Scroll Compressor Functional Checks

As the cooling season begins to wind down, I’ll have a few more posts on air conditioning service and diagnostics before we get into the heating season and discuss topics relating to heating  and service.

This post will address how to properly diagnose scroll compressors. (Information provided in this post is directly from Copeland )

So, continuing on with compressor diagnostics, scroll compressors need to have the following functional tests performed to determine if a compressor is bad. 

A functional compressor test with the suction service valve closed to check how low the compressor will pull suction pressure is not a good indication of how well a compressor is performing. Such a test will  damage a scroll compressor.  Also keep in mind, that you CANNOT pump down a unit with microchannel coils. The holding capacity of the coil will not allow pump down of refrigerant into the coil.  this can also lead to damaging the compressor.

 The following diagnostic procedure should be used to evaluate whether a Copeland Scroll compressor is working properly:

1. Proper voltage to the unit should be verified.
2. The normal checks of motor winding continuity and short to ground should be made to determine if an internal motor short or ground fault has developed. If the protector has opened, the compressor must be allowed to cool sufficiently to allow it to reset.
3. Proper indoor and outdoor blower/fan operation should be verified.
4. With service gauges connected to suction and discharge pressure fittings, turn on the compressor. If the suction pressure falls below normal levels, the system is either low on charge or there is a flow blockage in the system.
5. If suction pressure does not drop and discharge pressure does not rise to normal levels, on 3-phase scroll compressors, reverse any two of the compressor power leads and reapply power to make sure compressor was not wired to run in reverse direction. If pressures still do not move to normal values, either the reversing valve (if so equipped) or the compressor is faulty. Reconnect the compressor leads as originally configured and use normal diagnostic procedures to check operation of the reversing valve.
6. To test if the compressor is pumping properly, the compressor current draw must be compared to published compressor performance curves using the operating pressures and voltage of the system. If the average measured current deviates more than ±15% from published values, a faulty compressor may be indicated. A current imbalance exceeding 15% of the average on the three phases may indicate a voltage imbalance and should be investigated further.
7. Before replacing or returning a compressor: Be certain that the compressor is actually defective.

And Finally, if you do find a “bad” compressor, be sure to determine the cause of the original failure and correct it.  Make sure all wiring is correct and connections are “tight”. Make sure the run (and start gear) capacitors are the correct size and the µf rating is within the tolerance rating on the capacitor and the voltage rating of the capacitor matches the new compressor (on single phase units). Verify correct air flow both across the evaporator and the condenser coil. Just replacing a compressor without correcting what caused the original compressor to fail will only lead to another compressor failure in the future.

Believe it or not, more than one-third of compressors returned to Copeland for warranty analysis are determined to have nothing found wrong. They were misdiagnosed in the field as being defective. Replacing working compressors unnecessarily costs everyone. 



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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13 Responses to Scroll Compressor Functional Checks

  1. Kent Gregory says:

    In response to Point#5:
    The compressor may not be faulty if you find it running and not pumping. See the Copeland Application Publication below:

    ZP16 to ZP44K3E and ZP14 to ZP61K5E R-410A
    1.5 to 5 Ton Copeland Scroll® Compressors
    The Therm-O-Disc® or TOD is a temperaturesensitive
    snap disc device located between the high
    and low pressure side of the scroll or on the muffl er
    plate. It is designed to open and route excessively hot
    discharge gas back to the motor protector when the
    internal discharge gas exceeds 290°F (144°C). During
    a situation such as loss of charge, the compressor will
    be protected for some time while it trips the
    protector. However, as refrigerant leaks out, the mass
    fl ow and the amperage draw are reduced and the
    scrolls will start to overheat. A low pressure control
    is recommended for loss of charge protection for the
    highest level of system protection. The low pressure
    cut-out can provide protection against indoor blower
    failure in cooling, outdoor fan failure in heating, closed
    liquid or suction line service valves, or a blocked
    liquid line screen, fi lter, orifi ce, or TXV. All of these
    can starve the compressor of refrigerant and result in
    compressor failure. The low pressure cut-out should
    have a manual reset feature for the highest level of
    system protection. If a compressor is allowed to cycle
    after a fault is detected, there is a high probability
    that the compressor will be damaged and the system
    contaminated with debris from the failed compressor
    and decomposed oil. If current monitoring of the
    compressor is available, the system controller can
    take advantage of the compressor TOD and internal
    protector operation. The controller can lock out the
    compressor if current draw is not coincident with the
    contactor energizing, implying that the compressor
    has shut off on its internal protector. This will prevent
    unnecessary compressor cycling on a fault condition
    until corrective action can be taken.

