Compressor Electrical Diagnostics

A compressor is an electric motor.  Since it is a motor, it has windings just like any other motor.  These windings can be checked for shorts, grounds, or opens. 

To check compressor windings, first shut off the power to the unit.  Next, go to the compressor electrical connection box on the compressor and open the cover. At this point you will see the terminations of the windings where they enter the compressor.  Whether the unit is single phase or three phase, there should be three terminals from inside the compressor. These terminals will be marked (C) common, (S) start, and (R) run on a single-phase unit or T1, T2, and T3 on a three-phase unit.  On the single-phase unit it is important to note which wire went to which terminal so they can be properly replaced.  On a three phase reciprocating compressor, it does not matter which wire goes to which terminal.

CAUTION: On a three-phase SCROLL compressor, the rotation must be checked to verify proper rotation when finished with electrical checks. 

To properly check the windings, the wires must be removed from these terminals.  


With the wires removed, take an ohmmeter and set it on the R X 1000 scale. Take one probe from the meter and find a good ground. (You may need to scratch the paint on the compressor or scrape the copper tubing at the compressor to assure a good connection).  Take the other probe and touch it to EACH terminal in the compressor.  ANY reading to ground from any terminal indicates a short to ground and the compressor must be replaced. 


Next we want to check for an open winding in the compressor.  Again, with your ohmmeter set on R X 1000, take the probes and go between pairs of terminals. If an “infinite” reading is obtained between pairs of terminals, there is an open winding in the compressor.  This does not necessarily mean the compressor is bad.  Compressors have internal overloads that open due to temperature or high amperage.  Feel the compressor.  If it is hot, chances are an internal overload may be open.  Note:  on a single-phase compressor, if the open is read between the start and run winding, the compressor is bad and needs to be replaced.  If the compressor is hot and an open winding was found, it may take an hour or two for the compressor to cool down and the overload to reset.  

While waiting for the overload to reset, check the contactor for pitted or worn points.  Check the capacitor on a single-phase unit and make sure it is good. Check all wiring connections for loose terminations.  Check the condenser coil to make sure it is clean.  Verify that the unit has proper voltage present at the disconnect.  All of these things can cause the internal overloads to trip. 


With an ohmmeter set on the R X 1 scale, we can check the windings for internal shorts. On a single-phase compressor, the windings should always “add-up” by pairs.  What this means is when reading the resistance between windings on a single phase compressor, Common (C) to Start (S) plus Common (C) to Run (R) should always equal Start (S) to Run (R).  (C-S) + (C-R) = (S-R).  Common to run should be the lowest reading.  Common to start should be the mid-range reading. Start to run should be the highest reading.  Example: (C-S)= 3 ohms and (C-R)= 1 ohm then (S-R) should = 4 ohms.  If these readings cannot be obtained, chances are there is an internal short in the windings of the compressor 

On three-phase compressors, all the windings should read the same. Again, if there is a variance in the readings, there could be an internal short in the windings. 

Since these checks are electrical checks of the MOTOR of the compressor, it does not matter if it is a reciprocating, scroll, hermetic or semi-hermetic compressor.  All the windings checks, for opens, shorts, and grounds are performed the same.


About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
This entry was posted in HVAC Tech Support. Bookmark the permalink.

76 Responses to Compressor Electrical Diagnostics

  1. Abdu Hafeez says:

    It’s really very informative.

  2. raymond says:

    what does infinite mean it says it the reading is infinite on the meter or no reading the compressor is ok but im not sure what infinite means

    • the scale on an ohm meter reads in the reverse direction of most conventional measuring scales, that is, less resistance is to the right (all the way to zero), and more resistance (infinite) is to the left. Zero resistance should be observed when your probes are connected directly to each other – aka dead short.

  3. Ben says:

    I have a single phase 1/3 HP Copland hemetically sealed compressor that has trouble starting. I have eliminated the thermostat by jumping it. I can hear it click and current draw then 2 seconds later it clicks off. I feel like it is the internal over current device but how would I know? My resistance readings total up correctly according to your blog. The start capacitor is good and the external relay seems ok.

