How to Tell If an AC Compressor Is Bad: A Copeland Compressor Field Checklist

Before You Condemn the Compressor

I work as a quality/compliance manager for a refrigeration equipment supplier. I review every compressor shipment before it goes to a customer—roughly 600 units a year. In our Q1 2024 quality audit, I rejected 2.4% of first deliveries because the model, electrical data, or quoted scope did not match the order.

This checklist answers two questions. How to tell if an AC compressor is bad—and what to check before you blame it. It is written for Copeland compressor models in commercial refrigeration, AC, and refrigerated air dryer applications.

My experience is based on about 600 mid-to-large orders per year. If you're working on small hermetic AC compressors or mobile refrigeration, some electrical details will be different.

Here's the 7-step checklist I use.

Step 1: Identify the Copeland Compressor Models on the Nameplate

The model number encodes the compressor family, capacity, voltage, phase, and refrigerant. I know it is tempting to order the same one we used last time, but one letter difference can mean a different motor protection or starting method. Write down the full model number, serial number, electrical characteristics, and refrigerant.

Take a photo of the nameplate and the wiring diagram before you do anything else. If the unit is single-phase, pull the Copeland compressor wiring diagram single phase for that exact model from the service manual. Do not rely on a diagram from another model that looks similar. It will cost you time.

Looking back, I should have insisted on a photo of the nameplate before we released an order to a customer. At the time, the part number looked identical. It was not. The compressor was one digit off, and the start components were different.

According to AHRI Standard 540, compressor capacity and power are published at standard rating conditions. That means you should compare replacement models using the same conditions, not by body style or eyeball tonnage.

Step 2: How to Tell If an AC Compressor Is Bad (Without Guessing)

You can't tell by touching the shell. A hot shell only tells you the overload has been running. The reliable way starts with electrical checks.

1. Winding resistance. Measure between the start, run, and common terminals. Compare the readings to the service manual for that model. For a single-phase motor, the run winding usually reads lower than the start winding. An open pair can mean a burned winding or an internal overload that needs time to cool before you re-test.
2. Insulation resistance. Use a megohmmeter. The reading between the terminals and the shell should be above 1 MΩ on a used compressor. A lower reading means the winding is compromised. (Honestly, this is the test most technicians skip.)
3. Amp draw. If the compressor starts but keeps drawing locked-rotor amps, and the voltage is within nameplate range, that is a bad compressor. If it never starts, check the start circuit first.

Until 2022, we did not have a formal compressor failure reporting form. The third time I saw a compressor returned as no start that was actually a failed start relay, I created this diagnostic checklist. Should have done it after the first time.

Step 3: Pull the Copeland compressor wiring diagram single phase for your model

Most single-phase compressor failures I audit are not compressor failures. They are electrical failures around the compressor. A PTC relay can fail, a run capacitor can open, a contactor can weld.

Before you order a compressor, verify the voltage at the contactor, the relay resistance, and the capacitor microfarad rating. Use a capacitor tester. An old capacitor can look fine and still test far outside its rating. A universal capacitor might fit physically but be wrong electrically.

If you have any doubt, get the exact Copeland compressor wiring diagram single phase. It will show you where the overload, relay, start capacitor, and run capacitor belong. It is a checklist, not a suggestion.

For the branch circuit, check the conductors and overcurrent protection against NEC Article 440. This matters especially when someone has already repaired the wiring once.

Step 4: Check the System Before You Blame the Compressor

A compressor can trip and still be fine. The symptoms that mimic compressor failure are: dirty condenser coil, dead condenser fan, restricted filter-drier, overcharge, undercharge, and non-condensables.

In one Q1 2024 audit, a technician replaced a Copeland scroll compressor because of high head pressure. The condenser fan motor was dead. The old compressor was still good—though it had been overheated long enough to darken the oil. That was an expensive mistake: compressor, refrigerant, labor, and a second visit.

Check condenser airflow. A household circulation fan—even a Woozoo fan—is not a condenser fan replacement. It might move air across a desk, but not the 3,000+ CFM a condensing unit needs. If the condenser coil is dirty, clean it. If the motor is dead, replace it.

This is also the time to check the refrigerant charge with superheat and subcooling. A compressor is part of a system. A locked-out system is not always a locked-out compressor.

Step 5: Check Moisture and Acid If the Compressor Failed

If the compressor is truly bad, assume the system is contaminated. Motor burnout overheats the refrigerant and oil. If you don't remove the decomposition products, the new compressor will die early. This applies to a standard refrigeration system and to a refrigerated air dryer with a small Copeland compressor.

Replace the liquid-line filter-drier with a high-acid-capacity core. If the burnout was severe, replace the expansion valve and flush the evaporator and condenser. Use an acid test kit if you have one. It costs about $20. The second compressor costs a lot more.

One replacement compressor failed in eight months because the contractor skipped the filter-drier. That is not a mystery failure. That is a process gap.

Step 6: Start It Up and Measure at Stabilized Conditions

After a replacement, verify the wiring against the diagram and run the system for at least 20–30 minutes. Take a performance log: suction pressure, discharge pressure, liquid line temperature, superheat, subcooling, and amp draw.

A new compressor can be damaged quickly by an overfeeding TXV or a blocked evaporator. If the pressures are not stable, don't sign the job. It's tempting to close the panel and head to the next call. The next call will still be this unit.

In my opinion, a diagnostic log is more useful than a listen to it test. Sound gives you clues; pressures give you answers.

Step 7: Ask for a Line-Item Quote Before You Authorize the Replacement

The transparency rule I live by: ask what's NOT included before what's the price. The quote should list the compressor model number, start components, wiring harness, refrigerant, filter-drier, recover/reclaim, freight, and disposal of the old compressor.

The vendor who lists all fees upfront—even if the total looks higher—usually costs less in the end. A low number that grows on the invoice is not a better price; it's a different price. If a supplier refuses to put these line items in writing, that is a red flag.

From my perspective, the total cost of a compressor replacement is the written scope plus every fee that appears later. You want the written scope to be the final scope.

Common Mistakes I See in Quality Audits

Three mistakes show up again and again:

• Ordering a compressor by photo instead of by model number.
• Replacing a compressor without checking the start components or capacitor.
• Ignoring the filter-drier after a burnout.

There is also the cheaper part trap. It might save $40 on a relay or a capacitor. In one audit, that $40 substitution cost $1,400 in additional labor and refrigerant. That is not saving.

If the system is dirty, the compressor is not the problem. If the quote is incomplete, the compressor is not the only unknown. Run the checklist before you take a compressor out of service, and make sure the invoice tells you the whole story before you put one back in.