE page, and if this doesn't resolve your issue, please post to this thread.



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May 03, 2015
Compressor won't restart under load
by: Fadi (MA)

I know this is an old thread. But I had exactly the same issue today on a brand new compressor from Harbor Freight (21 Gallon). The compressor will not restart unless the pressure in the tank is below 40 PSI, or if I press the unloader nipple with a flat screw driver while the compressor is restarting. I do not use extension cord, and the circuit breaker is not tripping.

My solution to the problem was simple. I used a 20A outlet from the house and the compressor worked like a charm. It would restart with no effort at all.
I think the Outlet in my garage is far from the main panel, and that's what caused the problem.

Jun 27, 2011
Problem Fixed.
by: Jay

Homedepot had a compressor on sale. Bought new one. Old one is now a bubble tank. Problem resolved. Thanks all for the time and effort.

Jun 24, 2011
New unloader valve and start capacitor same issue.
by: Jay


The unloader valve is fully mechanical. As pressure builds in the tank the unloader valve is closed by pressure. When the tank reaches pressure and shuts down the unloader valve is mechanically pushed open. The valve is also spring loaded to the open position so without any pressure it is open.

This problem has actually gotten worse with a new start capacitor.

So I have installed new start capacitor and new valve with the same issue remaining. I don't think the run motor capcitor would cause this. Any ideas?


Jay, I think your unloader valve is mechanically opened and closed by the movement of the pressure switch.

Normally, unloader valves are closed all the time the compressor is running, to force the air into the tank, and not out the unloader valve itself.

When the compressor tank pressure reaches the normal cut out pressure setting, the pressure switch trips the power off, and, at the same time, releases the pin and spring pressure opens the unloader valve. This immediately dumps the air over the piston with the characteristic "pssssst" you hear.

What keeps the air in the tank at this point is the check valve. The unloader valve is normally open all the time the compressor is idle.

Are you certain that the unloader valve actually opens when the pressure reaches the cut out pressure setting? In other words, does air blow out when the compressor stops? That has to happen or there may be too much load on the motor when it tries to start again, it pulls too much power, and the breaker pops.

What is the compressor plugged into? Extension cord? Dump the cord. Any other items pulling power from the same supply circuit the compressor is on.

If there's too little power flowing to the motor, it can't start against almost any pressure, as there isn't enough inrush current and the breaker pops. If the tank is empty and the unloader has worked, there is lower load on the motor, which might allow it to kick off even if the start capacitor is weak.


Jun 12, 2011
Ne part no joy.
by: Jay

Changed controler with unloader valve built in. I issue remains.

I took apart the old controler and removed the valve and directly installed it in line. If I press the brass nipple in. (unloader valve open)I can get the compressor to start and keep running. It seems the valve is closing too early but I installed a new valve and same issue. Just can't seem to get to sustaining speed without a little help.

Could it be the start motor capacitor?


Could be the capacitor, though, you say if you open the unloader the compressor starts. Why isn't the pressure switch toggling the unloader valve to open when the compressor stops? That's what allows the air trapped over the piston to bleed off, making the restart easier.


Jun 12, 2011
New unloader valve
by: Jay

Ok so I installed a new control with built in unloader valve. Same issue. I took the old controler apart to get the unloader valve out of it.

If I rig it to the direct to the check valve the side where it is supposed to attach (remove unloader valve from assembly). If I press the little nipple in (unloader valve open) compressor runs. The valve seems to close just a little too soon or the motor just doesn't have enough power to to get to RPM with out the little bit extra relief.

Any ideas?

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the compressor will not cut back in

by tony

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

Bill says...

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.

Let us know, K?


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Aug 22, 2013
won't start under any load
by: Anonymous

compressor pumps to 110 psi and bleeds when compressor stops. Will not re-start until "0" pressure is present.original issue was a leaking pressure switch, replaced with a new single stage switch and now unit won't resart under pressure
I suspect your leaking pressure switch was actually a leaking tank check valve, but then, I've been wrong before. :-)

That the compressor bleeds now with a new switch supports the leaking check valve theory.

I'm not sure what you mean by "single stage switch" but if, when the pressure drops to below cut in, if power is then not flowing across the switch to the motor circuit, then either the switch has failed or it has been wired incorrectly.


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Unloader valve operation

by Marty
(Salt Lake City Utah)

Air compressor unloader valve

Air compressor unloader valve

Customer - Question

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

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, 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, it 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.


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