Table of Contents
- the difficulties in troubleshooting a hermetically sealed compressor
- differences in various compressor manufacturers and HVAC manufacturers
- the commonality of single-phase compressors
- basics of troubleshooting an air conditioner or heat pump compressor
- Compressor capacitors and compressor mechanics
- basic tips for compressor repair
- Air Conditioner Compressor Repair - Heat Pumps are included
- lots of resource and related links that allow you to go in-depth and learn about HVAC compressors
Air Conditioner Compressor Troubleshooting
What Causes the Air Conditioner Compressor to Stop Working?
Trane Heat Pump Condenser
Air Conditioner Compressor Troubleshooting can be challenging. The average compressor in many residential condensing units is hermetically sealed compressors. Furthermore, the only visible working parts on the compressor are the electrical connections and the copper discharge and suction lines. That makes it difficult to repair any compressor once a diagnosis is made to the air conditioner or heat compressor.
Troubleshooting air conditioning and heat pump compressors are not complicated once you understand the basics of the compressor. It doesn’t matter what kind of residential air conditioner or heat pump compressor you are working on. As long as it is single-phase for use in a residential heat pump or air conditioner. Furthermore, this article will cover the basics of troubleshooting a compressor when the circuit breaker trips continuously.
A compressor can stop working for two reasons:
- A mechanical problem
- Or an electrical problem
In either case, we will cover both types of problems that cause compressors to stop working. Additionally, how to figure out which problem you have with the compressor. That does require basic knowledge of refrigeration and electrical circuits and components. Finally, you will also need some tools for the job.
Air Conditioner Compressor Troubleshooting - Different Air Conditioner or Heat Pump Compressors
Scroll Compressor Inside Heat Pump Condenser
Many residential heat pump or air conditioner compressors are black. A few are red, depending on the manufacturer of the compressor. Trane is the only OEM in the USA that manufacturers their compressors. That falls to American Standard because the same company owns Trane and American Standard. In any regard, the color does not matter as all the components inside the compressor are the same. That is, a scroll compressor is a scroll compressor.
There are new scroll compressors out that can unload. Other electronic scroll and rotary compressors can fully modulate. This article does not address these newer compressors. Copeland initially owned scroll compressors and the scroll technology. Emerson Electric owns Copeland. There are also inverter duty compressors in some split system air conditioners and heat pumps. Nordyne et al. manufacture those compressors. And are different than the compressors we are talking about here. These newer systems require a different compressor troubleshooting analysis and technique than described here. The newer modulating systems are now in many different brands of HVAC equipment. Lastly, they use rotary compressors.
So now that we know this and that it doesn’t matter what color the compressors are or who manufactured the compressor, that every residential air conditioner or heat compressor is the same except the air conditioner or heat pump compressors mentioned above.
We have many other related articles. Use the search feature to the right to help you find other related articles.
Air Conditioner Compressor Troubleshooting – Compressor Basics
The Compressor Electrical Basics
Compressor Start & Run Windings Whether it is a scroll compressor or a reciprocating compressor (of which scrolls and reciprocating compressors make up the bulk of residential air conditioner or heat pump compressors in use today) there are two major components to the compressor. Each compressor has an electric motor and an assembly of mechanical components which are driven by the electric motor.
A compressor electric motor has two separate windings inside for the average residential air conditioner or heat pump single-phase compressors. The windings inside the compressor are the run winding and the start winding. When troubleshooting an air conditioner or heat pump compressor one should see three terminals on the outside of the compressor.
These three electrical terminals are called or should be labeled in some manner. – Common Terminal or C, Start Terminal or S, and the Run Terminal or R. Somewhere near these terminals, there should be some labels……….sometimes they say CSR and CSR stand for Common – Start – Run. Those are the compressor basics. Lastly, the other item in the electrical circuit for the compressor that you can troubleshoot with the compressor is the run capacitor.
Start Capacitors | How Do I Troubleshoot AC Compressor Capacitors?
There are some compressors that also have start capacitors. The capacitors are external to the compressor. Always check the OEM wiring diagram for correct wiring. Start capacitors will have a relay in the circuit. The start capacitor will only be in the start circuit for a split second. Then the relay, usually a potential relay and sometimes a current relay, will open the relay contacts. Lastly, this takes the start capacitor from the start circuit.
