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Air Compressor Motor - Start Capacitor Bad or is it something else?


May 5, 2024
I have a 2003 Craftsman Professional Twin-Cylinder 7 hp 80 gallon 175 psi air compressor. The motor without the belt to the compressor attached will start turning but will kick out the breaker. At my attempt to start the motor - there is a fair amount of noise coming from the motor prior to immediately kicking out the breaker. I can turn the motor over by hand without bad bearing drag. The start capacitor has corrosion coming from it - see photos. Would this be causing the problem? How does one test the start capacitor to confirm if it is bad? Thank yoU!IMG_4664.jpgIMG_4659.jpgIMG_4660.jpgIMG_4662.jpgIMG_4663.jpg
If you have a VOM that does capacitance and the proper diagnostic skills, great, but consider capacitors a wear items and the cheapest easiest and most likely thing to go bad so I would replace it unless the motor had smoke coming out of it. Amazon, < $20
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I would suspect the run capacitor if the compressor runs for more than a second or two and the centrifugal switch is kicking out properly, not the start capacitor. If it's instant, may be the start cap or centrifugal switch.

Check capacitance, but the only good way to test a cap is with an ESR meter.
If you have an analog ohm meter, you're good to go. A down and dirty test.

Discharge the capacitor, then hook the leads to the terminals. Resistance should start at infinity, then rapidly go to a lower ohm resistance. Some capacitors will immediately return to infinity when charged by this method. (your ohmmeter is charging the capacitor).

If the reading goes to 0 ohms, and stays there, you have a shorted capacitor. If the reading stays at infinity, you have an open capacitor.

Another test involves using a 9v transistor battery. Discharge the capacitor, then momentarily hook the battery to the terminals to charge the capacitor. Wait a bit, then put a voltmeter on the terminals. The voltage should start out at around 9v, then rapidly descend to 0 volts. This tells you that the capacitor is storing voltage. Your voltmeter is discharging the capacitor.

You can also use a multimeter with capacitance test mode. Just make sure you buy one that will test the range of your capacitor. I've got one that goes to 100,000uf because I need to test capacitors in welders. Smoothing capacitors tend to have large capacitance. I don't believe your capacitors should be this beefy(the capacitance is on the label on the capacitor)
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I get a lot of noise/ vibration when the start switch contacts get stuck together. This condition will take out the start cap. It is only meant to be energized until the motor comes up to speed.