How to Test if Your AC Compressor is Bad: A Comprehensive Guide

by Anna

Air conditioning systems are essential for comfort in both residential and commercial settings. At the heart of these systems lies the compressor, often referred to as the “heart” of the AC unit. The compressor is responsible for circulating refrigerant throughout the system, enabling the cooling process. When the compressor fails, the entire air conditioning system can be rendered ineffective. Knowing how to test if your AC compressor is bad can save you time, money, and discomfort. This comprehensive guide will walk you through the steps necessary to diagnose a faulty AC compressor.


Understanding the AC Compressor

Before diving into the testing procedures, it’s important to understand the function and components of an AC compressor. The compressor’s primary role is to compress low-pressure refrigerant gas into a high-pressure, high-temperature gas. This compressed gas then travels through the condenser, where it dissipates heat and converts into a high-pressure liquid. This liquid moves to the evaporator, where it absorbs heat from the indoor air and cools the environment. Without a functioning compressor, this cycle is interrupted, and the AC system fails to cool effectively.


Signs of a Bad AC Compressor

Identifying the symptoms of a bad compressor is the first step in diagnosing the problem. Here are some common signs:

Reduced Cooling Efficiency: If your AC is not cooling as effectively as it used to, the compressor might be failing to circulate refrigerant properly.

Unusual Noises: Grinding, screeching, or clunking noises from the outdoor unit can indicate internal damage or wear and tear within the compressor.

Hard Starting: The compressor struggles to start or takes longer than usual to turn on.

Tripped Circuit Breaker: Frequent tripping of the AC circuit breaker can signal electrical issues within the compressor.

Refrigerant Leaks: Visible leaks around the compressor area or low refrigerant levels without a visible leak elsewhere can point to compressor issues.

Overheating: The compressor unit feels excessively hot to the touch, indicating it is overworking or malfunctioning.

Tools Needed for Testing

To test your AC compressor, you’ll need the following tools:

  • Multimeter
  • Screwdrivers
  • Pliers
  • Manifold gauge set
  • Safety gloves and goggles

Safety Precautions

Working with electrical components and refrigerants can be hazardous. Always follow these safety precautions:

Turn off the power supply to the AC unit before performing any tests.

Wear safety gloves and goggles to protect against electrical shocks and refrigerant exposure.

Ensure the area is well-ventilated if you need to discharge refrigerant.

Step-by-Step Guide to Testing Your AC Compressor

1. Visual Inspection

Start with a thorough visual inspection of the compressor and surrounding components. Look for signs of physical damage, oil or refrigerant leaks, and loose or burnt wiring. Addressing these visible issues can sometimes resolve the problem without further testing.

2. Check for Electrical Integrity

Use a multimeter to check the electrical integrity of the compressor:

Turn off the Power: Ensure the AC unit is completely powered down.

Locate the Compressor Terminals: Find the terminals labeled “C” (Common), “R” (Run), and “S” (Start) on the compressor.

Set the Multimeter to Ohms: Measure the resistance between these terminals.

  • C to R: Record the resistance.
  • C to S: Record the resistance.
  • R to S: The sum of the resistances between C to R and C to S should equal the resistance between R and S. If not, the windings are likely damaged, indicating a bad compressor.

Check for Continuity to Ground: Place one multimeter lead on a terminal and the other on the compressor housing. If there is continuity, the compressor is grounded and should be replaced.

3. Inspect Capacitors

The start and run capacitors are critical for compressor operation. To test them:

Discharge the Capacitor: Use an insulated screwdriver to discharge any residual power.

Remove the Capacitor: Disconnect it from the compressor.

Set the Multimeter to Capacitance Mode: Measure the capacitance and compare it with the rating printed on the capacitor. A significant deviation indicates a faulty capacitor that might be preventing the compressor from starting properly.

4. Measure Refrigerant Pressure

Using a manifold gauge set, measure the refrigerant pressures:

Connect the Gauges: Attach the low and high-pressure gauges to the respective service ports on the AC unit.

Turn on the AC Unit: Run the system and observe the pressure readings.

Low Pressure: Consistently low pressure can indicate a refrigerant leak or a malfunctioning compressor.

High Pressure: Abnormally high pressure may suggest a blockage in the system or a failing compressor that can’t properly circulate the refrigerant.

5. Conduct a Voltage Test

Verify that the compressor is receiving the correct voltage:

Turn on the Power: Restore power to the AC unit.

Set the Multimeter to Voltage Mode: Measure the voltage at the compressor terminals while the system is running.

Compare with Specifications: Ensure the voltage matches the specifications provided in the unit’s manual. Significant discrepancies may indicate electrical issues rather than a compressor failure.

6. Perform a Hard Start Kit Test

If the compressor struggles to start, a hard start kit can temporarily boost its starting capacity:

Install the Hard Start Kit: Follow the manufacturer’s instructions to connect the kit to the compressor.

Test the System: Turn on the AC unit and observe if the compressor starts more reliably. While this isn’t a permanent fix, it can confirm that the issue lies with the compressor’s start function.

When to Call a Professional

While these tests can help diagnose many issues, some problems may require professional expertise. If you’re unsure about any step, encounter complex electrical issues, or suspect refrigerant handling is needed, it’s best to contact a certified HVAC technician. Professionals have the tools and experience to safely and accurately diagnose and repair compressor problems.

Preventive Maintenance Tips

Regular maintenance can extend the life of your AC compressor and prevent future issues:

Annual Inspections: Have a professional inspect and service your AC system yearly.

Clean the Coils: Keep the condenser and evaporator coils clean to ensure efficient heat transfer.

Check Refrigerant Levels: Regularly monitor and top up refrigerant levels if necessary.

Replace Filters: Change air filters every 1-3 months to maintain proper airflow and reduce strain on the compressor.

Monitor Electrical Components: Periodically check capacitors, wiring, and connections for signs of wear or damage.


Testing an AC compressor involves a combination of visual inspections, electrical tests, and pressure measurements. By following the steps outlined in this guide, you can diagnose many common compressor issues and determine if a replacement is necessary. Always prioritize safety and consider seeking professional assistance for complex or hazardous tasks. Regular maintenance and timely interventions can keep your AC system running smoothly and efficiently, ensuring comfort and reliability for years to come.


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