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In this HVAC Training Article, I discuss how to perform electrical troubleshooting on an HVAC/R compressor to determine if it is bad. Too many times, compressors are condemned without taking the proper measurements to verify that they are damaged. To help prevent this, I wanted to cover how to check and test various compressors. A multimeter is needed in order to measure electrical resistance at each of the following steps.
Before doing any testing on the compressor, make sure that the electrical power to the outdoor unit is off at the electrical disconnect (In this example, we are looking at a single-phase split system where the compressor is located in the outdoor unit.). Lock the disconnect box out for safety if it is a distance away from the outdoor unit.
Initial Testing
Initial wire testing is performed in the electrical compartment of the outdoor unit. This is done prior to opening the unit up to get to the actual compressor tabs.
1. Test for voltage with the multimeter to make sure that electrical power is not present at the contactor.
2. Take a picture of the wiring or note their location before removing the compressor wires.
3. Identify the wires connecting to the compressor.
4. There are three wires on the compressor. Two of which should be on the contactor (one on each pole) along with one wire attached to the Herm tab of the capacitor. Because this compressor is installed in a single-phase unit, a capacitor is used. Disconnect the compressor wires from the contactor and this capacitor.
5. Next, measure the electrical resistance of the windings by measuring each pair of wires, three pairs total.
6. In this example, between black and yellow, we measure 0.6 ohms. Between black and blue, we measure 1.7 ohms. Between blue (start) and yellow (run), we measure 2.3 ohms.
7. The pair with the lowest resistance measurement is always common and run. In this case the lowest measurement was taken between black and yellow.
8. The pair with the medium resistance is always common and start. In this case the medium resistance measurement was taken between the black and blue.
9. The pair with the highest resistance is always start and run. In this case the highest measurement was taken between the blue and yellow
10. From these results we can determine which wire goes to which tab on this compressor: -The Run tab on the compressor is the yellow wire -The Start tab on the compressor is the blue wire -The Common tab on the compressor is the black wire
This initial test will serve to confirm that the resistance between common to run and common to start is equal to the resistance from start to run.
0.6 ohms (Common to Run) + 1.7 ohms (Common to Start) = 2.3 ohms (Run to Start).
Additionally, this test indicates that common can be measured and therefore the thermal limit is intact.
A version of the thermal limit is seen on the compressor motor above. This is considered a protection device and is also referred to as an internal thermal overload or compressor protector. This device is mounted against the motor windings on the inside of the compressor shell. Unfortunately, you cannot get to these sensors on larger hermetically sealed compressors. However, you can get to a compressor protector from the outside of a small fractional horsepower compressor. On a small fractional horsepower compressor, it is mounted to the outside of the shell or on the common terminal.
If the motor has a hard time starting, the windings will get heated up, but this device will help protect them from overheating by shutting off the compressor when the temperature reaches a high limit. The goal is to prevent the resin on the outside of the wire loops to melt down, because this would allow the wires to touch and short, causing permanent damage.
When performing the initial wire testing, if the internal thermal overload for the compressor was open, the measurements would be the following:
OL Ω between Common and Run
OL Ω between Common and Start
2.3 Ω between Run and Start
If no resistance (OL) is measured between any of these pairs, it could indicate an open winding. In this case, it is best to wait for it to cool down just to make sure that the internal overload is not wired differently and just needs to reset. Be sure to have the power off when attempting to cool down the compressor with water. After the compressor has cooled down, the initial test can be redone. If no resistance is measured between any of the pairs after the compressor has cooled, it is likely that the compressor windings have been damaged and are open.
If the windings appear to be intact, there are additional tests that should be performed.
Testing if Compressor is Shorted to Ground at the Wires
In order to see if your compressor has shorted to ground at the wires, perform the following steps:
1. Clean off a bit of copper on your line set tube. The copper tube is brazed into your compressor, so this is a great spot to use as a ground.
2. Take a resistance measurement between the tube (ground) and one of the wires. Any of these wires can be checked, as long as you have checked the pairs and they have good resistance values between them.
When checking for a short to ground, the measurement of OL is a good thing. This means that the compressor is not shorted to ground.
Make sure to measure the electrical resistance in Kilo/Mega Ohms between the tube and wire, you will still need to check your compressor plug down at the bottom. Additionally, we also check at the plug terminal if we measure OL or a high resistance value between Common and Run, Common and Start, or Start and Run, and we believe that the compressor’s thermal overload is closed.
Pulling the Plug
Remember, we always start at the top of the unit and work down, when testing for a bad compressor. The plug should be the last location where we perform testing. Be very careful and wear safety glasses when pulling out the plug. Some Fusite plugs may be weak and end up leaking refrigerant.
