It’s always a capacitor

-January 29, 2016

Back in 2006 I wrote about how my friend Alan Martin over at Texas Instruments had a saying: “It’s always a capacitor.” This was based on his observance that there is a lot of test equipment that goes bad due to dried out electrolytic capacitors, or failed tantalum capacitors. Alan explained his experiences in a subsequent post.

Alan's comments were in regards to electronics. Yet his maxim that “it’s always a capacitor” was proven out when my central air conditioning went out recently. I went to the compressor on the side of the house and the fan on top was not spinning. I spun the fan with a bread-knife and it was free. I looked up "AC fan not working" on YouTube and a video there said it was the start/run capacitor. In fact there were several videos about bad start/run capacitors. I opened the unit up, and sure enough, the capacitor is swollen. That is a sure sign of trouble.


Figure 1 

This capacitor is a dual run capacitor for both the fan and the AC compressor. There is a separate black start capacitor for the compressor, needed due to the high starting load of a compressor under pressure.

The compressor was making a horrible noise, like the one in that first video, but I suspected that was due to the lack of cooling. What I didn't like was the little chip in the flange of the black capacitor, and the fact that it was 16 years old. If one capacitor went bad this one would be next.


Figure 2 The compressor run capacitor was damaged and old.

Even though the label of the silver capacitor was obscured right at the part number, it did not take long to figure out this was a 5uF/25uF GE capacitor part number 27L33 also known as Trane part number CPT00695. Neither Home Depot nor ACE carry start capacitors in the store. I had a local company service the AC a month ago, but they didn't have the same capacitor. Neither eBay nor Amazon has the exact same capacitor. Then I find this great place called Shortys HVAC in Indiana. They have the exact same capacitor, with the right picture on the website. They have it at a cheap price, and they offer free shipping. Best yet, they take Paypal so I don't have to expose my credit card number. I also replaced the run cap on the compressor, just for good measure. It was Trane part number CPT00091. It too was the exact same capacitor, not just value, but also size and appearance. It did come with a bleed resistor across the terminals. Snip snip. Don't try this at home.

Look, we're engineers. And we know that getting the exact part is better than just close. The straps that hold the capacitors are intended for an exact physical size.


Figure 3 I labeled the wires for the capacitors with duct tape.

You can see where I used duct tape to mark the wires. I figured they would take the rain if it came. Thing is, Sharpie ink will rub off duct tape, so maybe masking tape would have been better. The best part of the experience was that when I wrote an email to Shorty's HVAC at 7:00 PM on Thanksgiving Saturday, Mike Short responded in 15 minutes. He said he could air ship the capacitors if I wanted them Tuesday, but that his FedEx ground shipping would have them here by Wednesday. I doubted that, but I figured free shipping was better than expensive shipping. To my amazement, the capacitors came at 10:00 AM Wednesday morning. I had them installed fifteen minutes later.


Figure 4 Here’s the AC condenser with its new capacitors.

Ahhhhhhh it’s cool again. Thankfully this happened in November here in Florida, when it is "only" 80 degrees out. At first I was in a panic trying to get the central AC working. A good lesson, never rush when troubleshooting or fixing things. Instead, I used a portable unit I bought for my shop to get me through the few days without central AC.


Figure 5

 This field expedient kept the office cool while the central AC was being repaired.

This field expedient was a great way to lessen my panic over not having air conditioning. It kept my little office comfortable and the whole house dry. I think everyone south of the Mason Dixon line should have one of these portable AC units in the garage, just in case. It took all the pressure off to get the parts immediately.

You might note in that picture of the installed capacitors that there is a big contractor relay to the right. This powers up the compressor and fan. The contacts were pretty worn after 16 years of continuous service. Figuring out that part number was a puzzle as well.


Figure 6 Replacing the AC relay was more of a challenge.

I will write this up in a subsequent blog. It took hours to figure out this was a Potter Brumfield relay, since bought by AMP/Tyco and sold as Products Unlimited. There were all kinds of knock-offs and “replacement” units on the Web. But I wanted a “real” relay made by someone I trusted. Well, Newark Element 14 to the rescue. They had the exact unit, a real Tyco part, in stock. And rather than $60, Newark wants $13.

This brings up a peeve of mine. I just paid a hundred dollars to the company that installed this AC system in 1999. A nice young fella came and took the squirrel-cage blower down from the attic, cleaned it, and reinstalled it. He unplugged the water drain. He checked the refrigerant. But you would think that a service call on a 16-year-old AC system would at least offer to replace all the capacitors. Now I wonder if that blower the young fella pulled out had a run capacitor on it. I sure would want that replaced too.

I know to replace old capacitors because Alan Martin explained it, “It's always a capacitor.”

I dropped Alan Martin a note about this bad capacitor. He replied:
"The capacitor quotation may have been taken a bit out of context since Tektronix never made air conditioners. I'm not aware if oil capacitors of the type used here are prone to rupturing and causing a fire, but the thought of it happening makes me glad I don't need AC here. That's one scary bulge. You said you used a bread knife to spin the fan. Wrong knife. My HVAC manual says it should be a butter knife because it can be used to lube the bearings with Plugra as part of the preventative maintenance."
Another friend, the International Man of Mystery Martin DeLateur, wrote me:
“I had the starter cap go bad on my air compressor after continuous use for 12 years. I also found the exact replacement on eBay for $8. After replacing it, the compressor fired right up. I did not bother to change the run cap since it tested good. Two weeks later, it stopped working again. I have not worked on it yet because I have since bought two more compressors which I am using as the backup and backup/backup.”

So let us know if you have had to struggle with motor start/run capacitors to keep your stuff working. I can tell you, it sure feels good to fix something and to save a bunch of money in the process.

Also see:

Loading comments...

Write a Comment

To comment please Log In

FEATURED RESOURCES