Teaching an Old Compressor New Tricks

24 Jan

When it gets cold out the low pressure warnings start popping up on the cars and of course you need to pump up those tires a little to get rid of the nagging yellow light (and to keep the tires at a safe and efficient pressure, of course). For a while we were relying on my very handy CO2 tank which is very portable and quick to fill, but through experience we have found that CO2 does not stay in tires for the long term. It sounds like nonsense but apparently the molecules are small enough to seep through the rubber faster than the nitrogen molecules that make up most of compressed air. CO2 is amazing at filling tires quickly in an emergency or for track side or trailhead use, and great for powering air tools but will not sustain tire pressure in the long term. This was very evident when we were trying to use CO2 to maintain the bike tires. I got tired of going down to the local gas stations where you have to either wait on line or pay quarters or deal with unreliable equipment and even sprinklers. (At one point I had a slow leak in my tire and was stopping there every day at around 5am and the sprinklers were on at that time so it was a wet endeavor).
Finally, the other day I decided it was time to get back into the compressed air business. We were on the way home from Townline BBQ and I decided to pick up a cheap air compressor at Harbor Freight to at least have some source of compressed air. Once we got it home I read further into the reviews online and decided it really wasn’t up to snuff and maybe I should return it.
This brings us to my old air compressor.


I have a 25 gallon Craftsman air compressor that is probably going on close to 20 years old, and it’s served me well, but developed a problem. When we brought it to the new house we tried to use it for the nail gun and it would blow the circuit breaker. We figured it was just the house wiring and went to the CO2 tank to power the nail gun and put the compressor on the back burner. I decided now that I should try to look into what was really wrong with this compressor or if it was just the house wiring after all. The first step was to bring the compressor inside and down to the basement and park it right next to the new sub panel in the workshop with its heavy gauge wiring and 20amp breakers. We plugged it in and hit the switch, and it worked! For about 5 seconds before blowing the circuit breaker. A second attempt yielded similar results but it was still encouraging that it was starting and spinning and not just seized up or something. I removed the covers and took a look and started doing some research. The compressor pump is driven by an electric motor that has 2 capacitors on it; a start capacitor and a run capacitor. I am still not totally clear on what they do but essentially a capacitor holds an electrical charge that is available for equipment when it needs a surge of power, like to start an electric motor spinning. That’s what the start capacitor is for, it kind of gives the motor a big push in the right direction and helps reduce the big inrush of current from blowing the circuit breaker. I tried to see if there was a way to test if the capacitors were bad, and I found lots of articles and videos on the matter but there were two problems. One was that a lot of the test methods use a meter that has a function that neither my Fluke or Hioki clamp meters have, which is basically a capacitance measurement. The other problem was that some of the tests used continuity across the terminals as a test (which I have the capability of testing) but the articles didn’t agree on whether continuity indicated the capacitor was good or bad. Just for reference the start capacitor did not have continuity and the run capacitor DID. It was my feeling that because the compressor was actually starting that the start compressor was therefor good, and perhaps the run capacitor was at fault.

I also read of some people experiencing an issue where the centrifugal mechanism that is supposed to disengage the start capacitor was faulty and therefor the start capacitors would stay active for more than the brief moment it is meant to and cause a large draw on the electrical system which would then blow the breaker.
I had been disconnecting and reconnecting the leads on the capacitors to do the continuity tests and this gave me an idea; I would let the motor start and then quickly unhook the start capacitor and if it ran then I would know that it was not disengaging. I tried it and it worked! Just for kicks I tried it a second time but this time I slipped and was unable to get it unplugged fast enough, yet it continued to run anyway! Then I made a shocking discovery, I saw that both leads to the run capacitor were not plugged in.


The run capacitor hanging from one of its leads

I plugged it back in and again we were back to blowing circuits. Unplugged it and we were off to the races. Apparently a run capacitor is not totally necessary for an electric motor to run, but if it is faulty it will stop it from running. Already I had found the correct parts online at Temco and had both capacitors in my cart so I just removed the start capacitor and for less than $9 my new run capacitor was on the way. Just a few days later it arrived in the mail and I was ready to install it.


The original GE capacitor with the new Temco replacement

It was a simple matter of unplugging the old one and plugging in the new one.


The capacitor fits snugly inside the cover and then the cover gets screwed down to the motor housing.

It does sound like it runs a little better with the run capacitor and I read it was bad for the motor and less efficient to do so but, in a pinch, it will certainly work. I did not see this as a possible solution anywhere in my reading so hopefully this will help someone else who is about to buy a new compressor or fork over a hundred bucks for a new motor or a service call. So if your air compressor is starting but blowing the circuit then try disconnecting your run capacitor and give it a go.

While I was at it I decided to give the compressor a little tune up so I added a new ball valve drain and a T fitting so I could run one air line to the garage and keep one line in the workshop.



One Response to “Teaching an Old Compressor New Tricks”

  1. Chrus February 4, 2014 at 12:22 AM #

    nice one!!!!!!!

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