Troubleshoot a Grounded (Shorted to Ground) Compressor
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When a breaker is tripped or a fuse is blown, the root cause may be the compressor or something else in the condensing unit. In any case, a blown fuse or tripped breaker indicates that a component is drawing significantly too much current. Those differ from overload conditions, where a small but sustained excess of current causes a motor to shut itself off. In many cases, a compressor winding that touches another surface and draws excessive current will result in a shorted to ground condition.
Bert starts his diagnostic process by resetting the breaker and seeing how quickly it trips. It trips immediately, indicating a significant short. Before working on a unit, he confirms that the power is off and that there is no electrical potential present. Then, Bert sets his meter to the ohm scale and checks the resistance between the high-voltage contactor and ground. One side of the high-voltage contactor has a perfectly clean path to ground with no resistance, indicating a short to ground somewhere in the system.
Once Bert has confirmed that there is indeed a short somewhere, he does a visual inspection to look for possible causes of shorts, including wire rubouts. Then, he removes his wire leads from the compressor terminals and measures the resistance between each terminal and ground with the multimeter and then a megohmmeter (which applies a set voltage to make the resistance more evident). Quickly, he realizes that the short is between the common terminal and ground.
Techs may forget to isolate the compressor by pulling the wires out, or they may measure resistance from leg to leg instead of from leg to ground. Measuring from leg to leg may give you an incorrect diagnosis and show you a perfect path when you’re supposed to be checking from leg to ground. Some scroll compressors also have an electrical connection between the windings and the casing, and the megohmmeter may pick that up and throw off your measurements. Scroll compressors, in general, generally result in lower resistance readings as well.
You can also do the “redneck compressor test,” which isolates the compressor from the system. You just run the system without the compressor and see how it works. If the system works without the compressor, then you can be sure that the compressor is the problem.