    After quite a few tries, sometimes, the compressor will run. I don’t seemt to find replacement parts for these small units. Do people just replace the whole compressor? Seems like a waste, but if the parts are internal there is nothing I can do.

    • when a single phase compressor won’t start and the winding resistance is correct and there are no open windings, it is usaully external to the compressor. the “run” capacitor could be bad or the “start gear” (potential relay and start capacitor) could be bad. Lastly, verify that you have the proper voltage to operate the compressor.0
      If all of this is good, then you could have a bearing inside the hermetically sealed can binding. By trying to start and stop, it frees up some times and it runs. Idf this is the case, yes, you are looking at replacing the compressor.

      • Ben says:

        Thank you for your quick response! The proper voltage is going to the compressor. On the outside of the compressor connected to the three pins there are two devices connected.. on one pin there is what looks like a relay that has a wire from the start capacitor. The other two pins are connected to what looks like a plastic connector with a small coil of thick wire. I’m assuming my problem lies within these parts, but I don’t seem to find any recplacements online. My start capacitor readings are good too..

        Sorry for my lack of proper terminology, but I am a beginner technican. I work for a hotel and recently became universally certified.

      • if the compressor ohms out good, then it is probably one of thos external components.

  4. Zia says:

    I have a compressor and when i try to connect it,I got confused.
    Three cables comming from pannel brown black and blue. I dont understand which one is start run and common.Please advise a way find out

    • common goes straight from the contactor to the C on the compressor. The other side of the contactor goes to R on the compressor and one side of the capacitor. S on the compressor comes straight from the other side of the capacitor.

  5. Silvester Wanjala says:

    Very helpful,thanks.

  6. what if C + R = 1 ohm, C + S = 2 ohms, and S + R = 5 ohms?? still ok??

  7. Tim B says:

    I have an older unit.
    C+S= .06
    C+R= .05
    R+S= .07
    is this acceptable?

  8. Justin T says:

    I am working on a 3/4 horse copeland hermetically sealed reciprocating compressor I tested the overload ,relay and compactor all checked ok. I tested from each terminal to the case none ohmed. I then checked for resistance between the terminals none had resistance they all showed zeros. I used a digital meter set to the highest setting. The compressor will try to start for a few seconds then stop. I also hooked my gauges to the low side I have a reading of 50lbs before and during startup.

    • ZERO ohms indicates a dead short. Windings should read some resistance. Try it on you LOWEST OHM setting on your meter and see what it reads. If still ZERO — you compressor is shorted between the windings. If the ohms add up properly (C to S)+(C to R) = (S to R) you have locked rotor and either the bearing are bad in the compressor or you can try a start kit on it to see if that will do it.

  9. Ben says:

    Hi I have a single phase compressor (small freezer) but their ohms come as 7.3, 7.4 and 6.4. I guess it is obviously not normal, but I wonder whether there can be any other explanation.

    • c-s + c-r = s-r it’s that simple on single phase compressors. what do you get from each leg to ground? Did you do a meg-ohm check?

      • Ben says:

        thanks. They were the Ohm meter readings. No terminals were shorted to ground. The wiring diagram shows the typical two windings in series (with a center tap) so I wonder chat kind of short would result in these readings.

      • a meg-ohm reading could indicate moisture in the system causing you “short”. Check with the compressor manufacturer to see what is an acceptable meg-ohm reading. Or recover the refrigerant and pull a very deep vacuum (min 500 microns) and then check the windings

  10. Will K says:

    I should start off by saying I am a helper (rookie currently attending school) and am having problems with a new compressor on a walk-in freezer (404a). We put in a brand new copeland 3 phase semi-hematic compressor today and when charged it and started it we were getting high head pressure no matter what how we tried to remedy it. One of the techs that i work with thought that the wires going from the contactor had gotten mixed up during installation and tried swapping them around to what he thought was correct. After we flipped the disconnect that time the compressor wouldn’t kick on. We opened up the connections to the windings, disconnected everything, and ohm’d it out to find that they were all the same (indicating that it is good electrically?). It was hot and we tried to cool it down and waited until the place we were working closed (about 1 1/2 hours) but we couldn’t get the compressor to come back on. Is this part of the internal overload? What could be the issue? Any help would be much appreciated. Thanks in advance.