    There has been a failure on the part of most if not all manufacturers to list this in their equipment manuals. I have personally seen scroll compressors in the field run for 20 minutes before this feature trips the overloads in the compressor. I believe that are many compressors changed under warranty that actually had a system problem that caused this running characteristic, like a faulty reversing valve, and not an actual problem in the compressor. I believe that this is why Copeland has stated that they have a 34% warranty return rate on scrolls with N.F.F. “no fault found”.

  2. Kent Gregory says:

    I read it, very good post..I wish Copeland would tag the 1-5 ton compressors. The 6.5 ton and larger are the only ones with tagging on the body of the compressor.

  3. Bill Dawson says:

    On several compressers I,me getting a fluctuating reading from run to start ? why do I get these readings, the compressers do run and work well, but diagnosing tough when I can,t get a good ohms reading ? part two I have had 2 instences now that I a small ohms reading from a terminal to case ground ? nothing that trips the breaker but still a continuity ?

    • Sounds to me like you have “moisture” in the systems. I would suggest recovering the charge, replace the liquid line drier, and evacuate to 500 microns. Then do an ohms reading to ground — no refrigerant. If you still get a reading, compressor may be on its way out. If readings are good. Check your refrigerant pressure in your recovery tank. At a given temperature, the refrigerant should be at a specific pressure. If it is higher — refrigerant has non-condensibles in it (moisture) put virgin refrigerant back into the system.

  4. Mahendra says:

    Thanks. sir point no 6 further I would like to know how much should be the perfect pumping for a healthy compressor and what is the right method to check.

    • To test if the compressor is pumping properly, the compressor current draw must be compared to published compressor performance curves using the operating pressures and voltage of the system. If the average measured current deviates more than ±15% from published values, a faulty compressor may be indicated. A current imbalance exceeding 15% of the average on the three phases may indicate a voltage imbalance and should be investigated further
      Use your AMP Meter and check current draw. Also check for correct voltage (see post on voltage balance)

  5. daniel says:

    went on service call goodman 3 ton r22 with copeland compr high side not pumping but 105 and low side would increase from 70to 80 psi then could hear a hissing or unloading of pressure after and back down to 70 psi, had amp clamp on line volt wire and read 14 then-16 amps, tested 2 start ups same result, compressor running and could feel high and low side lines getting cool not cold and high side definitely not warm or hot as if all pressure was bleeding back into system thru compresssor, but it happens or unloads after just a few seconds like maybe 10 seconds, is this enough tme to build up to 300 degrees farenheight approximately to trip the thermal disc and unload?
    or is it running backwards maybe, on residential, how to tell if wiring is backwards, just a polarity check? its single phase in usa residential, wouldnt it be obvious and compr makes unusual noise, and pulls into vaccum or tries to although i dont think scrolls can like hermetics,
    i found out later that customer had changed compressor week before and wanted a diagnostic as to what was wrong, i asked customer if he had ran the system and he said it was working fine, is this possible with a scroll?

    • Is the compressor getting HOT? This would be an indication of the internal pressure relief opening or the thermal disc opening. In both cases, if they open, they dump discharge gas back into the shell of the compressor.
      You mention amp draw but did not say what the name plate amps should be. Compareyour reading to the name plate and proceed accordingly. Was the capacitor(s) replaced when the compressor was replaced? Is there a TXV on the indoor coil that may be stuck closed? Is there a start assist kit on the unit.
      Check all of these.

  6. joseph says:

    my compresser model no.ZR94KC-tfd-522.compresser 5min running but over heat &compressor trip.olease why check for comprsser

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