If you’re an HVAC technician or a DIY enthusiast, you know that compressors are one of the most important parts of an air conditioning system. A malfunctioning compressor can cause your AC unit to stop working altogether or lead to other costly issues. In such a scenario, it’s crucial to know how to ohm out a compressor. By measuring the resistance of the compressor windings, you can determine if the compressor is working correctly or if it’s time to replace it. In this article, we’ll take a closer look at how to ohm out a compressor and ensure that your AC runs like a charm.

1. Understanding the Basics of Compressor Ohm Testing

Compressor ohm testing is an essential diagnostic tool for HVAC professionals, especially when dealing with cooling systems that aren’t functioning efficiently. Testing the compressor’s resistance, or ohms, can help identify potential issues within the system, such as faulty windings or damaged wiring, preventing further damage and costly repairs.

What is Ohm Testing?

When measuring ohms, you’re testing a device’s electrical resistance, which refers to the opposition of the flow of electrical current within a circuit. Ohm tests can provide information on the integrity of the device’s conductors. In the case of compressor ohm testing, it helps detect issues such as short circuits, open circuits, and ground faults.

Why is it Important to Test a Compressor?

Testing your compressor’s ohms can reveal potential problems before they cause significant damage or total system failure. It enables you to identify the root cause of your system’s issues, whether due to a compressor failure or another part of the system. By isolating the problem, you can reduce the risk of unnecessary repairs and replacements, which can be costly.

What do Compressor Ohm Readings Tell You?

Compressor ohm readings can interpret the following:

  • High-Resistance Readings – indicating damaged insulation, issues with motor windings or damaged terminals.
  • Low-Resistance Readings – could be interpreted as short circuits between motor windings, or continuity between motor windings and the compressor case.
  • Zero-Resistance Readings – this indicates a complete circuit between the windings and the case, which can cause over-amperage and eventually overheating.

Overall, is an essential skill for HVAC technicians and can help diagnose and resolve issues within the system quickly. With the right tools and knowledge, you can prevent costly repairs and prolong the lifespan of your system.

2. Essential Tools for Accurately Ohming Out a Compressor

Before you start ohming out a compressor, make sure you have all the right tools necessary to get an accurate reading. Here are some of the essential tools you need to effectively ohm out a compressor:

A Multimeter

A digital multimeter is an essential tool for ohming out compressors. It is used to measure resistance, voltage, and continuity. With the help of a multimeter, you can easily determine if the motor windings are shorted out or open. Having a multimeter will help you get a more accurate reading of the compressor’s resistance.

A Properly Rated Insulated Screwdriver

You must have a properly rated insulated screwdriver to avoid electrical hazards. Make sure the screwdriver is rated as per your local safety standard and can handle the voltage being measured. A properly rated insulated screwdriver can help you avoid electrical shock when testing the compressor’s windings.

A Wiring Diagram

A wiring diagram is necessary to understand the electrical connections of the compressor. This diagram will help you to identify where to connect the multimeter leads to test the compressor’s electrical windings. It can also help to determine if any other components like start or run capacitors, relays, or overload protectors are connected to the compressor.

With these tools in hand, you can effectively ohm out the compressor and identify any issues that may be harming its performance.

3. Step-by-Step Guide to Ohming Out a Compressor: Preparation, Safety, and Testing Procedure

Ohming out a compressor is a necessary process that an HVAC technician must undertake to determine the compressor’s internal condition. This process will help you identify potential issues before they lead to severe damage. To ohm test a compressor, you will need proper safety equipment, testing tools, and basic knowledge of electrical circuits. In this section, we will take a closer look at the step-by-step guide to ohming out a compressor, starting with the preparation stage.


Before testing the compressor, you must turn off the electrical power supply. This way, you avoid electrical shock and prevent any damage to the testing equipment. Work with an assistant to ensure they don’t turn on the power supply accidentally while you work.


Safety should always be your top priority while performing any diagnostic tests on HVAC equipment. Start by putting on rubber gloves and protective goggles. Inspect the compressor for any damages, such as oil leaks or swollen capacitors. If you notice any visible damage, do not continue with the testing procedure. Instead, advise the owner to replace the compressor immediately.

Testing Procedure

Now you are ready to start the testing process. Begin by setting your multimeter to the Ohms (Ω) function. Touch one of the multimeter wires to a clean metal surface on the compressor body to check for grounding. Touch the other multimeter wire to each of the compressor’s terminal pins, and record the readings.

Next, remove the wires that link the compressor to the capacitor and take a reading between each terminal pin of the compressor. Record all readings that you get and compare them to the manufacturer’s specification. If any of the results are out of the recommended range, the compressor may be faulty.

In conclusion, Ohming out a compressor requires proper preparation, safety measures, and testing procedures. Follow the steps outlined above to ensure maximum safety and accuracy and to identify potential issues with the compressor. Remember that safety always comes first, and if you encounter any visible damages, do not continue with the testing procedure.

4. Troubleshooting Faulty Compressors Using Ohm Testing Results

Once you have conducted the ohm test on your compressor, you can obtain useful information about the system’s health. Having accurate readings can provide clues for troubleshooting the problem.

