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Air conditioner or heat pump compressor burn-out diagnosis & repair:
This air conditioning repair article discusses the
how to diagnose and replace a burned out air conditioner compressor, including
evaluation of air conditioner compressor noises, hard starting, lost cooling capacity, and detection of a burned out
compressor or A/C compressors at or near end of their life.
Here we catalog and describe the causes of air conditioner or heat pump compressor failures and we suggest compressor motor diagnostic steps.
We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.
Diagnose a Burned-Out Air Conditioning Compressor
When is a compressor at or near end of its life? Criteria for deciding that an air conditioner, heat pump, or refrigeration compressor needs to be replaced.
Hard starting air conditioner compressor. Increased air conditioning electrical bills. Air conditioner compressor defect diagnosis & repair guide.
Air conditioning condenser unit fan diagnosis/repair.
When to replace a failing or failed A/C or heat pump compressor. Common causes of A/C or heat pump compressor motor failure
When a sealed compressor motor has "burned out" this means that the internal wiring of the motor has become
irreparably damaged: the compressor motor windings may be burned and shorted together or shorted to the steel shell
of the motor, or the windings may have burned and simply become "open" or disconnected. If the motor has burned
out in either of these ways it needs to be replaced.
When an air conditioner / heat pump compressor can no longer develop proper pressures, even if its electric motor has not burned out, the unit will still need to be replaced. Below we give refrigerant pressure details that help make this decision.
Basic Diagnostic Clues Indicating a Failed A/C or Heat Pump Compressor Motor
Air conditioner failure warning signs: Before an air conditioning compressor fails solid you may notice that the unit is hard-starting, particularly when starting against a head pressure (someone switched the system off while the compressor was running, then tried turning it right back on).
The compressor won't start, perhaps just hums, and the motor star/run capacitor(s) are ok or you've tried replacing that part
As soon as the compressor tries to start the circuit breaker trips or fuse blows. The compressor motor has burned out, is internally shorted or gounded. A service technician will use a VOM and typically will find low resistance (low Ohms) between one or more compressor terminals and ground.
A VOM connected to the compressor circuit indicates that no current is being drawn, but the condenser fan is running properly.
Watch out: before assuming that the compressor motor is bad, when the unit won't start at all, check for a loose or disconnected electrical connection or a bad contactor relay. An overheated motor may also leave the compressor motor "off" due to a tripped internal overload switch or a switch that has simply failed.
If this is the problem, the motor should start normally once the unit has cooled down. (Some readers describe spraying water on the condenser unit to speed cooling - (don't wet electrical switches & components.)
Basic Electrical Tests on Air Conditioner or Heat Pump Compressor Motors
When an air conditioning compressor has "burned out" by shorting of internal components - it will fail to start at all. This failure is detected
by disconnecting all power and wiring from the unit and measuring resistance (ohms) between the motor start/common and run/common terminals.
Meaning of Zero resistance: If there is zero resistance there is (probably) no break in the wire or circuit being tested. Zero resistance also can mean that wires are shorted together or are shorted to ground.
Watch out: while finding very low or zero resistance would be normal for a simple length of electrical wire, for some circuits and equipment, finding low or no resistance where there should be some can indicate a short circuit. Another problem to watch for is testing electric motor windings; a winding may appear intact, with low resistance, but it may open (showing high or infinite resistance) only when the motor begins to spin.
Meaning of Infinite resistance: If you measure the resistance across a compressor winding and your meter's needle is stuck over at infinity,
or "OL"/"OVER" on a digital meter, that would indicate the compressor winding is open (burned through).
The same effect can be observed from simply connecting the meter to absolutely nothing.
Typically if just one widing is "open" you'll see infinite resistance at one check point and in contrast (for the non-open windings) you will read zero resistance ("continuity") between the Common terminal and the Start or between the Common terminal and Run terminal.
Low resistance: If the resistance measured across the air conditioning compressor winding is too close to 0 ohms, it's shorted.
The compressor should blow the fuse or trip the circuit breaker when power is turned back on.
But watch out: we get field reports of equipment burn ups and even fires when the air conditioning circuit breaker for the
compressor happens to be an old FPE Stab-Lok or Zinsco unit.
If there is
resistance but not infinite resistance between the motor terminal and the motor casing, the motor has become shorted to ground internally and the unit needs to be replaced.
If there is no resistance between the start and run terminals to common, but there is resistance between the start and run terminals, this means
that the internal motor overload protection circuit is open. In this last case, allow the motor to cool and re-test it before replacing it.
Question: very low ohms readings at air conditioner compressor?
2016/09/10 johnteh said:
I had the summer A/C inspection done was told the compressor capacitor was indicating a short life because the compressor mega ohms was 200 which is very low for a unit that is only 2 years old. recommendation was to replace compressor capacitor with a hard start to prolong compressor life.
