How does a tech know the a/c compressor he or she is diagnosing is actually failed and that the problem is not a crankcase heater, condenser fan, or rubbed out wire?

According to the contractors The NEWS spoke with, there are a few simple steps one can take on a job site to comprehensively ensure the problem lies within the compressor.


Bryan Orr, co-owner and vice president of service, Kalos Services Inc., Clermont, Florida, and founder of, said it’s imperative to completely isolate the compressor when checking for a short.

“Unless you pull the compressor from its terminals, you cannot know it’s shorted,” he said. “If you’re reading at the contactor or anywhere else, there could be a problem with the wire itself.”

Orr advises connecting one lead to a compressor terminal and the other directly to the compressor.

“I scratch a spot in the suction line and check right there to the terminals,” he said. “I will often use a megaohmmeter and look for a conclusive short between the terminals and the copper, keeping in mind that scroll compressors can give false positives if you are not careful to understand the readings.” 

When a compressor shorts, it often starts with a mechanical problem, which can lead to an electrical problem later.

“Inside a compressor, there are windings that are immersed with oil and refrigerant,” Orr said. “When something breaks and bounces around in there, it can damage and short out the windings. When the windings short out, carbon and acids are often created. This carbon creates an insulator. When you’re reading it with a 9V battery on your meter, you won’t always read to ground even though you have a shorted compressor. However, if you turn the power on, you’ll see it trips the breaker.”

If you have a shorted circuit, your meter will read low or zero ohms, Orr said.

“We don’t want a good path to ground inside that compressor,” he said. “You want a very poor path between those terminals and the suction line that shows the windings and insulation are in good shape and intact.”

Even once a shorted compressor is diagnosed, it is a good idea to remove it from the electrical circuit and test the other components, according to Orr.

“Once the compressor is isolated, tape off the leads and turn the power back on with the fan and everything back in place,” he said. “Make sure everything else is running without the compressor.”

Greg Fox, president, Fox Family Heating and Air Conditioning Inc., Sacramento, California, said his first step is to check and see if the compressor is getting voltage using a multimeter.

“Check the line side of the contactor first,” he said. “You want to make sure all the fan and compressor terminals on the load side of the contactor are getting voltage. Once you’ve established you’re getting voltage to the compressor at the contactor, trace those high-voltage wires to the compressor and its terminals. Those wires lead to a plug at the compressor. Assuming the unit is unplugged, you’re going to pull that plug or cover off the compressor and visually inspect the terminal leads. If they’re bad or burnt beyond repair, the compressor needs to be replaced. If the terminals and wires are still good, then we know the voltage is getting to the compressor. If the unit is getting voltage, then it’s likely either a seized-up compressor or the internal overload switch is open.

“Let’s assume you’re not getting voltage and nothing’s running on the a/c,” Fox continued. “Work backwards by checking the service disconnect. If you have voltage getting to the line side but not the load side, then your fuses could be blown. More testing needs to be done before you replace those fuses. Usually, when fuses are blown in the disconnect or the breaker trips, I start thinking there may be a short at the compressor. What I don’t want is continuity between the compressor wires and the chassis of the unit. I check each contactor wire to ground. Then, I further verify that by taking the plug off the compressor and test the lugs to ground or on a clean piece of copper pipe, such as the suction line. If we have continuity, we know there are some wires inside the windings of the compressor that are touching the shell of the compressor and sending that continuity all the way back to where I’m testing. Then, I isolate the compressor by unplugging it completely from the contactor and capacitor and run the condenser fan and blower motor. If all is well with those two things running, I know the compressor is shorted to ground and is bad.”

Mike Matta, service manager, Unique Indoor Comfort, Conshohocken, Pennsylvania, said his techs strive to gather as much information on the issue as possible before arriving on a job site.

“When we get to the house, we interview the homeowner and gather as much information about the complaint as we possibly can,” he said. “From there, we try to reproduce the issue and start checking boxes on potential issues. We verify what’s working and what’s not working, check the air filter, ensure the equipment is properly charged, examine the ductwork to ensure the equipment can operate when properly charged, etc. We do this to make sure we’re properly addressing the issue and to make sure whomever was there before us did things correctly.”


Another common compressor failure occurs when the internal overload is triggered due to extreme internal compressor temperatures.

“The thermal overload opens,” Orr said. “This happens by design and is a protective matter. You’ll read an ohm reading between run and start, and you won’t have an ohm reading between common and run and start and common. Of the open-winding circumstances, this is the most common. There will be circumstances when you see an open winding between start and run. If that’s in conjunction with shorting — a tripping breaker — then that’s a shorted compressor first and foremost.

“If it’s a compressor that’s out on internal overload, cool it down with water with the power off and leave an ohm meter on the terminals between common and run on ring mode so that when the internal overload resets the meter, it will ring to let you know it’s sufficiently cooled down,” he continued. “In the rare case you find a compressor is stuck open, and it’s not resetting and open between common and run and start to common, you have an internal overload that’s stuck open. You don’t see it a lot, but I suggest you shake it around a bit or tap it with a rubber mallet to try to get the snap-action disc to reset. If it’s still not resetting, tell the customer it’s not resetting. Now, that said, it could reset at some point in the future. I’ve seen cases where we quote a compressor, and before we can get it put in, the internal overload resets. It’s good to have that conversation so that it doesn’t appear you’re trying to sell snake oil.”

