Is The Compressor Start Capacitor Working?

This is page three about troubleshooting a Craftsman air compressor that will not start and this page focuses on – is the compressor start capacitor working?

If you want to review the steps from the beginning, here is page one on figuring out why your Craftsman air compressor will not start.

What in heck is a start capacitor?

It’s a device (or sometimes two) normally found on the exterior of a compressor motor, often covered in a metal shroud, near the shaft end of the motor. The purpose of the start capacitor is to help the compressor get going. If your compressor motor also has a run capacitor, that one helps to keep the motor running.

Start capacitor cover installed on electric motorThe start capacitor cover on your compressor motor may look the same as the one shown in the photo, or might differ slightly.

As we have been trying to find out why our Craftsman air compressor will not start, in previous pages we’ve traced the power supply successfully to the motor circuit.

If the compressor motor start capacitor has failed, the power supply to the motor stops there, and the motor will not start.

If you are comfortable with doing so, remove the cover of the start capacitor. Your capacitor may look something like the one in the next photo.

compressor motor start capacitorWhen you get the cover off, examine the capacitor. We are expecting that you have pulled the compressor power cord at this point. Please be careful not to touch the capacitor terminals as a capacitor is a high discharge device, and you could get quite a jolt from the power that’s stored in it.

Look for discoloring, blisters on the outside of the capacitor body, connected terminals that don’t have a lot of corrosion on them, anything that would give you the impression that all is not right with the capacitor.

Even if the motor capacitor looks good, you will still want to check it to make sure that it’s working properly. Please see the embedded video about “How to Check Motor Start and Motor Run Capacitors

 Replacement compressor motor capacitor

You will, unless the label is worn off, be able to see writing on the side of the capacitor. It will show a rating in MFD (mircro-farads) and a voltage range.

As long as you acquire a replacement motor capacitor that has the same MFD rating as the old, and the same voltage range, and you can connect the terminals, then you can use that capacitor, regardless of the shape or size. It’s best to get one that fits inside the existing capacitor cover, of course.

If you cannot see the writing on the side of the capacitor, then it’s time to check out the motor specs to find what capacitor is needed, and you can use your search browser to query what the capacitor is for such and such a motor HP, with xxx voltage etc.

 Now what?

If you’ve checked the capacitor and it’s good, then possibly the electric motor itself has failed, particularly if, when you power up the air compressor with the tank empty, and power flowing to the tested start capacitor, your compressor motor makes no sound at all.

How to test a compressor motor is a fairly complex subject and beyond the scope of many DIY among us.

This will give you some idea about the steps in testing an electric motor.

What we would do at this point is take the electric motor off the air compressor and take it to a local repair shop for testing and, if necessary, for repair.

Comments

  1. Derek Wang says:

    When I turn on my compressor, it sounds like an engine that just wont turn over. It sort of tries to turn, then slows down, then tries again. After a few tries, it trips its own fuse. This just happened when I tried to start it just after winter. Thanks for any help.

  2. Just a question. I have a mastercraft multi meter and just can not figure out which is best setting to test a capacitor. See image attached.

    • Your meter won’t read capacitance so you need to use a different meter. You have AC/DC voltage, DC current, and resistance. Capacitance is measured in Farads so you will usually see uF for microFarads, etc.

      Some capacitors are labeled MFD for microFarads.

      As an example, the starting capacitor on my air compressor is 120 uF.

  3. Hi there,

    My compressor (condor cosmos 2.4 – 240v european) motor wont run properly.

    I have disconnected air supply to tank, and removed cylinder head, so it should be running free with no load. However when I flick the switch on it chugs a few times, before overloading the power (2.6kw generator). It’s like it is trying to swim through mud…

    Any ideas? Anything more I can test?

  4. When I turn on my compresser on it is making a humming sound and not turning on. Do you have any ideas?

    • If you are using an extension cord, don’t. Try it plugged directly into a wall socket. Does that help? If not, empty all the air from the tank and turn it on again. Did that help? If not, see the page on how to check the capacitor as that would be the next likely thing that is causing this humming air compressor.

  5. i have the same problem as greg. i disconnected the drive belt and the compressor alone turns freely.i then started the compressor without the compressor drive belt attached and the motor came up to speed with out any problem and i could hear the centrifigual switch kicking out. when i reattached the belt the motor and compressor ran for a few seconds and the circuit breaker popped.it should be noted that i am using an extension cord to provide power to the compressor????

    • As per our suggestion to Greg, try plugging the compressor directly into a wall socket, no extension cord, and see if that helps.

  6. Hi, My father in law has a HF 2hp 8 gal compressor that is having problems. The motor/compressor runs for about 8 seconds then shuts off. Regardless or how much air is in tank. After reset trips, a few seconds, I can press it and motor starts up again. Then it shuts off after 8 seconds or so. i tried to run it with empty tank and drain valve open. No difference. I realize it is a cheap compressor, but I would like to figure it out. Not many hours on unit. Any ideas? Thanks

    • We suggest it’s an electrical problem. Whether that problem is due to a too long and too small extension cord, a pressure switch not working, or a failing start capacitor, we don’t know. Make sure the compressor is plugged directly into a wall socket, no extension cords of any kind. If the problem persists, the next time the compressor stops use a multi-meter to determine if, even with the compressor off, power is still passing the pressure switch. If the switch is working and the air pressure is not above cut out, then power should be passing the switch. If it is not, the switch may be faulty. If power is passing the switch, it’s time to check the functioning of the capacitor… make sure you ensure that all wires are connected. The vibration on smaller air compressors does, from time to time, dislodge a capacitor line.

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