Compressor Electrical Diagnostics — Revisited

In reviewing posts from time to time that have the most views i feel it is a good thing to bring back a post that was previously written.  The following was originally written back in 2011 but there have been a lot of views so here it is again.  Hopefully, this will remind you of some basic checks before we really get into the air conditioning season. Keep in mind that this is true of all electric motors and compressors.

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
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