During troubleshooting, if you determine the compressor is getting the proper voltage and the motor is not turning on, one of two things may be going on — the internal overload switch is open or the compressor is seized up, Fox said.

“An internal overload switch is built into the compressor,” he said. “It can only take so much pressure before it shuts itself down. You can touch the compressor, and if it’s hot to the touch, that switch is very likely open and will remain open until it cools, which could take as long as two hours. Then, the compressor will try to run again until it overheats and shuts down. This happens over and over. This internal overload switch can’t be cooled manually, though it can be cooled with cold water. Cover the terminals and take a hose and pour water over the compressor for a half hour or more. It has to be cool to the touch before the compressor will run again.”

Matta said his techs see a lot of open windings in the dog days of summer.

“If we feel it may be a winding, we’re obviously checking the capacitors first and then ohming out the compressor,” he said. “We ohm start to run, start to common, run to common, and the ground to make sure nothing’s shorted to ground. If we find that there’s an open circuit, we’re going to take a garden hose to it and try to cool it down. Sometimes, if it’s a rooftop unit, and it’s 130°F up on that roof, it’s going to take a while to cool it down. In fact, the compressor may not feel hot at all up there, so make sure you’re giving it your full attention.”

Sometimes, an open winding, or other fault, may completely lock the compressor.

“If you’re dealing with a tell-tale internal overload, the unit is drawing high amps, and you walk up to the compressor and hear a humming noise when it attempts to start up, it could be a locked compressor,” Orr said. “One cause may be that oil has migrated out of the compressor. Using a hard-start kit can get the oil flowing properly. In some cases, you have to leave the hard-start kit on to keep it working, and that’s not a good sign. This is a sign that the system may not have a lot longer — its days are numbered. If the compressor will not unlock, you must replace it. Do your due diligence to get it unlocked. I’ve gone to some pretty extreme measures to unlock a compressor. When it’s locked, you want to show a customer that it’s locked. Any compressor that’s locking — that’s not a good sign.”


Orr suggests techs utilize a high-quality multimeter or even a megaohmmeter when troubleshooting compressors.

“I often use megaohmmeters because they use a higher voltage than typical multimeters,” he said. “A typical meter may give you a false negative.”

Orr said a megaohmmeter is most useful when utilized over a period of time.

“You really need to check a megaohmmeter’s reading against itself,” he said. “Theoretically, you could go there and see what the ohm reading is to ground and check it a year later. If the ohm reading is decreasing, it’s a sign that the windings are breaking down, the oil has moisture in it, and the unit is contaminated. It’s a meaningful measurement at that point. You can’t simply walk up to a system that reads in the middle range and recommend a replacement compressor. You can potentially say there are signs that something is going on and recommend a line drier, but you can’t request a new compressor simply because the reading is slightly lower than what you’re used to seeing.”

Orr also noted that some compressor manufacturers recommend against using megaohmmeters, especially on scroll compressors.

“In these cases, a good multimeter will do the job,” he said.

Fox said his go-to tool is a standard multimeter.

“I want to replace the unit or not replace it — does it meet specs or not?” he said. “I’m not going to use a megaohmmeter to verify a compressor is going bad or is on its way out. When a compressor fails to meet specs, that’s when we pursue a replacement.”

Matta also prefers to use a multimeter when troubleshooting a faulty compressor. Megaohmmeters are great for long-term monitoring, though we never want to tell a customer he or she needs to replace a compressor prematurely, he said.

“I also like to have an acid test kit handy, which is helpful in pointing us in the direction of what the best fix may be,” continued Matta. “Additionally, a hard-start kit can be useful. If a compressor is a little off on an ohm test, a hard-start kit can sometimes get it going. That’s a good time to talk to the homeowners and let them know that while the unit may get them through the hot season, it may not last much longer.”


One of the worst mistakes you can make in a refrigeration system is to diagnose a compressor as failed when it has not failed.

“You have to learn and understand the theory behind how compressors work and apply it,” Orr said. “There are a lot of guys in the field who never learn the theory. They’ve always applied it but have never really understood why, which is a problem.”

Fox said techs often need to slow down and be patient when working within a condensing unit.

“Don’t jump to conclusions,” he said. “Some techs might condemn a compressor when it’s really a tripped internal overload switch. It’s important to touch that compressor and see if it’s hot. You never want to replace a compressor and later learn it was just an open internal overload switch. When that occurs, you’re back to square one, because you’ve replaced the compressor, and the system is still faulty.”

Honesty is a virtue that goes a long way when dealing with compressors, Matta said.

“Unfortunately, in our industry, there are a lot of companies out there that will use any form of scare tactics to sell customers anything they can,” he said. “When they’re well maintained and the system is designed properly, compressors should last 20 years or more — the entire life span of the unit. And, when they go bad, it’s up to us to make sure we understand why and only replace them when necessary.”   

Publication date: 3/26/2018

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