Testing Capacitors


I am, along with many others, grateful for the following information about testing capacitors, from an anonymous visitor, who took it upon him/herself to provide this information for me to share with you. Need to test your air compressor capacitors? Here's how!

The information about testing capacitors is copied here exactly as received. He/she and I both hope you find it useful and helps you resolve one of the many reasons why, from time to time, your air compressor will not start.

Testing Capacitors


"I'm probably too late to help the original poster, but someone might find this thread and be thankful for a little insight.

There are various ways to test capacitors, but for your average handy person, there's an easy way to tell whether they are good or bad.

Generally, capacitors themselves fail in one of two ways: they become an open circuit, or a dead short. When they're a dead short, usually they trip a breaker or explode, so most likely it's not dead shorted. I'll tell you how to test it after letting you know about the other "capacitor failures" that are just as common as the capacitor itself.

The start capacitor for one model of a Bostitch air compressor is shown below.


testing capacitors - air compressor start capacitor

Capacitor Connection Failure


That's a connection failure. Look closely at the terminals where the capacitor connects. Are they rusty? Are the push-on connectors loose? Are they discolored, like they were overheating? Is there melted plastic around the area? All those things could mean a bad connection... I even had one where the terminals and wire connectors were fine, but the rivets that held the terminal (push on tab) on the capacitor were rusty enough that there was no connection. The electrical test to the rivet showed a good capacitor, but not to the connector.

You Need At Volt Meter / Ohm Meter


OK, to check the capacitor you need a volt meter/ohm meter. Do not touch the terminals of the capacitor with your fingers or metal objects before you're sure there is no charge on the capacitor! It could have up to 1.4 times the voltage that the motor takes (the actual peak voltage reached by a 120V ac line is actually over 170! 120v is just the equivalent DC voltage, or the root mean square of the sine wave... but that's another story).

Step One in Testing Capacitors


Step one - CHECK VOLTAGE on the capacitor with your volt meter set on DC Volts. Make sure to check all the terminals on your capacitors, and push your probes hard enough to dig in through the crud or surface oxidation. Does it read close to zero? Great. If you read that accurately, then it won't melt the tip off a screw driver that you will short across the terminals to make extra sure. Again, push it hard enough to make a good solid contact with both terminals, but no other metal (and make sure you are not touching the screw driver or anything that could lead to a shock). If you were wrong about the reading, or didn't have the voltmeter turned on, there will be a really strong "zap!" - a spark that might melt a chunk out of your screw driver.

Step 2 in Checking Capacitors


Now that you're sure there's no charge on the capacitor, if you haven't already unplugged the wires to it, note where they went and unplug them.

Step 3


Switch your volt meter to the ohms range, maybe around the hundred ohm range if it isn't auto-ranging. Check the meter by touching your two leads together - the meter should go immediately to about zero ohms if everything is set right, and you have a good battery in it.

A good capacitor charges up to that voltage, so when you connect the meter to a good capacitor, you'll see the ohms start kind of low and go up really high as the capacitor charges up to the 1.5 volts or whatever your meter uses.

This can happen in a few seconds, or as fast as a half second.

A run capacitor from a Campbell Hausfeld air compressor is shown below.

air compressor capacitor

Step 4


Here's the trick - switch your probes to the opposite terminals on the capacitor. Now the capacitor is charged the opposite way from what you're measuring, so the ohm meter will momentarily read a negative number! which will move through the range to zero and back to the high number or infinity like it did before (or if your meter is not digital, it'll slam the needle to the peg as it discharges the capacitor and recharges it the other way).

I usually switch the probes back and forth a couple of times to make sure I'm getting that capacitance effect that sends the number negative and slowly rises back up. Only a capacitor does that. And it won't do it if the capacitor is shorted or an open circuit.

That will give you about a 95% certainty that the capacitor is good... again, the value of a capacitor rarely changes significantly enough to affect the running of a motor. Also, most motors have a start capacitor, which is only used to START the motor. The capacitor and the start windings are only used to get the motor up to speed. A few motors (usually the bigger ones) also have a run capacitor. While the method of checking the capacitors is the same, this whole discussion is about run capacitors because it's about a motor not having the power to keep running under higher loads...

Blowing Breakers


And one thing I would check if you're blowing breakers is whether you have too long or too light of an extension cord. You may be losing a lot of power across the cord, so your motor draws more amps to keep running, and that could be blowing your breakers.

To see if that's an issue, try to check your voltage at the compressor while it is running, especially when it is bogging down. If it is significantly below 110 or 220 volts, then you might get better power to your motor and see if it still does that.

A Weak Capacitor?


If the capacitor is just weak, and not blown entirely, diagnosing that is difficult. Another anonymous contributor offered the following advice.

"It's rather difficult to test a capacitor without a multimeter that has a feature that allows you to test for capacitance directly.

To see if your capacitor is within specification set the meter to the capacitance mode and apply the test leads to the terminals - it will take a few seconds as the meter is actually charging the capacitor and then reading it in farads. Be careful when testing capacitors because they do hold a charge which sometimes can be hazardous.

A continuity tester will tell you if it's blown open or shorted so if the only meter reads Infinity or almost zero owns the capacitor is toast. If it is simply week an old meter will be very very difficult to interpret correctly. Hope this helps."

Still have a question about compressor motor capacitors? Please use the forum below.


Compressor Motor Capacitor Questions

Having an issue with your Compressor Motor Capacitor? Here is the place to ask about it.

I work on responses to information and technical requests daily. Please allow up to 24 hours for a response. Thanks. Bill

Other visitor's issues with Compressor Motor Capacitors

Please see below.

What Capacitor for a Speedway? 
I got a speedway series 25 gal compressor given to me without a capacitor, i think thats all it needs. what type does it need? Thanks (Question has …

The last 4 letters on capacitor? 
Do the last 4 letters have to be the same, I have a 61B4D220189NNEM

Wiring compressor capacitors 
I have a Sears Craftsman Motor. Model #113.12440, CAT 1244, 5 HP, 1725 RPM ,230 Volts, A24.0 , MFG Model-F-9777-07-163. It has 6 wires coming out of …

Click here to write your own.