How to Check an AC Compressor

Co-authored by Victor Belavus

Last Updated: October 13, 2024 Fact Checked

An air conditioning compressor creates cool air by circulating refrigerant through your AC unit. If your AC isn’t working, there’s a good chance that a faulty compressor could be the culprit. You can diagnose problems with your compressor by using a multimeter, or a device that can measure volts, amps, and ohms. Once you determine what the problem is, contact a heating and air specialist or an automotive mechanic to get the problem resolved.^{[1]
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U.S. Department of Energy
Official site for the U.S. Department of Energy, which provides resources related to energy safety, conservation, and efficiency

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Method 1

Method 1 of 3:

Inspecting a Home AC Compressor for Damage

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1
Turn off the power leading to your air conditioning unit. It’s important that your power is off so that you don’t shock yourself when checking the compressor. Many central air units will have a power shutoff switch on the wall next to the unit. Find the switch and switch it to the off position.^{[2]
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- If there is no switch, find the circuit breaker that controls the power to the area of the house where the AC unit is. Flip the circuit to the off position to turn off the power.
- You can unplug the AC to shut down the power if it’s a window unit.
To inspect the compressor, you’ll have to remove the faceplates of the unit to see the internal components. One side of the unit will have a faceplate that has ventilation holes and screws attaching it to the unit. Use a Phillips head screwdriver to remove the screws and place them in a safe place for later. Repeat the process on the top of the unit as well. Once all the screws are removed, you should be able to pop out the faceplates to reveal the inside of the unit.^{[3]
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Once you remove the faceplate, you’ll need to remove another panel to unveil the wiring of the AC unit. Use a screwdriver and loosen the screws on the access panel, just like you did AC the faceplates. Once they are removed, pop out the access panel to reveal the unit’s wiring.^{[4]
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4
Look for damage to the wires and compressor itself. The compressor will look like a cylindrical metal tank. Examine the compressor and make sure that none of the wires are frayed or damaged. If you do notice frayed or damaged wires running from the compressor, the wires are likely what’s causing it to fail. Take notice of the compressor itself. If there is visible damage to the compressor, you will most likely need to replace it.^{[5]
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- If you need to repair your compressor’s wires, you should call a certified air and heating specialist to replace them. This will be cheaper than replacing the compressor itself.
The terminals are the metal nodes that the wires connect to and are usually on the side of the compressor. Make sure that there are no burns or damage to the terminals. If there isn’t visible damage to the terminals or wires, you’ll have to test the terminals with a multimeter to make sure that they still can hold a current.^{[6]
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Carey Brothers, Home Improvement and Maintenance Experts

An AC compressor is the heart of the air conditioning system, pulsating with the refrigerant flow. Like any heart, it needs to be checked regularly to ensure it's healthy and functioning properly. A malfunctioning compressor can lead to a domino effect of issues in your AC system. So, don't ignore the signs. Regular maintenance and prompt attention to potential problems can save you from a hot and uncomfortable situation.

Method 2

Method 2 of 3:

Testing an Exposed Home AC Compressor

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1
Wait for the unit to cool down. Touch the top of the AC unit with the palm of your hand. If the unit is still warm, it may throw off your multimeter settings. Wait until the AC unit completely cools down before trying to test the voltages.^{[7]
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- Remember to make sure that there isn’t any power running to the unit or you’ll shock yourself.
There should be a dial on the front of your multimeter. Set this dial to continuity so that you can test each individual terminal to determine where the fault is. Continuity tests whether there is an electrical flow between the terminals. If there is no flow, it’s likely that the terminal is broken or damaged.^{[8]
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Find the terminals marked C, R, and S. Your compressor should have three terminals on it or in the access panel. The terminals should be marked C, R, and S.^{[9]
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- These letters stand for common, run, and start.
Place the pins on the black and red wire from your multimeter onto the terminals. Once you place the pins on the terminals, your multimeter should have an ohm reading of less than 30. Anything significantly above this reading indicates that your compressor may be broken.^{[10]
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Repeat the process, this time checking the continuity between the common and run terminals. Again, the continuity should read under 30.^{[11]
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The final test is between the run and start terminals.The continuity between these terminals should be under 30.^{[12]
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If your ohm readings are above 30, it means that your compressor motor is likely faulty and you’ll need to get it replaced. A heating and air specialist may be able to repair the broken compressor motor instead of having to buy an entirely new compressor.^{[13]
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If your ohm readings are near 0, it means that the connection between your terminals is faulty and you’ll need to get a replacement compressor. If you want to make sure that this is indeed the case, refer to a heating or air specialist to get a second opinion.^{[14]
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Method 3

Method 3 of 3:

Checking a Car’s AC Compressor

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Turn off your vehicle. Turn off the engine so that the moving parts under your hood are stationary. If your engine or internal parts are hot, wait for them to cool down before checking your compressor.
The compressor clutch will usually be located under your hood on the left side near the front of your car. It will look like a cylindrical metal piece with tubes and wires running from it.
Compressors are prone to rust on older vehicles. Check the compressor itself for holes or rust damage. You should also check the tubing and wires to make sure that they aren’t disconnected, frayed, or damaged. If they are, you can replace them more cheaply than it would cost to get an entirely new compressor.^{[15]
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There should be wires running from the top or side of the compressor. The white wire is your live wire that carries electric current, while the black wire is your ground wire. Press on the sides of the plastic fittings and pull on the wires to remove them. This should reveal the contacts that you’ll use to test the compressor.^{[16]
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Set your multimeter to continuity and place the pins on the contacts. Setting the dial on your multimeter to continuity will test if current is properly running through your compressor clutch.^{[17]
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If you get a reading of 0 ohms, it means that your wires are bad and you’ll have to get them replaced. If your reading is over 5 ohms, it most likely means that you have a bad coil, and your diodes will need to be checked by a mechanic.^{[18]
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Connect your negative, or black battery cable with the grounded contact in the compressor. Then, attach the black wire to the negative side of the battery. Connect the positive, or red wire, to the positive side of the battery. Touch the metal end of the wire to the live compressor clutch wire. This should engage the clutch back and forth. If the clutch does not move or make a noise, it means there is an issue with the contacts or the clutch itself.^{[19]
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