    • a 3-phase semi-hermetic compressor should not matter which way the motor turns as it is just driving the pistons. With that being said, if the overload is tripped, do you have power on each leg ? What were your pressures? Is it possible the TXV/solenoid is closed and you are not getting enough refrigerant back to the compressor to cool it?

  11. kyddo88 says:

    The contactor is working properly (as far as I can tell) and I have voltage on each leg coming and going off of the contactor. The pressures we were reading were about 20psi on the low side and about 330psi on the high side. And with it being about 50°f yesterday, the tech I was working with concluded that the high side was running higher than it should. We haven’t looked at the expansion valve yet. Could that keep the compressor from kicking on tho? Thanks for your help.

    • high head/low suction, even with a 3-phase compressor could cause it to not be able to start. When checking voltage, not only check between pairs to see if you have voltage but check ecah leg to ground to make sure you have power on all 3 legs. Some where you have a “high side restriction” so definately look at the txv.

  12. shah zada says:

    my refrigerator compressor start after few sec compressor stop.i check the continuity b/w compressor pin and ground no continuity and check resistance b/w pin top right to bottom 15ohms and top left to bottom 14 and top left to top right 29ohms.capicator also show voltage 5volt.relay also seen to be good.what the reason to run the compressor? How to check ptc relay and capacitor?

    • you are probably on the right track. The compressor appears to ohm out correctly. More than likely it is either the capacitor or the start device (PTC). See my post on capacitors. The ptc should be “closed” and it then opens the circuit after a certain amount of time from voltage being applied.

  13. Luke says:

    Hi I’ve recently installed a new 3 phase compressor, contactor and thermal overload for a freezer room. It was running fine for 3-4 days getting down to temp fine with an empty room. Got a call saying there is no power to the room and that the breaker has tripped.. I reset the breaker and it trips straight away.. I disconnect the compressor terminals and everything works fine. I tested the compressor to see if it goes down to earth and it doesn’t, each pin on the compressor reads 4 when testing continuity. I’m not sure how to tackle this is as the compressor was brand new and now it’s tripping the breaker.. Any help would be appreciated!!

    • Luke. What are your pressures? If the compressor ohms out correctly to ground and between windings, then you could be looking at “locked rotor” in the compressor.
      Try switching 2 of the wires in the compressor to reverse rotation. If it is a recip compressor, and it starts, you can leave it like that. If it is a scroll compressor, if you get it to start, you WILL need to change the wires back since scrolls only work in one direction.
      Then find out why the compressor “locked up”! Unequal pressures can prevent a compressor from starting. Liquid slugging can cause this also. Just some suggestions.

      • Luke says:

        pressures were very normal when intitially tested and fitted compressor.. Is it possible that too much liquid has flooded back to the compressor through a faulty solenoid? Thanks for your suggestions.. I’m about to go on site shortly

      • Fauly solenoid allowing flood back is possible. Could also have stuck TXV (either open or closed) causing liquid flood back or creating a “pump down” situatiuon. I would suggest getting all of your readings before trying to start compressor and then try reversing it if it is still stuck. Lastly, could be a bad bearing (bad compressor) but that only happens rarely.

  14. Eric Williams says:

    I have a single phase compressor…when ohming the windings I get: C to R = 2.1 ohms; C to S = 2.1 ohms; S to R = 3.8 ohms. Since C to R should read at least 1/2 of C to S, would it be reasonable to say that the Run winding is open?