High Resistance Readings

If your ohm readings show abnormally high resistance, it is an indication that your compressor is not working efficiently and is about to fail. This can cause damage to the system as well as consume more electricity than necessary. To fix this problem, you may have to replace the compressor.

Low Resistance Readings

Low readings indicate that your compressor is experiencing an electrical short circuit, due to a damaged insulation layer. This can cause the compressor to overheat and damage the system. It is recommended to replace the unit in this situation.

Intermediate Resistance Readings

Intermediate readings indicate that a portion of the compressor’s internal circuit is damaged or failing. You can follow some basic troubleshooting steps to identify and resolve the issue. First, isolate the compressor from the system and test again. If the readings remain the same, the problem is within the compressor. If not, the problem is within the system. You can use a process of elimination to find the faulty component.

In conclusion, ohm testing is an effective method of troubleshooting your compressor’s health. Proper readings will provide insights into the cause of inefficient performance. If you are unsure or inexperienced with these procedures, seek assistance from a professional technician. Remember to adhere to safety procedures while working with electrical components.

5. Tips to Avoid Common Mistakes When Ohming Out a Compressor:

Ohming out a compressor is a sophisticated task that requires precision and attention to detail. Even a tiny mistake can lead to erroneous readings and faulty diagnosis. Therefore, it’s essential to follow a few tips and best practices to avoid common errors when ohming out a compressor.

1. Check the Plugs and Cables Before Testing:

Before you connect your multimeter to the compressor, ensure that the plugs and cables are in good condition. Any loose connection, damaged wire, or corroded plug can affect the readings and give you a false result. Also, make sure that the multimeter probes are clean and free of dirt or debris that can impair their conductivity.

2. Be Careful with the Multimeter Settings and Probes:

The multimeter is a sensitive tool that requires proper handling to produce accurate results. When ohming out a compressor, set your multimeter to the appropriate ohm range to match the expected resistance level. Also, be careful when inserting the probes into the compressor terminals, making sure not to touch the outer shell or the adjacent terminals.

3. Verify the Ohm Reading with the Compressor Specifications:

After you obtain the ohm reading, cross-check it with the compressor specifications to ensure that it falls within the expected range. Different compressors have different resistance values, so it’s crucial to know the specs of your compressor to compare the readings accurately. If you get a reading that deviates significantly from the designated range, double-check your connections, retest, and verify before proceeding with any troubleshooting or repairs.

6. When to Replace a Compressor Instead of Attempting Repair through Ohm Testing

It’s essential to understand that compressor ohm testing isn’t the ultimate solution to all compressor-related problems. There will come a time when ohm testing won’t suffice, and the only option left is to replace the compressor entirely.
While ohm testing can identify possible issues with a compressor, it doesn’t reveal its root cause. Here are tell-tale signs when you should replace your compressor instead of attempting repair through ohm testing:

1. High Amperage: If your compressor is registering a high amp draw, it is a good indicator that the compressor is not functioning efficiently. High amperage for an extended period results in overheating and possible damage to the compressor’s internal parts.

2. Burnt Smell: If you detect a burnt smell coming from your compressor, it is best to shut the unit down immediately. A burnt smell usually indicates that the motor windings have overheated or the insulation has burned off. These are catastrophic failures that are beyond repair and require a replacement compressor.

3. Refrigerant Leak: If there is a refrigerant leak, it indicates that the compressor’s internal parts have failed and corroded. Such a leak is visible in compressor oil stains, ice buildup near the compressor, or hissing sounds. A replacement compressor is the only viable option in such scenarios.

Remember that attempting to repair an already failing compressor may not work, even with ohm testing. It’s best to consult a professional HVAC technician if you experience any of the above mentioned problems. They can help you diagnose and solve compressor problems efficiently and quickly.

People Also Ask

1. Why is it important to ohm out a compressor?

Ohming out a compressor is important to determine if the compressor is functioning properly or if it needs to be replaced. By measuring the resistance of the compressor, you can identify if there are any open or short circuits, which can affect the performance and efficiency of your HVAC system.

2. What tools do I need to ohm out a compressor?

To ohm out a compressor, you will need a multimeter, which measures voltage, current, and resistance. You will also need a set of electrical leads, which connect the multimeter to the compressor terminals.

3. How do I ohm out a compressor?

To ohm out a compressor, you must first isolate the compressor from the rest of the HVAC system. Then, using the multimeter, you measure the resistance across the compressor terminals. You should refer to the manufacturer’s instructions for specific guidelines on how to ohm out your particular compressor.

4. What readings should I expect from ohming out a compressor?

The ohm reading should be in the range specified by the manufacturer. If the reading is zero, it indicates a short circuit, while if the reading is high, it indicates an open circuit. Either of these problems will require a compressor replacement.

5. Can I ohm out a compressor without turning off the power?

No, it is not safe to ohm out a compressor without first turning off the power to the HVAC system. Working on live electrical equipment can be dangerous and could result in injury or death.


Ohming out a compressor is a simple but important task that can help you determine if your compressor is functioning properly or if it needs to be replaced. By using a multimeter and following manufacturer’s instructions, you can measure the resistance of the compressor and identify any open or short circuits. It is essential to turn off the power before starting this task to avoid any potential hazards.