Are compressor capacitors difficult to replace/install? They don't seem to be, since they have only 2 wires.
But I'm confused about the compressor diagnosis. Very low resistance across a compressor motor is not a diagnostic I understand. High current draw (measured in amps not ohms) is a well-known indicator that a motor is struggling and may indicate failing or seizing bearings and thus end of life.
Ron Walker, over at hvactrainingsolutions.net gives this nice summary of AC Compressor troubleshooting by measuring resistance across air conditioner or heat pump motor windings from which we excerpt and expand.
Mr. Walker notes that these measurements are difficult to obtain as the compressor motor wiring connector can be itself hard to access and he suggests obtaining an old motor wiring harness and connector plug to extend the wires out to where you can reach them - a great idea.
Ohms measurements across the windings terminals of a good electric motor such as is used in an air conditioner or heat pump system will read something like this:
Pin 1 to Pin 2 – 3 ohms
Pin 2 to Pin 3 – 6 ohms
Pin 1 to Pin 3 – 9 ohms
The lowest electrical resistance reading should be below 10 ohms (this is the [electric motor] run winding).
The second highest reading should be about 2-4 times higher than the lowest reading (this is the [electric motor] start winding).
The highest reading should be the sum of the resistance read across the two smaller [electric motor] windings
Mr. Walker adds that:
If any of these readings read infinite or over range on your ohm meter, you have an open winding. A reading of zero ohms indicates a short in a winding.
Very high or infinite resistance across the windings of a motor would typically indicate that a winding wire is broken. What's tricky about this motor widing resistance measurement in ohms is that a winding wire may be damaged but may "open" or "break" only when the motor is trying to spin or only when the motor gets up to full speed.
So you can see that NONE of these air conditioner or heat pump motor ohms test measurements is even close to 200 Ohms and that to me, 200 ohms sounds high, not "very low".
Before trusting such a measurement I'd be checking my ohm-meter, probes, quality of contact, and other measurement conditions including of course that power is OFF and disconnected from the motor.
A different air conditioner / heat pump compressor (or fan) motor test that involves measuring resistance is a check for shorted motor windings. Measuring between each motor pin and ground (typically the compressor motor case) should give "infinite" resistance, meaning no current is flowing. If you see some lower resistance with the -to-ground measurement then there's an internal short.
Before changing the start/run capacitor - which may be fixing the "wrong problem"
- I suggest that you give the service manager at your AC repair company a call and that you ask for clarification and if needed, ask for a visit from a senior, experienced HVAC technician.
Either there has been a misunderstanding or what your tech told you needs some further explanation vis-a-vis what other experts say about AC troubleshooting by measuring ohms across motor windings.
Walker, Ron, "Troubleshooting Air Conditioning Compressor Motors Like A Pro", [Web article],
HVAC Training Solutions, LLC
4475 U.S. 1 South #607
St. Augustine, FL 32086
retrieved 2016/09/10, original source http://www.hvactrainingsolutions.net/troubleshooting-air-conditioning-compressor-motors-like-pro/
Other (Non-Electrial) Refrigeration Compressor Failure Modes
Watch out: for a mechanically frozen compressor: a compressor may pass all of these electrical tests and still require replacement. The tests above only test electrical connections and windings. An air conditioner or heat pump or refrigeration compressor that has jammed up mechanically internally will still refuse to start (perhaps will hum) when all of the electrical tests, contactor relay, start capacitor, etc. are tested as perfecliy fine.
A compressor with broken internal parts may also not be frozen, that is its internal electrical motor may start and run, but the compressor fails to produce any refrigerant pressure at its outlet side. In this case internal parts or valves in the unit have broken without jamming the motor itself. In this case, all of the electrical components and tests will look "OK".
A compressor with broken internal motor mounts may make a rattling or clanking sound and needs replacement. It will continue to run but could become shrapnel at any time.
A compressor with bad internal valves will continue to run but is inefficient and should be replaced. The symptom is very quick equalization of high and low side pressure as soon as the motor stops.
When to replace a sealed-unit air conditioning or heat pump compressor?
Burned up electric motor in the HVAC compressor
Some compressor motor failures are so apparent that there's no question: an electric motor burnout that draws high amps or is internally shorted, for example.
Abnormal HVAC compressor pressures
But what about a compressor motor that may be just "worn" ? An HVAC compressor should be able to pull at least 15" of vacuum against 100 psi head pressure or else it is inefficient. Of course to make this test you must be able to isolate the compressor from the rest of the cooling or heat pump system, so this test is not rapidly made in the field.
