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  1. #14
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    Quote Originally Posted by Sabr7 View Post
    Hey it’s ya favorite HVAC noob here.

    So I got a single phase scroll compressor from the job today. (It’s scrap, we replaced with a new one)

    Instead of throwing it away I brought it home because I’m curious. So when I set the multimeter to ohms and touch any two combinations from common, start, or run the numbers jump. I thought I wasn’t getting a good connection so I attached alligator clips from my leads to the terminals and the same thing. Any insight?


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    My first inclination is the meter is either cheap, low battery, or set on the incorrect sensitivity level. My second inclination is who cares? The compressor is good or it's bad. Do you need to know anything more than that? It's not like you're going to re-wind the motor. Resistance is a great diagnostic tool, but you either have it or you don't. If the numbers don't add up then they don't add up. Your $200 Fluke meter isn't going to tell you anymore than good or bad and it doesn't have to. Let's face it. We don't get called out when things are working properly. Were you to megg an operating compressor and told the owner the tolerances are off...but the unit is still cooling, are they going to take your word and replace the compressor? My folks have a unit that eats capacitors annually. I talk to their hvac guy on the phone every spring. He says the megger says the compressor is bad. I tell him every year to replace the capacitors and leave it go. This is probably going on 8 years now.
    Officially, Down for the count

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  2. #15
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    Thread Starter
    Quote Originally Posted by Snapperhead View Post
    when you cranked up the new compressor was there any issues ? or did the replacing go smooth
    No issues.


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  3. #16
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    Quote Originally Posted by 2sac View Post
    My first inclination is the meter is either cheap, low battery, or set on the incorrect sensitivity level. My second inclination is who cares? The compressor is good or it's bad. Do you need to know anything more than that? It's not like you're going to re-wind the motor. Resistance is a great diagnostic tool, but you either have it or you don't. If the numbers don't add up then they don't add up. Your $200 Fluke meter isn't going to tell you anymore than good or bad and it doesn't have to. Let's face it. We don't get called out when things are working properly. Were you to megg an operating compressor and told the owner the tolerances are off...but the unit is still cooling, are they going to take your word and replace the compressor? My folks have a unit that eats capacitors annually. I talk to their hvac guy on the phone every spring. He says the megger says the compressor is bad. I tell him every year to replace the capacitors and leave it go. This is probably going on 8 years now.
    Changed the batteries and the numbers are holding steady now...

    Im doing this for personal growth and understanding. Ive learned a lot from this thread already.


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  4. #17
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    Quote Originally Posted by Sabr7 View Post
    Changed the batteries and the numbers are holding steady now...

    Im doing this for personal growth and understanding. Ive learned a lot from this thread already.


    Sent from my iPhone using Tapatalk
    I was going to suggest that it may have been the meter auto ranging but it sounds like you found the solution.

    Thanks for posting the solution that you found!


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    Quickly, I must hurry, for there go my people and I am their leader!

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  6. #18
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    Quote Originally Posted by Sabr7 View Post
    No issues with new compressor


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    well then Id say the old compressor is definitely shot

    maybe the pump went bad , but the electric motor is ok

  7. #19
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    Quote Originally Posted by pecmsg View Post
    Too ground should be ZERO always!
    Zero ohms would mean he is shorted to ground. With a typical multimeter he would want OL (no continuity). If he gets a megger he will need to learn how to use it. Not sure what good a megger is for residential work. If I was him I would invest in a good HVAC multi meter.

  8. #20
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    Quote Originally Posted by Push It View Post
    Zero ohms would mean he is shorted to ground. With a typical multimeter he would want OL (no continuity). If he gets a megger he will need to learn how to use it. Not sure what good a megger is for residential work. If I was him I would invest in a good HVAC multi meter.
    Correct, OPEN or OL

  9. #21
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    Quote Originally Posted by Sabr7 View Post
    Changed the batteries and the numbers are holding steady now...

    I’m doing this for personal growth and understanding. Ive learned a lot from this thread already.


    Sent from my iPhone using Tapatalk
    If you are interested in learning grab a grinder and cut it open. Take it apart piece by piece. Might even find an issue.

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  11. #22
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    Quote Originally Posted by Sabr7 View Post

    Well let me try this again..


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    True to ground it SHOULD be OL or "open loop" meaning electricity can not flow from one lead to the next... but checking from terminals to casing is checking if windings is shorted to case. With old compressor still in old unit and installed casing would've been grounded... out of system and compressor sitting by itself un-grounded, then ohm between terminals to case would show if maybe any one or more terminals is shorted to casing. So, if you have an OHM reading from ANY terminal to casing that winding is shorted to casing and compressor needs to be replaced. Even grounded it would read OHMS, if shorted. With readings he is getting without using my own meter, sounds like windings are shorted to each other.

    Zero ohms on all terminals= bad compressor. OL readings with compressor cool means internal thermal overload switch is stuck open = change compressor. Good readings would be run to common + common to start = start to run .
    Does not sound like stuck internal overload.

    Windings shorting to themselves can happen because a few factors. 1. Extreme natural age of system. 2. Incorrect incoming voltage. typical compressors allowance is around +/- 10%. Too much or too little will cause excessive heating of windings. 3. Compressor being used with bad capacitor for long periods. 4. wrong voltage capacitor. using a 370 v cap on a 440 v required cap compressor typically kills the capacitor first, but if wrong volt cap is repeatedly used, could cause windings shortage.

    The windings in a compressor operates exactly like a step-up transformer during start. Thus reason for 370 and 440 caps. Even though its being fed with ~220 volts.

    Remember, proper use of a volt meter is understanding how the meter works. A meter even multi-meters measures the difference of voltage... not voltage. This is why if you put two leads on same leg, it reads Zero. Because there is no difference. This is why to get a reading using a meter you must either check between ground and a hot line or between 2 different lines.... to read a difference between your two leads from meter. Again, OL means open loop and electricity can not make it from one lead to the other.

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