Compressor Won’t Run Diagnosis

Bryan explains and demonstrates how to reach an accurate diagnosis when a compressor won’t run. In this case, the compressor doesn't trip a breaker or blow a fuse; it simply doesn't run at all.

The compressor is the "heart" of the HVAC or refrigeration system; it increases the pressure of the refrigerant and moves it through the circuit. It is often a loud component, so you might be able to determine that a compressor isn't running if you can't hear it. However, checking the amperage is the only surefire way to tell if a compressor isn't running; you can put the amp clamp around the common wire or one of the compressor leads.

Compressors may come in the three-phase and single-phase varieties; the former is more common in commercial applications, and the latter is common in residential and light commercial HVAC. You will need to take different diagnostic steps depending on if the unit uses a single-phase or three-phase compressor.

Compressor Won’t Run Diagnosis? Before starting your diagnosis, allow the compressor protector (or thermal overload) to reset. In three-phase compressors, the thermal overload breaks all three legs of power. In single-phase compressors, the thermal overload will break common or directly behind common. In any case, a solid visual inspection will be your first main step; make sure that there is no evident damage to the compressor or capacitor(s).

Then, you check the voltage. Start checking voltage at the contactor (in single-phase applications), and then measure it at the compressor terminals. If the voltage is good in both cases, then you can check the amp draw. If you detect excessive amperage, then the next step would be to check the winding resistances and resistances to ground. An ohmmeter can tell you if a compressor is tripping on thermal overload. When measuring resistance, make sure you have an accurate meter and that your leads make adequate contact with the terminals; otherwise, you might get an inaccurate measurement. If you find a resistance well outside the manufacturer's parameters, you have enough evidence to condemn and replace the compressor. Before you condemn a compressor, you need to know exactly which measurement was out of the specifications.

If the resistances are in range, re-check the wiring and the start gear. If all of the readings are normal, make sure that you have the right compressor for the application; you can verify it by checking the manufacturer data. Ensure that the pressures are equalized, and then you can restart the system. If the compressor doesn't start, replace it.

In some cases, a scroll compressor will measure improper resistance from the terminals to ground. Copeland states that anything less than 0.5 megohms is a failure, but anything between 0.5 and 20 megohms is NOT a failure; those values are more likely to indicate oil contamination or mild winding breakdown than a total failure.

If you read no amps, you need to allow the overload to reset. If it doesn't reset, then you need to replace the compressor. In the case of a single-phase compressor, an open thermal overload will result in no continuity between common and either run or start. However, you WILL read an ohm value between run and start, so be aware of which terminals you're measuring.

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Compressor teardown class with Mike Nipper:
https://www.youtube.com/watch?v=3aVMfR4QLgc&t=904s

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