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AssuredElectrical, Home Specialist
Category: Home Improvement
Satisfied Customers: 4241
Experience:  35 years home improvement
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I have a Sears Creaftsman 150psi 1.5 hp 20 gal air compressor

Customer Question

I have a Sears Creaftsman 150psi 1.5 hp 20 gal air compressor which has worked fine until last weekend. I transported it (upright by the way) to a neighbors house to use with my roofing nail gun. When I plugged it in and tried to turn it on, it just hummed like it was stuck. any thoughts on how to make it run again? Thakns
Submitted: 3 years ago.
Category: Home Improvement
Expert:  AssuredElectrical replied 3 years ago.
Welcome. My name is XXXXX XXXXX would be glad to assist.

What is the Model of the compressor so a detail parts diagram can be found?

Customer: replied 3 years ago.
Expert:  AssuredElectrical replied 3 years ago.
Thanks for the additional information.

I was surprised at the manual and parts book, usually Sears has a great breakdown on the motor but not in this case.

Let see if we can work through the components.

Can you see if the motor has any rounded covers on top? Look similar half moon shaped?

Can you gain access to disconnect the hose connected to the check valve going into the tank from the compressor?
Customer: replied 3 years ago.

the motor is mounted sideways on top of the tank - the cover on the end of the motor is plastic and is round, as the motor is round. I emptied the tank and opened the valve on the bottom of the tank, so there is no pressure in the tank. I removed the cover on the motor and piston and attempted to start the compressor. It did momentarily start then stopped and began to hum like the piston or the motor was stuck. I unplugged the compressor and tried moving the fan and the piston and they moved freely. So I think there may be something wrong with the motor, but it is only a guess. Any thoughts?


Expert:  AssuredElectrical replied 3 years ago.
Ok, thanks.

The reason I asked on the motor having any rounded covers was to determine if it has capacitors for starting or running.

Sears parts only shows and references "a motor", nothing more.

The motor diagram picture resembles one having capacitors and so I thought it is possible.

A capacitor helps a motor overcome the inertia allowing it to come up to speed and run with the added power.

The other item was accessing the check valve where the hose connects to the tank.

It is supposed to open when the compressor is pumping and close when it isn't.
if the check valve sticks, it keeps back pressure on the compressor and the motor will not be able to come on and run at the set speed.

Those are the 2 most common issues.

If you can disconnect the hose at the tank, you could eliminate one of them.
Customer: replied 3 years ago.

OK I disconnected the hose from the tank before the check valve this morning and attempted to start the compressor. It started running for about 5 seconds then quit. So I am assumming it might be one of the capacitors attached to the top of the motor. One says "starting" and the other does not say what type it is.( I assume it is a running capacitor) How do I test either one of these capacitors?

Expert:  AssuredElectrical replied 3 years ago.
To test a capacitor properly, you need a multi meter with the Cap setting.

It will have a uf--mfd---or cap sitting on the dial. That is for capacitors and it actually measures the capacitance as shown on the capacitor.

If you do not have one, then a trial and error would be needed, but 5 seconds it should be up to full speed, so I doubt it is the start cap.

If it is just barely rotating in the 5 sec, then it would be the start cap.
Customer: replied 3 years ago.

OK, I bought a meter to test them from Lowes. I called the manufacturer to find out the proper way to use the meter. I just came in from testing the capacitors and the "start" capacitor registers 296 on the meter ( the capacitor shows it is rated for 250), so I assume it is more than sufficient. It does not give a + or - amount on it. I also tested the "running " capacitor and it showed 51. It is rated at 50 with a + or - of 5. So I assume this one is also sufficient. Any more suggestions?

Expert:  AssuredElectrical replied 3 years ago.
So, the capacitors are rated at 250uf? and 50uf?
And your measurement showed the equivalent to those?

Make sure no wires are connected to the capacitor when testing also.
Customer: replied 3 years ago.

Yes that is the rating on the capacitors. According to manufacturer, I was to leave only one of the leads off the capacitor as I was testing it. But I will try it again later with both off of each.

Expert:  AssuredElectrical replied 3 years ago.
That is the normal, at least one lead off is supposed to keep from reading downstream, I prefer all wires off to guarantee a correct reading with no interference.

I would re-test with all wires off to insure the proper reading.

Now, when you start the motor, does it actually come up to full speed in the 5 sec?
Then stops?

Or does it just lag along at a very slow speed in the 5 sec and stop?

With the meter, after checking the capacitance of the the caps, put the multi meter on OHMS

Then check each contact point on the cap to the metal case of the capacitor.

That will verify that it isn't shorted at all.

If they are plastic caps, then you do not need to do that test.

They can still fail under power in some instances even if they read properly, so it is not always a definite outcome.
Customer: replied 3 years ago.

I retested with all the wires off and got a reading of 296.9 on the "start" capacitor and 51.5 on the "running "capacitor.


to your question about the running - when I did try that yesterday, it ran for the 5 seconds at a slow speed.


The cases of the capacitors are plastic so I didn't bother with that ohm test. My meter does not have that capability anyway.


Now what?

Expert:  AssuredElectrical replied 3 years ago.

Ok, thanks.


I am surprised a meter without continuity, never seen one without it and measure voltage and capacitors.

Well, the only option at this point is to try a new START capacitor.

Possibly it is breaking down on the surge while voltage is applied, even though the testing without voltage is good on the capacitance.
I have seen caps fail when voltage is applied.

The only reason I suggest a new cap, is it is not getting up to speed.
The windings appear to be good since it does not trip a breaker at startup.
If they were shorted then it would trip immediately.

If it never reaches full rpm, the centrifugal switch cannot changeover to the run windings.
It must start and reach full rpm , then the internal switch breaks the start windings in the motor and switches over to the run windings.

It is not making it up to speed (if it runs slow for the 5 sec and stops), so there are only 2 things to stop it, bad winding or bad capacitor.

Before I would call it a bad winding and motor replacement, I would want to get a $5-10 cap and try it to be certain.

Now on another note, since you had transported it before the problem started I assume, I would check all connections inside the motor for tightness to be sure a wire did not slip off or cone loose anywhere inside the wiring compartment.