    • Actually. if it were open there would be no reading. My guess is you have a shprted run winding. S-R = 3.8 and C-S = 2.1 then C-R should = 1.7 or less. Since it is up where it is, you have an internal short of the windings bridging between windings — bad compressor

  15. Paul says:

    I have a Sante Fe Force Dehumidifier. It was a costly unit and was only used for two seasons. Its a single phase compressor. I tried to turn on the dehumidifier after the winter season and the compressor would not turn on. The fan just ran. So, I troubleshoot it to find out the capacitor was bad and the humidistat. I replaced both of them.
    Readings are c to s = 2.7, c to r = 1.0, s to r = 3.5 ohms. All my reading looks good and it is not grounded.
    When I turn on the unit, the compressor kicks in and then blows the breaker about 10 seconds later. Could the compressor be bad even though all of the readings look good? Or, could it be the start relay? I looked at the start relay and there was no burning. I checked the resistance on the relay and that tested good, but did not check the resistance of the relay when it was running. I hear you can hook up hard start kit to give the compressor a boost before start up. Would that be recommended? Thanks..

  16. Paul says:

    Ill do that. Should I get a two or three wire hard start kit for my compressor? They have many different types.
    The relay has four wires coming off of it from four terminal ends (three black and one white) and the start capacitor has two wires (black/brown together and brown). The brown wires goes to the thermostat. How would I wire the hard start kit? Thanks again..

  17. I have a walk in cooler condenser unit I have now started to work on “ICS.” No schematics, and looking up the model# to the skid is rather a joke finding one. “Any help there?” My ohms checks good, C-S .9, C-R .9, S-R 1.8. I cant find nothing grounded either but It’s blowing the fuse instantly. I was thinking that it might need a boost with a hard start kit, “but,” all three lines ties straight to the contactor so I do not see a way to hook one up without burning one up due to. The previous tech had left all the wires unhooked for me to rewire and troubleshoot this, so I followed the wiring diagram behind the cover plate on the compressor to T-1, 2, n3. I have never had to worry about an electricians side at the breaker panel but is there something I need to look for there with the line voltage? Should I reverse a leg and see if it stuck so i can continue finish troubleshooting from this point? I always like to meet my specs every time and did my math knowing it would take a 40 uf capacitor if It’s missing somewhere? What should and can i do at this point? Please sir’ Thanks.

    • if the compressor is a single phase compressor, C-R should be lower that C-S. Both of these have the same reading which lead me to believe you have an internal short between the windings. If the compressor is 3-phase, then all the windings should read the same. Based on the info you gave me, I have to think you have a bad compressor either way.

      • chriswyld says:

        Thank you. that is much help that is needed. I am going to go back in my books and make sure I understand all the readings across a single phase and three phase again. I do have a couple of 1/3 horsepower compressors for refrigeration in my storage so would there be a way I might can use one of them. the only reason I’m asking is because the compressor info does not tell you any design pressures, just phase, some thermally protected, volts and LRA. I pulled a cross reference guide for the model number to find out the tons. Is this possible if all matches or are the suppliers right and not trying to tell me I need the exact one making a sale. Lol. thanks again’

      • as long as tonnage/HP, voltage and phase are same, you should b able to substitute compressors

  18. chriswyld says:

    Oh’ i did not see that you had sent me all of the information verifying the compressors, thank you again’

  19. Renz says:

    Hi I am new beginning tech and I recently came a cross a compressor with these ohms. Readings:
    System is a 2 ton r410a. Rheem with TXV
    R-S=3.1 but it should add up to 3.3. Amp 6 with a max FLA of 13.5 A

    Capacitor is 35 uf but it reads 34.61ohms within -+6 % of swing .

    Vapor pressure are lower than normal 130 psig with 62 vapor line temperature and liquid pressures are 370 psig with a 102 liquid line temperature. Both vapor and liquid pressures fluctuates up and down between a 30 to 40 psig interval.)

