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.
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.
When 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.
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.
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.