The run capacitor remains in the circuit for the duration of the compressor run time. For electrical compressor troubleshooting, you will need:
- an Ohm Meter
- basic knowledge of electrical safety
- and to know the information above. One word of caution! Capacitors hold a charge of electricity. You can be seriously injured or worse by coming into contact with a charged capacitor. Discharge the capacitor using a properly rated resistor.
When troubleshooting the electrical system of a compressor three different problems can occur with the windings of the compressor.
Related Link: Learn more about start capacitors for HVAC compressors (opens in a new window)
Air Conditioner Compressor Troubleshooting - Compressor Troubleshooting – Ohming the Windings
What are the Symptoms of a Bad AC Compressor?
None of these tests require any knowledge of Ohms Law.
1. Shorted Windings – It occurs when the windings touch one another. The problem usually occurs because the insulation on each winding has dissolved, allowing the copper windings to touch each other. Your Ohm Meter will likely read zero in this instance. Furthermore, the compressor is bad if you get this reading.
2. Open Winding – This occurs when a winding is open or broken. Your Ohm Meter will read infinity. Furthermore, if you get this reading the compressor is bad.
3. Grounded Winding – Grounded windings are nearly the same as an open winding except the winding is touching the casing of the compressor. Your Ohm Meter reading will be zero from the terminal to the casing of the compressor. Touch one lead of the meter to the copper pipe going into the compressor (discharge or suction). And then to one of the CSR terminals.
If any of those conditions exist, the circuit breaker for the condensing unit will trip when the compressor tries to start. When you perform this test, and everything seems normal, and the circuit breaker still trips, you likely have a mechanical issue. Furthermore, we will address this next.
Air Conditioner Compressor Troubleshooting - The Math
What are normal readings on the Ohm Meter, you ask? A good example of normal reading would be the resistance of say 7 Ohms on the start winding and 4 Ohms on the run winding. Your compressor may show a different measurement of Ohms, but as long as you get resistance, it is a good sign. If you measure between the run winding and the start winding, you should get the sum of the other two readings. Example – C -S = 7 Ohms. C – R = 4 Ohms. S – R = 11 Ohms.
Furthermore, the compressor capacitors must be functioning properly and are not bad. To properly check a capacitor to make sure it is good, you need a special meter that reads microfarads. Some multi-meters can read microfarads. Lastly, the reading from the meter should be plus or minus 10 percent of the rating on the capacitor.
Related Link: Learn more about troubleshooting compressor overheating (opens in a new window)
Air Conditioner Compressor Troubleshooting - The Compressor Mechanical Basics
Of the several types of compressors out on the market and that are in the average residential air conditioner and heat pump condenser, we will only focus on the two types of basic compressors. These compressors currently make up approximately 95% of compressors installed in the average residential air conditioner or heat pump.
Those two types of compressors are the scroll compressor and the reciprocating compressor. The scroll compressor has outpaced the reciprocating compressor in the volume of installations. This is in the average residential air conditioner or heat pump condenser or package unit.
To properly troubleshoot any compressor mechanically whether it is a reciprocating or a scroll you need to eliminate any electrical problems with the compressor. There is not much one can do if the compressor is mechanically or electrically broke except to replace the compressor. However, we must determine if there is a major un-correctable problem with the compressor.
Air Conditioner Compressor Troubleshooting | Additional Test
One other test with the mechanical system will be an oil test. Required for this function is a special kit. With the oil test kit, take a sample of the compressor oil. Then it is sent to a lab to determine that the oil contains all-natural properties required of refrigerant oil. Furthermore, you can purchase the compressor oil test kit at an HVACR supply store.
The oil test will only be necessary if you find the windings have shorted. That indicates the compressor is a burnout. If the compressor burned out, it will need a replacement compressor. When you are replacing a burnout, then the old oil will have to be flushed. Furthermore, flush the entire refrigeration circuit before putting the new compressor into service.
Air Conditioner Compressor Troubleshooting | The Mechanical Components
After you have checked out all the electrical components and eliminated any problems with the electrical circuit and you continue to trip the circuit breaker for the condensing unit the next step is to try to eliminate any mechanical seizure of the compressor mechanical components. Then, after I have checked the compressor electrical system and eliminated any problems with that I get an age-old tool that every man loves. Lastly, the hammer. This is true for reciprocating compressors but I cannot say I have found this works for any scroll compressors.