    Also subcooling hunts up and down erratically from 6.0 to 8.8 (manufacturer suggested subcooling is 7) as well as the superheat from 14 to 17.

    Can you tell me your analysis on what maybe wrong? Is the compressor mechanically or electrically damaged? TXV hunting? Or non condensable?

    Thank you.

    • My best guess is there is a txv issue. The bulb may not be insulated well on the line. The could bee loose. With that much “hunting” the first thisng to check would be to see if there are any issues with the txv sensing bulb. Non-condensables would cause high pressures.

      • renz says:

        Thanks for your reply .Then you think the compressor is good with the ohm reading I got?

      • I guess I should ask, why do you think there is a problem? What is the system doing or not doing?
        Now, depending on the type of ohm meter you are using, some do averaging. Your readings are close enough that I do not feel there is an issue with the motor in the compressor.
        As far as your second question, if you have non-condensables in the system, your pressures would be running higher. If your shut off the unit, get the pressures to equalize by warming the txv bulb, then compare your standing pressure to your temperature pressure chart for that temperature, again, if you have non-condensables in the system, the pressures will be higher than what they should be for that given temperature.

      • renz says:

        It is possible that non condensables be freezing and thawing at the TXV thus causing the hunting.

        Thanks for your help.

  20. Andy Juniyarta says:

    I think its a bit late to ask a question, however i have a compressor and the reading is C-S = 18ohm, and C-R 17 ohm, my question is, is it ok for the compressor to have almost same resistance between starting and running winding ? is it possible for 1 phase compressor for having same resistance between starting and running ?

  21. Rich says:

    I tested compressor and no matter what pair of pins I test the multimeter ends up at zero I was on 200 Ohm and tried 2000 same thing happens. I assume that means bad compressor. My freezer will not turn on at all. Thought it was switch (Temp) but now..

  22. Aditya says:

    What is the resistance between Compressor Terminal & Body of Compressor ?

  23. John says:

    I have a single phase fridge compressor which is tripping the circuit breaker after running for a few hours. Thought it would most likely be the defrost element but thought I’d check the compressor anyway and I’m getting 11.5 Ohms between all terminals. Does this mean I should replace the compressor?

  24. Abel says:

    we have 250 kilowatt compressor motor .what resistance should i be getting ..I have done an insulation resistance test between windings all where clear…and Im getting 1 ohm on each winding…is it okay?

  25. lboggie36 says:

    I have a Danby small refrigerator and the compressor ohms out to 0.00 on all terminal so is it bad I don’t get any other readings

    • What terminals are you testing? Internal windings? Windings to ground? What ohms scale are you using? Each terminal to ground should be checked with you highest scale (Rx100?). Between each terminal you should have 3_readings and those should be checked on your Rx1 scale.

  26. Tenny Thomas says:

    I have a single phase compressor reading C-S:1.9, C-R:1.8 and S-R:3.8. Also checked for ground fault and reading good. But when starting the compressor it draws high current and trips. I also checked the line voltage, gas charge,capacitor and condenser coil, everything is ok. Could you suggest any tips to identify the problem for the high current issue. The compressor was installed 1 week ago.
    Thanks in advance.

    • A weak run capacitor would be something to look at. Make sure it’s microfarads are within the allowable %.(+/-). You could have a “stiff” compressor. See what the manufacturer recommends for a start assist kit.

  27. John says:

    Hi I was working on a Goodman unit, condenser. When the contactor pulls in I lose a leg of power. Tried a different breaker, replaced wire from panel box, tried separating compressor and condenser fan, checked capacitors, fan was replaced and has separate run capacitor. Compressor does not ohm to ground, c-s = .06, c-r= .05, r-s= .09, I had to install a lug kit on this compressor about 3 years ago, it burnt the terminals off the wires. With the contactor pulled in I have 234 volt on breaker at panel box, 118 volt at each leg to ground at unit and disconnect, with contactor disconnected I have 234 volt at disconnect and contactor. Let me know what you think, Thank you

    • Why do you think you have lost a leg? You say you have 118 from each power wire to ground and 234 volts across the power wires with the contactor pulled in and with it disconnected. What voltage do you have at C and R at the compressor?

