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Compressor Short to Ground

I received an e-mail from a long time Wallie and he was asking me about compressor shorts to ground.

I'm not certain if he posted this here, as I have been out to the country for a couple of weeks with no access to the Internet or e-mail. Since compressor shorts-to-ground can be a perplexing and confusing issue, I opted to post the response to the e-mail here.

First, let me say that a short to ground on a compressor is most often not a defect in the manufacture of the compressor, but deficiencies elsewhere in the system.

Some technicians arrive on a job, see a defective component, replace the component, start the system up and leave. This can, at times, be perfectly okay but quite often can lead to future system problems. Let me give you an example.

A service technician goes on a service call where the customer is complaining that the air conditioning system is not cooling. The technician arrives at the job and finds the system operating, but hot air is coming from the supply registers. He gauges up on the system and finds the high side pressure at 170 psig and the low side pressure at 140 psig. He determines that the compressor is defective, as it is operating but obviously not pumping refrigerant through the system. After checking the compressor it is determined that the valves in the compressor are indeed leaking. The compressor is replaced and the system is started up. The system begins to cool and the technician leaves the job. The next day the sustomer calls and complains that the system is not cooling. Upon returning, the technician determines that the compressor is operating but not moving refrigerant through the system once again. Once again, the new compressor has leaking valves.

What caused the problem and what could the technician have done differently? Well, the problem with the system was that the thermal bulb on the thermostatic expansion valve had come loose from the suction line at the outlet of the evaporator.

When the technician identified the reason for the system not cooling, he needed to ask himself why. You also need to keep asking yourself why until you can no langer answer the question. When you have reached this point, you will have most likely identified the cause for the problem, and no the effect of the problem. The above example could have proceeded like this:

The system is not cooling... Why?

The compressor is operating, but the high side and low side pressures are the same... Why?

Because the compressor valves are leaking... Why?

Because liquid refrigerant got into the compressor... Why?

Beacuse the TXV was overfeeding the evaporator... Why?

Because the thermal bulb was loose...

Got it? GREAT!

Now, let's get back to the matter at hand... A compressor with a short-to-ground.

When a compressor has a short-to-ground, it is likely caused by damage to the insulation within the compressor itself. Damage to the insulation is likely caused by acid within the system. Acid in the system can result from improper system evacuation or the presence of air/moisture in the system.

Simply replacing a compressor that has a short-to-ground may not alleviate the actual cause for compressor failure; namely the presence of moisture or acid in the system.

Make certain that all system leaks are found and repaired and that the system is properly evacuated before putting the new compressor into service.

I would love to hear from others who have had trouble with shorts-to-ground and how they dealt with the situation.
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