If you have used this repair technique on a scroll and it worked for you please leave a comment below. We want to hear from you. I take a good hammer and hit the compressor a few good times and then restore power. Approximately fifty percent of the time the compressor comes to life and lives out a normal life. If that happens I always make sure there is a time delay in the compressor circuit. Furthermore, mechanical lock-ups are usually the result of short cycling.
- Run & Start Capacitors for HVAC Motors
- Start Capacitors for HVAC Compressors
- Troubleshooting HVAC Capacitors
- Air Handler Blower Motors
- Condenser Fan Motors
- HVAC Electric Motor Basics
- Air Conditioner Maintenance and Repair - HVAC Motors
Air Conditioner Compressor Troubleshooting - Resources
Air Conditioner Problems - Questions from readers:
AC Compressor hums but won't start?
Humming Noise
This sounds like a mechanical issue but could be an electrical issue. Follow the troubleshooting steps above to figure out what the compressor problem is and repair it. Many times, when you hear a loud humming noise coming from a condenser the noise is created by the compressor contactor. If it is too annoying, replace it.
AC condenser fan is working but the compressor is not running. What to do?
This could be one of many problems such as:
- A bad start or run capacitor
- A loose wire
- Poor wire connections
- A mechanical lockup
- A bad compressor that has shorted windings (burnout)
- Check for a high head pressure switch. If equipped, reset the switch if reset is available. Then fix the high head pressure problem.
You know the condenser has power because the fan is running. Make sure to turn the power off before troubleshooting the compressor.
How do I reset my AC Compressor?
Reset?
The only possible reset available that would stop a compressor from working is a high head refrigeration pressure switch or a low-pressure refrigeration switch. Typically, high head pressure switches are resettable. Low-pressure switches are not typically resettable. High head pressure is typically caused by poor airflow through the condenser. However, there are other reasons that can cause high head pressure. For low-pressure, that too can be caused by low airflow but not at the condenser. It is the result of low airflow across the evaporator coil. Maybe a dirty clogged filter? A bad blower motor? Or another problem such as a low refrigerant problem because of a refrigerant leak?
How do I know if my AC compressor is working?
- First, make sure you have clean air filters.
- Take the temperature of the air near the filter.
- Take the temperature of the air at a supply vent.
The temperature difference you should get is called the Delta T or ΔT. Depending on system design, it should be between 12° F and 20° F.
E.G., the air at the filter is 75° F and the air at the supply vent (any supply vent) is 60° F. That is a ΔT of 15° F. That means the air conditioning system is removing 15° F of heat from the air. It also means your compressor is working fine. On a sweltering day, a system of low capacity may struggle, and you will observe less of a ΔT. Anything less than 10° F, and you likely have a problem. The problem may or may not be with the compressor.
Air Conditioner Compressor Troubleshooting
Air Conditioner Compressor Troubleshooting
Technical Resource: Refrigeration and Air Conditioning Technology
When the compressor is shut down by the outside disconnect how long does it take for the hi and low gas pressures to equalize or be the same?
Typically a few minutes but give at least 5
i put a brand new ome compressor in my unit,i do this for a living,started it up no pressure change what so ever not even one pound.Think i got a bad compressor??????
Maybe, but also check the metering device.
Although the thermostat is in off position the compressor continues to run, and only shuts down when the circuit breaker is shut off??
I hope this helps you – Air Conditioner won’t Shut Off
I appreciate that you explained how to determine what part of an air conditioner is broken. I need help repairing the AC in my living room. Perhaps it would be best for me to find a repair expert for help.
Thanks for explaining how to identify a broken air conditioner compressor. My wife and I have noticed that our AC hasn’t been working lately. It may be best to find a repair service to help us fix the compressor.
anybody here knows the reason why the receprocating compressor stuck,when i start after 5 seconds the compressor stuck only the motor is running with the belt,,
In this article you make the following statement: “The run capacitor is wired with the run winding and start capacitor is wired with the start winding.” I work strictly on milk cooling compressors and every single phase wiring diagram I have seen puts the run and start capacitors in parallel with each other and in series with the START winding. The switch in the potential relay then opens the circuit for the start capacitor only and the run capacitor continues to delay the voltage rise in the start winding while running. You can confirm this by using a clamp-on ammeter on the start winding and see a large current during the start-up, then you will see a current in the start winding that is about half of what is going through the run winding.
the potential relay works on what is technically called back EMF or electro-motive force which is another way of saying volts. It is not a current relay. The are some applications that use a current relay to remove the start capacitor from the circuit after starting the compressor however most applications for heating and cooling use a potential relay. Another word for potential is voltage. Full circle. There is a difference between a current relay and a potential relay.
