The compressor can fail in only a handful of different ways. It can fail open, fail shorted, experience a bearing failure, or a piston failure (throw a rod), or experience a valve failure. That is pretty much the entire list.When a compressor fails open, a wire inside the compressor breaks. This is unserviceable and the symptom is that the compressor does not run, though it may hum. If the compressor fails open, and following the steps here does not fix it, then the system may be a good candidate for a new compressor. This failure causes no further failures and won't damage the rest of the system; if the rest of the system is not decrepit then it would be cost effective to just put a new compressor in.Testing for a failed open compressor is easy. Pop the electrical cover for the compressor off, and remove the wires and the thermal limiter. Using an ohmmeter, measure the impedance from one terminal to another across all three terminals of the compressor. Also measure the impedance to the case of the compressor for all three terminals.

When a compressor fails short, what happens is that insulation on the wires has worn off or burned off or broken inside the compressor. This allows a wire on a motor winding to touch something it should not touch - most commonly itself a turn or two further along on the motor winding. This results in a "shorted winding" which will stop the compressor immediately and cause it to heat up and burn internally.Bad bearings can cause a failed short. Either the rotor wobbles enough to contact the stator, resulting in insulation damage that shorts the rotor either to ground or to the stator, or end bearing wear can allow the stator to shift down over time until it begins to rub against the stator ends or the housing.

Usually when one of these shorts occur, it is not immediately a hard short - meaning that initially the contact is intermittent and comes and goes. Every time the short occurs, the compressor torque drops sharply, the compressor may shudder a bit visibly as a result, and this shudder shakes the winding enough to separate the short. While the short is in place, the current through the shorted winding shoots up and a lot of heat is produced. Also, usually the short will blow some sparks - which produces acid inside the air conditioner system by decomposing the freon into a mixture of hydrochloric and hydrofluoric acid.You should read low impedance values for all terminal to terminal connections (a few hundred ohms or less) and you should have a high impedance (several kilo-ohms or greater) for all terminals to the case (which is ground). If any of the terminal to terminal connections is a very high impedance, you have a failed open compressor. In very rare cases, a failed open compressor may show a low impedance to ground from one terminal (which will be one of the terminals associated with the failed open). In this case, the broken wire has moved and is contacting the case. This condition - which is quite rare but not impossible - could cause a breaker to trip and could result in a misdiagnosis of failed short.

Source: Industrial Air Conditioner, Modular Air Conditioner