Air conditioner compressors commonly fail due to one of two conditions: time and hours of operation (wear out), or abuse. There are numerous failures that may occur elsewhere in the system that will cause a compressor failure, but these are less mutual unless the system has been substantially abused.
Usually abuse is a result of extended running with improper freon charge, or as a consequence of improper service along the way. This improper service may include overcharging, undercharging, installing the wrong starter capacitor as a replacement, removing (rather than repairing/replacing) the thermal limiter, insufficient oil, mixing incompatible oil types, or wrong oil, installing the compressor on a scheme that had a major burnout without taking proper steps to remove the acid from the system, installing the faulty compressor (too small) for the system, or installing a new compressor on a system that had numerous other failure that was never diagnosed.
The compressor may fail in only a handful of dissimilar ways. It may fail open, fail shorted, experience a bearing failure, or a piston failure (throw a rod), or experience a valve failure. That is gorgeous 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 repair it, then the system may be a good campaigner for a new compressor. This failure causes no further failures and won’t harm the rest of the system; if the rest of the scheme 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 throughout all three terminals of the compressor. Also measure the impedance to the case of the compressor for all three terminals.
You ought to read low impedance values for all terminal to terminal connections (a few hundred ohms or less) and you must 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 affiliated with the failed open). In this case, the broken wire has moved and is contacting the case. This condition – which is rather rare but not inconceivable – could cause a breaker to trip and could result in a misdiagnosis of failed short. Be careful here; do an acid test of the contents of the lines before settling how to carry on with repair.
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 must not touch – most normally itself a turn or two further along on the motor winding. This results in a “shorted winding” which will stop the compressor without delay and cause it to heat up and burn internally.
Bad bearings may cause a failed short. Either the rotor wobbles sufficient to contact the stator, resulting in insulation harm that shorts the rotor either to ground or to the stator, or end bearing wear may concede the stator to shift down over time until it begins to rub versus the stator ends or the housing.
Usually when one of these shorts occur, it is not without delay 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 sufficient to discerned the short. While the short is in place, the current through the shorted winding shoots up and a lot of heat is produced. Also, normally the short will blow a heap of sparks – which formulates acid inside the air conditioner scheme by decomposing the freon into a mixture of hydrochloric and hydrofluoric acid.
Over time (possibly a couple of weeks, commonly less) the shuddering and the sparking and the heat and the acid cause insulation to fail speedily on the winding. Ultimately, the winding loses sufficient insulation that the inside of the compressor is in a literal sense burning. This will only go on for a few minutes but in that time the compressor destroys itself and fills the system with acid. Then the compressor stops. It may at that time melt a wire loose and short to the housing (which may trip your house main breaker) or it may not. If the introductory cause of the failure was bad bearings causing the rotor to rub, then commonly when the thing at last dies it will be shorted to the housing.
If it shorts to the housing, it will blow fuses and/or breakers and your ohmmeter will show a very low impedance from one or more windings to ground. If it does not short to the housing, then it will just stop. You still establish the type of failure using an ohmmeter.
You cannot directly diagnose a failed short with an ohmmeter unless it shorts to the housing – a shorted winding won’t show up with an ohmmeter though it would with an inductance meter (but who has one of those?) Instead, you have to infer the failed short. You do this by establishing the the ohmmeter gives normal readings, the starter capacitor is good, power is arriving at the compressor, AND an acid test of the freon shows acid present.
With a failed short, just give up. Change everything, including the lines if possible. It is not worth fixing; it is full of acid and consequently is all junk. Further, a failed short could have been initially induced by a heap of other failure in the scheme that caused a compressor overload; by replacing the whole scheme you likewise will get rid of that potential other problem.
Less commonly, a compressor will have a bearing failure, piston failure or a valve failure. These mechanical failures commonly just signal wear out but could signal abuse (low lubricant levels, thermal limiter got rid of so compressor overheats, chronic low freon condition due to un-repaired leaks). More rarely, they may signal another failure in the scheme such as a reversing valve problem or an elaboration valve problem that winds up letting liquid freon get into the suction side of the compressor.
If a bearing fails, ordinarily you will know because the compressor will sound like a motor with a bad bearing, or it will lock up and refuse to run. In the worst case, the rotor will wobble, the windings will rub on the stator, and you will wind up with a failed short.
If the compressor locks up mechanically and fails to run, you will recognise because it will buzz very loudly for a few seconds and may shudder (just like any stalled motor) until the thermal limiter cuts it off. When you do your electrical checks, you will find no proof of failed open or failed short. The acid test will show no acid. In this case, you might try a hard-start kit but if the compressor has failed mechanically the hard-start kit won’t get the compressor to start. In this case, replacing the compressor is a good plan so long as the rest of the system is not decrepit. After replacing the compressor, you will have to conservatively make an analyzation of the performance of the entire system to determine whether the compressor problem was induced by something else.
Rarely, the compressor will experience a valve failure. In this case, it will either sit there and appear to run happily but will pump no liquid (valve won’t close), or it will lock up due to an disability to move the liquid out of the compression chamber (valve won’t open). If it is running happily, then once you have established that there is without doubt a great deal of freon in the system, but not one thing is moving, then you have no choice but to modify the compressor. Again, a system with a compressor that has had a valve failure is a good nominee for a new compressor.
Now, if the compressor is mechanically locked up it could be because of a couple of things. If the compressor is on a heat pump, make sure the reversing valve is not stuck half way. Also make sure the elaboration valve is working; if it is blocked it may lock the compressor. Also make sure the filter is not clogged. I once saw a system that had a locked compressor due to liquid lock. Some moron had “serviced” the scheme by adding freon, and adding freon, and adding freon until the thing was exclusively full of liquid. Trust me; that does not work.
Should diagnosis show a clogged filter, then this ought to be taken as positive proof of a great deal of failure in the system OTHER than a compressor failure. Typically, it will be metal fragments out of the compressor that clogs the filter. This may only take place if something is causing the compressor to wear very rapidly, specially in the pistons, the rings, the bores, and the bearings. Either the compressor has vastly insufficient lubrication OR (and more commonly) liquid freon is getting into the compressor on the suction line. This conduct ought to be stopped. Look at the elaboration valve and at the reversing valve (for a heat pump).
Often an old system experiences sufficient mechanical wear internally that it is “worn in” and needs more torque to start out versus the system load than may be delivered. This system will sound just like one with a locked bearing; the compressor will buzz loudly for a few seconds then the thermal limiter will kill it. Occasionally, this scheme will get started right up if you whack the compressor with a rubber mallet while it is buzzing. Such a scheme is a good nominee for a hard-start kit. This kit stores energy and, when the compressor is told to start, dumps extra current into the compressor for a second or so. This overloads the compressor, but gives some extra torque for a short time and is oftentimes sufficient to make that compressor run again. I have had hard-start kits give me an extra 8 or 9 years in some old units that other than as supposed or expected I would have been replacing. Conversely, I have had them give only a few months. It is your call, but giving careful consideration to how cheap a hard-start kit is, it is worth attempting when the sensations or changes are as described.
And this, in a nutshell, is what may occur to an air conditioner compressor and what you may do when it comes to it.
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