Bad HVAC compressor refrigerant valves
Bad air conditioner reed valves will be unable to pull pressure down on the low side of the system. A leaky discharge reed valve (on the compressor output side) pulls hot gases back into the compressor cylinder and recompresses them, causing abnormally high head pressures at the compressor motor. And as a result the compressor won't be able to move vapor.
In sum, HVAC compressors do fail and need replacement, but only when you have tested and ruled out the other 80% of the causes of common air conditioning, heat pump, or refrigeration problems (usually electrical in nature) do you go ahead and replace the compressor unit.
General advice: Electrical Tests to Check HVAC Blower Fan Motor or Outdoor Compressor Fan Motor Winding on Heating or Cooling Equipment or on Other Electrical Motors
Air Conditioner Compressor/Condenser Fan Not Working?
Details of compressor/condenser unit fan inspection, diagnosis, and repair are
A failed compressor/condenser fan can cause the air conditioning system to shut down due to an overheating compressor or excessive pressures developed inside the compressor. If your compressor/condenser unit does not include a safety override switch to perform this shutdown and if the condenser unit fan is not working, your compressor motor may be permanently damaged.
at NOISES, COMPRESSOR CONDENSER we include some condenser fan noise problems are traced to the cooling fan motor, bearings, fan blades, obstructions, etc.
Watch out: if your compressor/condenser unit motor is running but the fan itself is not blowing air, the system will not work and may be seriously damaged. The outdoor cooling fan or condenser unit fan is needed not only to cool high pressure, high temperature refrigerant in gas form so that it can condense back to liquid form.
That cooling step also cools down the compressor itself and keeps the compressor internal pressure from becoming too high. Most modern compressor / condenser units include an overpressure sensor that will shut the equipment down if compressor pressures become too high. Some older models and some modern economy air conditioner compressors may lack this function.
Example: testing a blower fan motor winding: referring to the electrical diagram for your equipment, unplug electrical connectors at the fan motor. Measure the resistance between each lead wire with a multimeter or VOM. The multimeter should be set in the X1 range. For accuracy, don't measure when the fan motor is hot, allow it to cool off.
When the resistance between each lead wire are those listed in the specifications for your equipment the fan motor should be normal. Zero resistance or infinite resistance are indicators of a problem. More examples of checking wiring:
see BURNED-OUT COMPRESSOR.
Common Causes of Burned Out Air Conditioner Compressors
Contamination of refrigerant, piping, or devices in the refrigeration system: contaminants, including air, moisture carried in by air, and dirt can enter the refrigeration system as contaminants due to a leak on the suction side or improper service procedures such as mis-handling of service port valves or opening the system for repairs without adequately drawing a vacuum and without proper use of filter/dryer canisters.
Air in the system can also lead to refrigerant flooding discussed above.
Cooling problems - compressor motor overheats: a compressor motor that keeps on running when the compressor/condenser fan has failed or when air flow through the condensing coil has become blocked by debris or damage can cause abnormally high compressor head pressures and operating temperatures.
If the condenser fan is running but airflow is impaired the system may continue to "run" but at reduced cooling capacity and shortened compressor life. We found a stack of nine A/C compressors improperly installed without adequate cooling air flow at a wealthy client's home in Pawling, NY.
Compressor motors were being replaced every year or two because the owner installed a stockade fence to "hide the ugly equipment".
A compressor may also run too hot due to too-high temperatures at the suction line and low-side of the compressor system. Under-charged refrigerant levels or lack of insulation on the suction line piping can cause these problems as might a TEV that is not quite flooding the system but is releasing refrigerant too fast through the cooling coil. 
Refrigerant Pressure problems - excessive high side pressure may be caused by a restriction or blockage such as in the condensing coil, refrigerant metering device, or even at the cooling coil. Excessive high side pressure causes hot compressor operation, lost cooling capacity, and ultimately damage.
Refrigerant flooding - sending liquid refrigerant into the compressor motor is a quick way to destroy its moving parts or valves.
We discuss refrigerant flooding and six common causes of liquid refrigerant slugging the compressor
Aside from TEV (TXV in some literature) problems, a mistake like overcharging the system can lead to liquid refrigerant can end up in the compressor bottom where only refrigerant gas is expected - causing the same failure problems. Christopherson, 
Refrigeration oil lubricant - lost due to system leaks (most likely you'll know there has been a history of refrigerant leakage too); also refrigeration oil can travel in the refrigerant lines where it reaches a capillary tube or TEV , clogging it and causing abnormal system pressures that can damage the compressor.
On commercial refrigeration systems that us a separate oil pump to deliver lubricant to the compressor motor, an oil pump failure also leads to compressor motor failure.