  28. I have a Westinghouse (Electrolux RS662V) fridge, with a starting after disconnection compressor problem, that is, it starts fine on its first run, then when the compressor stops and tries to start again after a while, it goes locked rotor (it buzz) and can’t start until the overload protector takes it off, and it stays in this cycle (locked rotor then disconnected by the overload protector in 5-7secs). It only starts again if I take the power off once it buzz, wait for 5mins or so, then power it on again, only then it starts fine until it disconnects again.

    Compressor performance (when running) on the fridge is as good as before this problem started, that is freezer and fridge are on the right temperatures.
    The Compressor is single phase, low starting torque one.
    The resistance readings between the 3 windings adds up as expected, 12ohm-21ohm-33ohm, and no shorts to ground.

    Now, the Capacitor installed, is a running capacitor, 5uf 400v, and it is connected to a PTC starter which in turn is connected to the S and R windings of the compressor, and C is the overload protector. I changed the PTC and the capacitor, but still facing the same problem.

    What could be the problem? Or how to get around it?

    • A PTCR can take anywhere from 5 to 10 minutes to reset. If it is not reset, the compressor will not start. See if you can get a different start assist kit like a Supco super boost

      • First thank you for your effort with all of us asking you.
        Back to the problem, the PTCR resets fine in around 2mins, nothing wrong with it. The problem is with compressor trying to start, and cannot, and as I mentioned I already changed the PTC, which works in place of the start capacitor and the relay in other systems.

        One thing that I just tried now after reading on some books, is to remove the extension power cord that I was using temporarily when I pulled the fridge out and far from the wall power outlet, and now the fridge power cord is connected directly to the wall outlet.
        When I did that an hour ago, the compressor fails to start on its first restarting attempt, and starts fine on its second attempt after the PTCR resets, without the need of cutting off the power manually as I had to do before.

        This implies that the extension power cord was not drawing the LRA needed to start the motor?
        But why now it needs 2 attempts to start instead of starting on the first try?

    • Yes, an undersized power cord was probably the problem creating a power drop. If all power is correct and all components are correctly matched the only other possibility I can think of is the PTCR is weak or damaged from the low voltage.

  29. alghzyli says:

    Hi, i have a compressor model “AH5524E” would like to make it as a vaccum pump but it wouldn’t work, checked the winding and got:C-S=1.6, C-R=1.4,S-R=2.8, is it shirted or still good? Thank very much.

    • Probably still good. When you say it does not work, what is it doing or not doing? Just saying it doesn’t work is not enough information to make an intelligent guess.

      • Ludwig says:

        Hi there i have a problem with my compressor i got a resistance reading from (s-r) only and rest of the terminal like (c-r) and (s-c) is totaly zero is my compressor is totally bad?

      • There are usually overloads in the c-s and c-r windings. If the compressor was HOT when you took you readings, way until it cools down and take them again. With the compressor cool, you should get readings. I no readings with a cold compressor, it is bad.

  30. Colin says:

    Hi, I’ve got a Trane XE 1200 with a Copeland CR47KQ-PFV-980WB compressor inside of it. What I’m seeing is the following:

    R-S – 0.46
    S-C – 0.46
    R-C – 0.13
    R/S/C-Gnd – Inf

    What does it look like happened? Thanks for the help!

    • C-S + C-R should = S-R. Since C-R is .13 and C-S is .46 S-R should equal .59. It does not. You have a problem (internal short ) with the compressor windings. Will probably need a new compressor.

      • Colin says:

        Bummer, I figured that was the case but thanks for the heads up. This might just mean a whole new unit for me unfortunately. Take care!

  31. Kakakhel says:

    What does R X 1 means ? How it can read the values for short winding test?

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s