Air conditioning compressors generally fail due to one of two conditions: time and hours of operation (wear) or abuse. There are some faults that can occur in other parts of the system that will cause a compressor failure, but they are less common unless the system has been substantially abused.
Abuse is usually the result of prolonged operation with an incorrect freon charge or as a result of improper service on the road. This improper service may include overloading, undercharging, installing the wrong start capacitor as a replacement, removing (rather than repairing / replacing) the thermal limiter, insufficient oil, mixing incompatible oil types or incorrect oil, installing the compressor in a system that had a major burnout without taking proper steps to remove acid from the system, install the wrong compressor (too small) for the system, or install a new compressor in a system that had some other fault that was never diagnosed.
The compressor can fail in only a handful of different ways. It can fail to open, fail shorted, experience bearing failure or piston failure (throw a rod), or experience valve failure. That’s pretty much the entire list.
When a compressor fails to open, a wire inside the compressor breaks. This is useless and the symptom is that the compressor does not work, although it may hum. If the compressor fails to open and following the steps outlined here does not fix it, then the system may be a good candidate for a new compressor. This failure does not cause any more failures and will not harm the rest of the system; If the rest of the system is not decrepit, it would be profitable to install a new compressor.
Testing for a failed open compressor is easy. Remove the electrical cover from the compressor and remove the cables and thermal limiter. Using an ohmmeter, measure the impedance from one terminal to the other at the three terminals of the compressor. Also measure the impedance to the compressor box for all three terminals.
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 kiloohms or more) for all terminals in the case (which is ground). If any of the terminal-to-terminal connections have 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 in contact with the housing. This condition, which is quite rare but not impossible, could cause a breaker to trip and could result in a misdiagnosis of a failed short. Be careful here; acid test the content of the lines before deciding how to proceed with the repair.
When a compressor fails, what happens is that the insulation on the wires has worn, burned or broken inside the compressor. This allows a wire in a motor winding to touch something it shouldn’t touch, most commonly itself a turn or two later in the motor winding. This results in a “shorted winding” that will stop the compressor immediately and cause it to heat up and burn out internally.
Faulty bearings can cause a faulty short circuit. Either the rotor wobbles enough to come into contact with the stator, resulting in insulation damage that shorts the rotor to ground or the stator, or wear on the end bearing can allow the stator to drift downward over time until it begins to rub against the ends of the stator or housing.
Usually when one of these shorts occurs, it is not immediately a strong short, meaning that initially the contact is intermittent and comes and goes. Each time the short occurs, the compressor torque drops sharply, the compressor may visibly shake a little as a result, and this tremor shakes the winding enough to separate the short. While the short is in place, the current through the shorted winding trips and a great deal of heat is produced. Also, usually the short will produce some sparks, which produces acid within the air conditioning system by breaking down freon into a mixture of hydrochloric and hydrofluoric acid.
Over time (possibly a couple of weeks, usually less), tremors and sparks and heat and acid cause the insulation to quickly fail in the winding. Ultimately, the winding loses enough insulation that the inside of the compressor literally burns out. This will only last a few minutes, but in that time the compressor self-destructs and fills the system with acid. Then the compressor stops. At that point, you may melt a loose, short wire to the casing (which may trip your home’s main switch) or it may not. If the initial cause of the failure was faulty bearings that caused the rotor to rub, then usually when the thing finally dies it will short out the casing.
Shorting to the case will blow fuses and / or circuit breakers and your ohmmeter will show a very low impedance from one or more windings to ground. If you don’t short the case, it will just stop. Still establish the type of fault using an ohmmeter.
You cannot directly diagnose a failed short with an ohmmeter unless the case is shorted; a shorted winding will not show up with an ohmmeter, although it would with an inductance meter (but who has one of those?). Instead, you must infer the failed short. To do this, establish that the ohmmeter gives normal readings, the start capacitor is good, power is reaching the compressor AND an acid test of the Freon shows the presence of acid.
With a botched short, just give up. Change everything, including lines if possible. It is not worth fixing; it is full of acid and therefore it is garbage. Also, a failed short could have been initially induced by some other fault in the system that caused the compressor to overload; By replacing the entire system, you will also get rid of that other potential problem.
Less often, a compressor will have bearing, piston, or valve failure. These mechanical failures generally only indicate wear, but could indicate abuse (low lubricant levels, thermal limiter removed to cause the compressor to overheat, chronic low freon condition due to unrepaired leaks). More rarely, they can indicate another failure in the system, such as a reversing valve problem or an expansion valve problem that ends up letting liquid Freon enter the suction side of the compressor.
If a bearing fails, you usually know it 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 against the stator, and you will end up with a failed short.
If the compressor locks up mechanically and does not work, you will know it because it will be loud for a few seconds and may shake (like any stuck motor) until the thermal limiter turns it off. When you perform your electrical checks, you will not find evidence of a failed open or failed short. The acid test will not show acid. In this case, you can try a hard start kit, but if the compressor has mechanically failed, the hard start kit will not cause the compressor to start. In this case, replacing the compressor is a good plan as long as the rest of the system is not decrepit. After replacing the compressor, you should carefully analyze the performance of the entire system to determine if the compressor problem was caused by something else.
On rare occasions, the compressor will experience a valve failure. In this case, it will sit there and appear to be running happily, but it will not pump fluid (the valve will not close), or it will block due to the inability to get the fluid out of the compression chamber (the valve will not close). opened). If you are running happily, once you have established that there is indeed a lot of Freon in the system, but nothing is moving, then you have no choice but to change the compressor. Again, a system with a compressor that has had a valve failure is a good candidate for a new compressor.
Now if the compressor is mechanically locked it could be due to a couple of things. If the compressor is on a heat pump, make sure the reversing valve is not stuck in the middle. Also make sure the expansion valve is working; if it is blocked, it can block the compressor. Also make sure the filter is not clogged. I once saw a system that had a blocked compressor due to liquid blockage. Some idiot had “fixed” the system by adding freon, and adding freon, and adding freon until the thing was completely filled with liquid. Trust me; That does not work.
If the diagnostic shows a clogged filter then this should be taken as positive evidence of some system fault that is NOT a compressor fault. Usually it will be the metal fragments from the compressor that will clog the filter. This can only happen if something is causing the compressor to wear out very quickly, particularly on the pistons, rings, bores, and bearings. Either the compressor has very insufficient lubrication OR (and more commonly) liquid freon is entering the compressor in the suction line. This behavior must be stopped. Look at the expansion valve and the reversing valve (for a heat pump).
Often times, an older system experiences enough mechanical wear internally that it “wears out” and needs more torque to start up against the load of the system than it can supply. This system will sound like one with a locked bearing; the compressor will hum loudly for a few seconds and then the thermal limiter will kill it. Occasionally this system will start immediately if you hit the compressor with a rubber mallet while it is humming. Such a system is a good candidate for a difficult startup team. This kit stores energy and, when the compressor is instructed to start, discharges additional current into the compressor for approximately one second. This overloads the compressor, but provides additional torque for a short period of time and is often enough to get the compressor running again. I’ve had hard start kits that give me an additional 8 or 9 years on some old units that I would have otherwise been replacing. On the contrary, I have only given them a few months. It’s your decision, but considering how cheap a difficult starter kit is, it’s worth a try when the symptoms are as described.
And this, in a nutshell, is what can happen to an air conditioning compressor and what you can do about it.