Air Conditioner Compressor Troubleshooting

Here is a recent email I received this weekend:

“I have a repair question for you whenever you got time. I have a 4 ton R22 condenser with a good tested capacitor of 40uf for compressor. However, when thermostat calls for cooling the contactor engages but compressor doesn’t start, I do have good voltage of 240 across t1 and t2 and the start leg coming from capacitor to ground is 120v. However, no amps. I tested each leg to ground and nothing (no grounding). This is the original size of capacitor for the compressor. The compressor was working a week ago but a day ago stop working . Would you call this a bad compressor?”

First note that the technician needs to check the resistance across the leads to the compressor. It may not be shorted to ground but it may just be internally shorted. To test the compressor, I would first check the resistance across the leads. Common to Run (r1), Common to Start (r2) then Start to Run (r3). What you should see is r1 + r2 = r3. Also, check for continuity across the windings this way for any open winding. Make sure the power is off and the leads are disconnected from anything when you do this. If you find an open winding or the resistance isn’t adding up right, then your compressor may be bad.  Also do you have 120V on all 4 legs (top and bottom) of the contactor when engaged?

My hunch is that the compressor has an internal short and there is no continuity on one of the leads.

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