I cannot speak for milk compressors as this is geared for heating and cooling systems but this information is correct and has been vetted by others.
Your response did not address the comment Fred Gaalswyk made, correctly, that both the run capacitor and the start capacitor are wired with the run winding. Thi is true for EVERY single phase motor with a run capacitor. The run capacitor causes the power flow in the start winding to be slightly out of phase with the run winding. It helps the run winding spin the armature. This makes the motor more efficient. The capacitance (microfarad) of the run capacitor determines the degree of the phase shift. The ideal run capacitor size will result in the lowest total motor current for a given load. Typically the current flow through the start winding, as Fred said, will be 1/2 of the current flow through the run winding when the motor is operating at full speed and under normal load.
The start capacitor and start switch are wiring in series and the run capacitor is wired in parallel with both of them. The start switch for a refrigeration compressor is typically a potential relay as you say. The relatively large capacitance of the start and run capacitor combined causes a large phase shift and high current flow in the start winding. The phase shift determines the direction of rotation and the high current flow gets the armature moving. The start switch disconnects the start capacitor from the start winding when the motor reaches a sufficient speed.
The information about the run capacitor being wired with the run winding is simply incorrect no matter who vetted it. If you take another look at your wiring diagrams I think you will see that Fred and I are correct. I would love to see any diagram you have from a working system that suggests we are wrong.
You guys are free to write an article and submit it provided you follow the editorial guidelines. We’d love to see what you have to offer.
This is a wiring diagram for a Lennox condensing unit and it indeed does show the run capacitor wired as you say it is supposed to be wired.
And I stand corrected…………..the run capacitor is wired to the common and the start terminals on the compressor. I will change this in the way it is written in the article above. I have also suspended the author of the article and now will go back to all his articles and check them for technical accuracy myself. Thanks!
Good info.
Thanks,
Tom Rueff Lexington, Ky. Retired HVAC&R
Any idea which stores carry oil test kits, like you mentioned in the article? And then how one would go about sending it off to a lab for testing? I’m assuming this is not something a consumer could do themselves? Probably needs to be done by a licensed hvac contractor?
Thanks for the info
This needs to be done by someone licensed in refrigeration as many HVAC contractors have licensed technicians in refrigeration. Unless you have had problems with your compressor which required it to be changed out then it is likely not needed. The test is to check for acid in the oil. Acid in the oil is detrimental to the windings in the compressor.
Hey, I have a 19 yr old Ruud unit that I have been frankenstiening along for several years.
Most recently, the condensing fan was running but not a peep from my scroll compressor, which was blazing hot.
I let it cool all day (got up to 82° in the house) and turned it back on…nothing.
All my electrical connections seemed to check out and just for S&G’s I tapped the scroll with a 3 lb hammer a few times. Tapped the top, then around the case and low and behold it suddenly kicks on.
Immediately started getting hot again, but still seemed to do its job…line set decently cool, so maybe okay on refrigerant.
I’ve been having a bad feeling about the system for a while, trying to schedule replacement now…but was pleasantly surprised the hammer worked…comfortable 76° in here now…don’t expect it to last.
One of two things happened when you hit the compressor with a hammer and I suspect most likely the following happened but there really is no way to know for sure: 1) you said the compressor was getting very hot. Inside compressors they have internal overloads in the windings. When the temperature gets too hot the internal overloads will open. This keeps the compressor from burning up. They are supposed to close automatically but sometimes they do not. A little rap with a hammer on the shell of the compressor and the internal overload snapped back closed again. 2) the compressor was mechanically locked up in some way because it is nearing its end of life and there is some wear in the bearings and other mechanical components in the compressor. If the compressor attempted a start up against higher than normal pressures then it is possible it locked up. This also happens sometimes when the unit short cycles. In that case it needs short cycle protection by way of a delay timer. Most digital thermostats have delay timers built in to prevent short cycling.
Lastly, it is time to start thinking about a replacement. 19 years is well beyond the average so it did its job. An upgrade will save you money on your power bill and offer you reliability.