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Manasota Air Conditioning Contractors Association

An Autopsy of a Compressor

Thursday, October 12, 2023 12:46 PM | Anonymous

Diagnosing the Failed Compressor

Picture this scenario: It's the end of a long day, and you arrive at your final call to find a malfunctioning HVAC unit. You hope it's a simple issue like a faulty capacitor, but after testing, you realize the compressor may be the culprit. To confirm this, several crucial checks need to be performed.

Photo Credit: Panchita Chotthanawarapong

1. Verify High Voltage and Capacitor: Check the high voltage from the contactor, ensuring it meets specifications. Additionally, ensure the capacitor produces the correct microfarads stated on the capacitor by measuring with a multimeter.

2. Assess Compressor Windings: For permanent split capacitance (PSC) compressors, measure the ohm values of the common, run, and start windings. The sum of the ohm values from common to run and common to start should match the run to start reading. For three-phase compressors, all windings should have similar ohm values.

3. Test for Resistance to Ground: Check the resistance between each compressor pin and ground using a multimeter. Any reading less than one megaohm indicates an electrical failure. Consider using a megohmmeter for more accurate results.

4. Amp Draw and Locked Rotor Amp Rating: Compare the measured amp draw of the compressor to the locked rotor amp rating stated on the unit's data plate. If the amp draw exceeds the rating, the compressor has failed to start.

Determining the Cause of Failure

While electrical tests can identify compressor issues, mechanical failures also occur. To determine if the compressor has mechanically failed, follow these steps:

1. Confirm Proper Voltage and Windings: Ensure the compressor receives the correct voltage, the windings are within manufacturer specifications, and there is no path to ground.

2. Check Start Components: Verify that the start components are functioning correctly, as a faulty component can prevent the compressor from starting.

3. Assess Temperature Change: Measure the temperature difference between the compressor's inlet and discharge pipes. If the temperatures are similar, it indicates compressor bypassing or internal mechanical issues.

4. Verify Phasing (for Three-Phase Compressors): Check the phasing using a phase meter in commercial units. Switching two incoming power wires can help identify if the compressor fails due to out-of-phase supply power.

The Wrap Up

By diligently performing these diagnostic tests and investigations, HVAC technicians can accurately determine the causes of compressor failures. Understanding electrical and mechanical aspects allows for better decision-making when replacing compressors, reducing callbacks, and ensuring customer satisfaction. Remember, thorough compressor autopsies are crucial to maintaining the integrity of HVAC systems and preserving professional reputations.


This article is adapted from the CE HVAC Tech Tips podcast hosted by CE Customer Assurance Managers. Available on Apple Podcasts and Spotify.

Jack Cauffman and Brooks Whitson

Note: All the information shared in this article is intended for licensed HVAC professionals. Only trained and qualified personnel should design, install, repair, and service HVAC systems and equipment. All national standards and safety codes must be followed when designing, installing, repairing, and servicing HVAC systems and equipment. The Contractor is responsible for meeting local codes, standards, and ordinances.

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