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I have a Lennox 2.5 ton a/c unit, model 13ACD-036-230-05, with a blown dual run capacitor (the run terminal for the compressor is burnt off the can, the fan and common terminals are still intact, fan still running).

I had the cap replaced a couple years ago when the fan quit working. I remember the 5/45 uf, 440 volt cap that was installed was close to the original cap that failed, but not exactly the same.

I ohm'd out the compressor and got 1.4 ohms S-C, 0.9 ohms R-C, 2.3 ohms S-R. These values seem possibly reasonable to me, but according to LennoxPros, "The resistance between start(S) to common(C) is typically three-to-five times higher than run to common winding..." but they didn't state what resistance is actually expected.

What is the original cap rating (uf and voltage) for this a/c unit and what are the normal compressor resistance measurements?

wiring diagram from inside the unit,  ref. C12

  • Look up a wiring diagram (or other documentation) to find the original values. – Tyson May 28 at 0:55
  • I checked the wiring diagram inside the unit. All it has is a label "C-12 CAPACITOR - DUAL". Checking several parts sources online, I found a number of different caps referenced for this model, ranging 300 to 500 volts, 5/30 uf to 7.5/60 uf. – Eric Simpson May 28 at 3:34
  • @Tyson, I extended my online search but only found the same diagram, an installation guide, and a 2 page marketing flyer, nothing with specs on the cap or compressor. Do you have access to more helpful info? The original cap lasted 7 years, this one lasted 4. How long would you expect a cap to last? – Eric Simpson May 28 at 11:31
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I went through the Lennox web site to find a "store" near me and found an actual Lennox service center. They confirmed the 5/45 uf 440v dual run capacitor was correct. They didn't have any specs on the compressor coil resistance.

The service center didn't have any caps in stock but I found one at a local electrical supply house for under $10. It can also be ordered from Home Depot online (not available in stores) for about the same price.

I checked the resistance from each compressor terminal to ground (29-30 meg ohm each), re-checked the coil resistances (no change), put in the new cap, and turned it on. I noticed the current draw is lower than expected (9.2A @ startup, 8.9A running @ 230V supply vs. 15.4A nameplate) but It's working fine so far.

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