Most reciprocating compressors have a cover over the plug, but scroll compressors usually just have the plug by itself. Either way, take care when pulling out the plug, as this is a weak point for the compressor. Avoid attempting to yank or force the plug off so an accidental refrigerant leak does not occur. Once the plug has been removed, inspect it to make sure no visible burn marks or melting are present. Also make sure it is tight when pushed in.
This is an example of a plug from a bad compressor. In this case, the compressor lost the terminal and had possible water damage. If a compressor has a melted or worn lug, or if the plug is damaged, there may be an issue with high current, a loose connection, or water dripping. If the lug terminal is still intact but not getting a good connection to the plug, a compressor lug repair kit may be used to obtain a better connection to the wire. Only do this as a worst-case scenario, because the OEM plug is designed to seal the connection from moisture. The lug repair kit will not. For this reason, if a lug repair kit is used, it is crucial to seal the plug cover if you tried to use a repair kit, since there is no real seal. Ideally, if a plug is damaged, replace the plug with a direct OEM replacement that will seal the terminals.
Be aware that even after repairing the electrical connection, if the damage was caused by high current, the issue will still need to be addressed, otherwise, another electrical connection problem may occur.
If no obvious damage is present on at the plug, continue to the testing steps below.
Testing if Compressor is Shorted to Ground at the Plug
In order to see if your compressor has shorted to ground at the plug, perform the following steps:
1. Use the section of the copper tube that was cleaned off earlier or a clean piece of tubing closer to the compressor to use as a ground.
2. Take a resistance measurement between the tube (ground) and one of the terminals (lugs). Any of the terminals can be checked.
If the compressor is NOT shorted, a measurement of OL will be present.
The example above shows a scroll compressor that has shorted the ground. This compressor is showing a measurement of 2.7 ohms of electrical resistance from the windings over to the tube. This is a big problem. Additionally, if when we check the pairs on this compressor, we are measure 2.6 Kilo-Ohms. Based on this information, we know that this compressor has or is going to burn out. This will be a problem for the refrigerant and the acidity within this system. To better understand why, let’s look at a burned-out compressor.
We have removed the scrolls from this compressor and the black soot on the inside is clearly visible. This acidic residue was pushed throughout the line set, evaporator coil and condenser coil, before the compressor stopped operating or was replaced. Keep in mind that even if you replace the whole outdoor unit, you still will have this black soot in the line set and the evaporator coil and even if the line set is cleaned, the evaporator coil itself, can’t really be cleaned.
Testing a 3-Phase Minisplit Compressor
Above is a 3-phase wound, mini-split compressor. These are commonly found on all single phase, inverter driven units. When dealing with and testing 3-phase motors, there are some differences from testing single phase motors. The most significant, we will see is when testing for an open winding.
Testing a 3-Phase Minisplit Compressor for an Open Winding
In order to see if your compressor has an open winding, perform the following steps:
Turn the electrical power off.
Take the protective cap or cover off.
Pull off the wires and set the multimeter on resistance.
Test for resistance between the terminal pairs.
All terminal pairs should present the same measurement.
When testing a 3-phase motor such as on a minisplit unit, expect that all the pairs of wires will have matching resistance values, if the windings are not open. In the example above, we are measuring 1.0 ohm of resistance on matching pairs of wires. If any of the pairs measured OL, that would indicate that a winding is open.
Testing if the 3-Phase Compressor is Shorted to Ground at the Plug's Wiring
Next we will check if the compressor is shorted to ground, and remember the 3 phase motor is going to have matching resistance readings between all 3 pairs. This means that any of those 3 terminals to the ground should be reading OL (open line). If they read OL, this means that the compressor is not shorted to the ground, as long as the plug or terminals are still connected to the compressor.
In order to see if your compressor has shorted to ground at the terminals, perform the following steps:
Clean a piece of tubing connected to the compressor to use as a ground.
Test for resistance between the tube and each of the terminals.
If the measurement is OL then the compressor is not shorted to ground.
I hope this helps you better grasp the process for testing compressors.
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Published: 04/13/2022 Author: Craig Migliaccio
About the Author: Craig is the owner of AC Service Tech LLC and the Author of the book “Refrigerant Charging and Service Procedures for Air Conditioning”. Craig is a licensed Teacher of HVACR, Sheet Metal, and Building Maintenance in the State of New Jersey of the USA. He is also an HVACR Contracting Business owner of 16 years and holds an NJ HVACR Master License. Craig creates educational HVACR articles and videos which are posted at https://www.acservicetech.com & https://www.youtube.com/acservicetechchannel & https://www.facebook.com/acservicetech/
