The overload protection has to be reset frequently to build pressure

by James Waldron
(Pierre Part, LA USA)

The overload protection button has to be reset frequently to build pressure. It will run for approximately 10 to 15 seconds and shutdown. I can reset and the unit will run again. The pressure will increase approximately 10 PSI each time I reset. The oil level is good. The motor does not feal or bind when rotated by hand.

This unit was given to me because of this problem. I do not know how this unit has been handled or maintained.

Can you give me any suggestions of things to check or parts you think could give me this problem?


James Waldron
Hi James. If by "overload protection button " you mean the thermal cut out that protects the motor, it is popping, presumably, because the motor is overheating.

By resetting so frequently, you are quite likely harming the motor.

You don't indicate in your post how your air compressor is powered? By that, I mean, are you using an extension cord or power bar into which you plug the compressor cord? If so, stop!

You need clean power to run an air compressor effectively, and that means if not a dedicated plug of, preferably, a 20 amp circuit, at least one that has no other applicances pulling power from the same circuit as the compressor is trying to run.

If the power supply is good, then I would next suspect the start / run capacitor is failing.

There is a page on this site explaining how to test a capacitor.



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Jan 07, 2014
Central Pneumatic 40400 Leaking Oil?
by: Tex

I am not very familiar with air compressors and am now just starting to get familiar with the model that I have. I have read the manual on a used Central Pneumatic 40400 air compressor that I purchased off of craigslist ($50 bucks?...but I guess you get what you pay for, right!?). It worked fine but after a couple of months (I only use it sparingly, no heavy usage) it now has problems and I would like to repair it myself if possible? Here is what is happening:

1.) When working, to start it I would pull up on the start switch to turn on, then allow it to build up pressure,once pressure (80 psi/2 hp) would get to it's point on the gauge I would switch the unit off. I was not aware that the unit would turn itself "on/off" because I would not allow the tank to build up pressure pass 100 psi. I was more concerned that I would do more harm than good? Any way, still turns on but don't know if the "auto on/off" switch is functioning as it should? How can I tell without damaging the air compressor? Is it a faulty or bad switch? Does it need to be replaced? If so where can I find one and can anyone recommend 'specifics' as to how to go about replacing it?

2.) Once turned on and starts to build pressure, oil starts to leak out of the "oil plug"! (this never happened before, it just started and yes it uses the correct oil recommended by the manufacturer). When the air compressor is first turned on and pressure starts to slowly build, oil slowly starts to come out of the oil plug then rapidly! I have to turn it off to stop it! Now it just started doing this (never did before) but would like to know is this compressor worth repairing? Can it even be repaired? If so can anyone assist me on how to go about repairing this? What parts would I need and where can I get them? Heck, are parts even available for this particular model!?

I only use this air compressor for putting air in tires and thats it nothing else! No heavy tool usage, not painting, etc.! I would like to know how to repair it and maintain it if possible. Really don't want to go out an buy a new one.

Anyway, all help is greatly appreciated and I hope to hear some postive feedback if possible!

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Model 40400 pressure switch leaks?

I have a model 40400, and the diaphram in the air pressure switch has a hole in it. Harbor Freight said it will take 3 months to get a replacement switch. How do I get this problem solved?
Dump the air in the tank, leave the compressor unplugged, remove the switch, and take it to the stores in the area that sell air compressors. You will find one that is similar, I am sure.

Many of the import compressors sold at a variety of big box stores use the same or similar make of pressure switch.

Or, ask the store where they send the compressors for warranty work, and go there.

Or, look at the pressure switch page on this site. Usually there are advertisers offering them.



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Central pneumatic 40400 manual?

Central Pneumatic Model 40400

Central Pneumatic Model 40400

Need a manual for a Central pneumatic 40400. Can anyone help?

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Oct 24, 2011
central pneumatic 40400
by: chuck

motor start/run capacitor is bad but part number/ value label is missing


Find out where your local Harbor Freight sends their air compressor warranty work. That shop should be able to help you.

Oct 16, 2011
Need Pressure Switch
by: Randall

Where can I find a pressure switch for a CP air compressor model #40400


Harbor Freight sells the compressors. Logic would suggest they sell the pressure switches. However, look at the brand on the switch. You may find that it's the same switch as on many other compressors, so you can pick one up at any of those outlets too.

Jul 05, 2010
Compressor 40400 will not start
by: Gerry

I ran my compressor 40400 yesterday. Today it would not start. It has power to it and through the switch, I acn't find prob. Thank you, for helping me.

Bill says...

Gerry, if you are getting power to the pressure switch, and then you have checked and you are getting full voltage through the switch, indicating that the pressure in the tank is at or below the cut in pressure level and the pressure switch has tripped, then power is getting to the motor.

When you turn the compressor on, does anything happen at all... any click, any noise?

If nothing happens at all, and power is getting to the start capacitor on the motor, then I'd be checking that or the motor itself.



Jul 03, 2010
Air Leaking from the Air Pressure Switch
by: Luis, L.

My compressor was working fine but lately it was not building pressure and I could hear an air leak coming from the Air Pressure Switch. I have not taken the switch apart to see what is wrong. Has anybody experience this problem? Is there anything that I can accomplish by taking this switch apart?
Luis, have a look at the page on leaking pressure switches found in troubleshooting or pressure switch sections on the sitemap page.

Jun 06, 2010
Motor lugs down
by: Sky

When I start the 40400 compressor it runs normally at a high speed for a few moments, but when the pressure reads about 18 PSI it starts to lug down and sometimes quits. The motor no longer just stops clean, it seems to be drawing lots of electricity (based on my dimmer lights) and sounds like the motor is under a heavy load. I was concerned that I would smoke the motor so unplugged it. About 30 seconds later I hear something like a valve move. I plugged it back in and the same thing happened again.
Any idea what the problem is? As it is I cannot use the compressor.
Thanks Sky

Jun 02, 2010
Oil leak
by: Anonymous

I am noting oil leaking onto the tank and it appears to be coming from the motor drive. Are there troubleshooting tips for this?

Jim, mine is leaking too. It's a seal replacement issue, I'm sure. In my case, I just make sure the level of oil in the sump is as it should be, and wipe off the housing as necessary. I don't feel like spending any money repairing the leak.

Of course, if it's bad enough, it's time to take it to the shop.

Dec 21, 2009
Central Pneumatics Compressor #40400
by: Mike Nickerson

here is a link directly to the Manual you want...

Thanks very much. A great help.

I have uploaded it to the Central Pneumatics forum page on this site for download by anyone.



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Correct Pressure Switch Wiring


I have a central pneumatic Model 40400 Air Compressor,

It requires a new pressure switch.

The new switch has the following designations next to the terminals. N, P, G. Im assuming the designations mean Neutral, Power and Ground.

The old switch was wired as follows: black wire connected to the N terminal, white wire connected to the P terminal and green wire connected to ground.

Seems to me it should be as follows: white wire connected to N, black wire connected to P and green wire connected to G. Am I correct?

I have verified with a volt meter that the black wire (plugged into an oulet)is hot.
Hi there... if the question is are your thoughts about wiring the pressure switch correct, or was the old switch setup correct, not being an electrician I cannot tell you which is correct.

It were me wiring up the new switch, however, I'd go with your version... :-)


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Nov 03, 2013
by: Doug in

I, too, agree, but it makes me wonder about the motor connections on the other side.

Assuming this is a single phase (120 or 240v) setup, then it's not critical, anyway.

Nov 03, 2013
Correct Pressure Switch Wiring
by: Anonymous


Thanks much.


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