How can I check fan capacitor on Lennox AC unit using digital multimeter I have no instructions for

micheric91's Avatar (by Gravatar) by micheric91 in  Climate Control » Air Conditioning 

I have absolutely zero knowledge on how to use a multimeter and need to check the fan capacitor on my home ac unit that is not working. I have access to digital multimeter - Etek 10709 is the brand/model. There is no manual with the one I have to use and I cannot find free download of one online either.
Its super hot here in Texas and need this AC back up and running ASAP

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Answer by homeowners

To test a capacitor properly, as if measuring its actual capacitance,  you need a multimeter that has the capacitance measuring mode or a specialized tool. Your Etek 10709 DMM (Digital MultiMeter) does not have the capacitance measuring mode. 

However, it is exceedingly rare for a capacitor to lose only a part of its original capacitance, and even then it would still work all the way down to at least 20%-30% of the original value. So, all you need to actually know about the capacitor in terms of operating an AC motor is these three things:

Does it have some capacitance?
Is it shorted?
Is it open?   

Only the positive answer to the first question and negative to the other two would mean a working capacitor. Any DMM that has a resistance measuring mode (and that's every DMM these days, yours included) can get you the answer to these three questions.
Note that the dual capacitor you're testing (borrowing info from your other question http://www.homeownershub.com/a... ) is actually two separate capacitors in one can, so you need to do the same test twice. The common end of the dual capacitor, the one with four blades for connector attachments, belongs to both capacitors. The remaining two ends are the second ends of the two capacitors.

How to do the test:
1. Switch your DMM into the 2 Mega Ohm  range, resistivity mode - two clicks to the right from the OFF position.
2. Touch the black test lead to the common of the capacitor.
3. While looking and the screen, touch the red test lead to one of the other ends of the capacitor.   

Here is the interpretation of what you will see:
1. If the DMM's screen does not go through any changes whatsoever, and it's still showing something like 0.L (L for large, in fact "infinity" in this case)   , then this one of the two capacitors is open
2. If the screen goes to zero (0.00 Ohm, give or take a few hundredths) , this one of the two capacitors is shorted
3. If the screen of the DMM quickly (in about a second, so pay attention) rolls through a number of readings starting with a 0, then something like 20K Ohm, then 200K Ohm and eventually settles on 0.L (as was in the "open" case), then this one of the two capacitors is fine. If it was too quick and you didn't catch it, reverse the test leads (red-black) and do it again.

4. Don't forget to repeat steps 1 through 3 for the other half of the dual capacitor.

This test is based on the way the DMM operated in resistivity testing mode - it provides a small voltage that can charge the capacitor and while being charged, it's exerting a changing resistance to the charging current. Once charged, it does not take in any more current and behaves as it were open.

Sorry for the long post, I just see you're in a pinch and although you can't properly measure the capacitor(s) with the tools you have, you can still determine if either or both halves of the dual capacitor are bad (i.e. shorted or open)

The power to the A/C unit should be disconnected and the capacitor removed from it's circuit for these tests! Be safe!

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