Air conditioning compressor motors are very simple to diagnose but can be confusing. If you find that an AC compressor is getting the proper voltages, has a good capacitor, and will not run or is tripping the circuit breaker, it could be that the motor windings are the problem.
There are only 3 motor winding failures:
Open Winding – This is when one of the wires that make up the motor windings is broken or separated.
Shorted Winding – If the insulation on the windings has failed, sometimes one part of the winding is making electrical contact with another part of the winding.
Grounded Winding – This is similar to the shorted winding but the wire with the broken insulation is making electrical contact with the compressor casing or ground.
Here is how to check the compressor motor if you suspect it has failed;
- Disconnect power
- Remove the top of the outdoor condensing unit
- Unplug the 3-prong plug on the compressor
- Set your multimeter to ohms
- Measure and record the resistance (ohms) measurement of each winding
Click the image below to download your free compressor troubleshooting chart!
A good motor 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 reading should be below 10 ohms (this is the run winding).
- The second highest reading should be about 2-4 times higher than the lowest reading (this is the start winding).
- The highest reading should be the sum of the resistance read across the two smaller windings
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.
If you find all the ohm measurements are correct:
- Set your ohm meter to its highest range
- Place on meter lead on the metal case of the condenser (make sure you scrape of any rust or paint to make sure good contact)
- Measure each pin to the ground
- Any measurement that does not read infinite indicates a grounded winding
These measurement are often times very difficult to make because there is usually a shroud around the compressor terminals and you have to stand on your head to get to them. If you can, find a scrap AC unit at your shop and grab a compressor plug and strip the 3 wire ends. Then when you remove the compressor plug you can plug-in your plug with the bare wires and easily make your resistance measurements.
Note – If you suspect you have an open winding, make sure the compressor isn’t overheated (touch it). If it is hot to the touch it could be the internal overload has tripped due to overheating. Use a water hose to cool it down and recheck your measurements. Many good compressors are replaced due to this mistake.