Electrical problems that can damage A/C or heat pump motors are cited by Christopherson, including improper voltage (hooking up to an incorrect voltage level supply (110V to 220V or 230V applied to a 208V motor), poor power quality delivered to the system such as at homes subject to significant fluctuations in actual voltage levels in the incoming mains, loss of voltage on one phase of a three-phase electrical hookup (more common on commercial than residential refrigeration equipment), and finally, unbalanced current across the individual phases of a three-phase electrical hookup. 
Worn out compressor internal parts - a compressor motor may fail due to mechanical wear, though in our OPINION and having seen some refrigeration compressor motors that ran for decades with no trouble, we think mechanical wear is likely to be traced to a refrigerant, lubrication, contamination, or perhaps mounting problem. See our contamination comments above.
The text above describing causes of refrigeration compressor damage is indebted to HVACR experts Norm Christopherson  and Joe Marchese , as well as Warren Hilliard .
Thanks to reader J.W. who corrected a typing error about zero resistance 2/11/2014
Thanks to Scott at SJM Inspect for suggesting this EPA document and for technical editing remarks regarding our air conditioning website,
SJM Inspection Service LLC, serves the entire state of CT, sjminspect.com 203-543-0447 or 203-877-4774
Thanks to Diaz, Domingo I. CIV NAVAIR Bldg.2118, rm. 131: firstname.lastname@example.org
- Ming Diaz, Great Falls, MD for editing help with the text about discharging air conditioning compressor capacitors - 3/07 DF]
Mark Cramer Inspection Services Mark Cramer, Tampa Florida, Mr. Cramer is a past president of ASHI, the American Society of Home Inspectors and is a Florida home inspector and home inspection educator. Mr. Cramer serves on the ASHI Home Inspection Standards. Contact Mark Cramer at: 727-595-4211 mark@BestTampaInspector.com
John Cranor is an ASHI member and a home inspector (The House Whisperer) is located in Glen Allen, VA 23060. He is also a contributor to InspectApedia.com in several technical areas such as plumbing and appliances (dryer vents). Contact Mr. Cranor at 804-747-7747 or by Email: email@example.com
Thanks to Jon Bolton, an ASHI, FABI, and otherwise certified Florida home inspector who provided photos of failing Goodman gray flex duct in a hot attic.
Air Diffusion Council, 1901 N. Roselle Road, Suite 800, Schaumburg, Illinois 60195, Tel: (847) 706-6750, Fax: (847) 706-6751 - firstname.lastname@example.org - www.flexibleduct.org/ - "The ADC has produced the 4th Edition of the Flexible Duct Performance & Installation Standards (a 28-page manual) for use and reference by designers, architects, engineers, contractors, installers and users for evaluating, selecting, specifying and properly installing flexible duct in heating and air conditioning systems.
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The printed manual is available in English only. Downloadable PDF is available in English and Spanish.
Owens Corning Duct Solutions - www.owenscorning.com/ductsolutions/ - provides current HVAC ductwork and duct insulating product descriptions and a dealer locator. Owens Corning Insulating Systems, LLC, One Owens Corning Parkway, Toledo, OH 43659 1-800-GET-PINK™
"Flexible Duct Media Fiberglas™ Insulation, Product Data Sheet", Owens Corning - see owenscorning.com/quietzone/pdfs/QZFlexible_DataSheet.pdf "Owens Corning Flexible Duct Media Insulation is a lightweight, flexible, resilient thermal and acoustical insulation made of
inorganic glass fibers bonded with a thermosetting resin."
 "Air Conditioning & Refrigeration I & II", BOCES Education, Warren Hilliard (instructor), Poughkeepsie, New York, May - July 1982, [classroom notes from air conditioning and refrigeration maintenance and repair course attended by the website author].
Warren Hilliard was an HVACR instructor as well as operator of an HVACR repair company in Dutchess County, New York.
 Christopherson, Norm, "Preventing Premature Compressor Failures, An Ounce of Prevention for Years of Service", web search & reader TN Goose referral, 08/12/2011, original source: http://www.hvacfun.com/a-once-prevent-years-service-comps.htm
Quoting: Norm Christopherson, CM, is a long-time hvacr instructor at San Jose City College, San Jose, CA
 Determining Compressor Amperage Ratings, Joe Marchese, Air Conditioning, Heating, Refrigeration News, 31 July 2003, web search 08/12/11, Joe Marchese is owner of Coldtronics of Pittsburgh. He can be reached at 412-734-4433, www.coldtronics.com, or email@example.com. Original source: http://www.achrnews.com/Articles/Feature_Article
/c229b8960ac5a010VgnVCM100000f932a8c0____ - quoting:
Books Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
"Air Conditioning & Refrigeration I & II", BOCES Education, Warren Hilliard (instructor), Poughkeepsie, New York, May - July 1982, [classroom notes from air conditioning and refrigeration maintenance and repair course attended by the website author]
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: firstname.lastname@example.org. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides extensive home inspection education and report writing material.
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