how to test a bad ac contactor


Why Testing a Bad AC Contactor is Important

The air conditioning contactor is an essential component of an AC unit that controls the flow of electricity to the compressor and the condenser fan motor. Over time, the contactor can become faulty due to wear and tear, electrical problems, or environmental factors. A damaged or malfunctioning contactor can lead to various issues, such as reduced cooling performance, frequent system breakdowns, and increased energy consumption.

Testing a bad AC contactor is crucial in diagnosing and resolving any issues before they escalate into more significant problems. By identifying a faulty contactor promptly, homeowners can save money on costly repairs or even full system replacements. In this article, we will explore various methods for testing a defective AC contactor, allowing you to determine if it needs to be replaced or repaired.

1. Signs of a Bad AC Contactor

Before delving into testing methods, it is important to recognize the telltale signs of a faulty AC contactor. Being aware of these signs can help you identify a potential issue and take proper action. Here are some common indicators of a bad contactor:

- Inconsistent cooling: If your air conditioner doesn't provide consistent cooling throughout your home or exhibits uneven cooling patterns, it could be a sign of a failing contactor.

- Frequent system cycling: A contactor that is going bad might cause your AC system to turn on and off more frequently than usual, resulting in higher energy consumption and reduced comfort.

- Loud buzzing sound: When the contactor becomes worn out or damaged, it can emit a buzzing or humming sound. This noise usually occurs when the contactor is energized or when the compressor is running.

- Failure to start: A faulty contactor may prevent your AC unit from starting altogether. If you notice that your AC is not turning on despite having power, a bad contactor could be the culprit.

- Burn marks or melted plastic: Inspect the contactor visually for any burn marks or plastic that appears melted. These signs indicate that the contactor has overheated or experienced an electrical arcing.

If you notice one or more of these signs, it is advisable to test your AC contactor to determine its functionality accurately.

2. Tools Required for Testing

Before commencing the contactor testing process, gather the necessary tools to facilitate a seamless and efficient examination. Here are the essential tools you will need:

- Screwdriver: A multi-bit screwdriver with interchangeable tips will come in handy for removing access panels and loosening screws.

- Multimeter: A digital multimeter enables you to measure voltage, continuity, and resistance. Ensure the multimeter is fully charged or equipped with new batteries to obtain accurate readings.

- Insulated gloves: To prevent electric shock, wear insulated gloves while conducting any electrical tests.

- Safety goggles: Protect your eyes from any potential sparks or debris during the testing process.

Having these tools readily available will save time and ensure a safe testing experience.

3. Turn Off Power and Discharge Capacitor

Before testing the AC contactor or any electrical component, it is crucial to prioritize safety. Always switch off the power supply to the air conditioning unit from the circuit breaker or main electrical panel. This ensures that you are not at risk of electric shock while working on the contactor.

Additionally, it is important to discharge the capacitor to prevent any residual electrical charge. Many capacitors store a significant amount of energy and can deliver a shock even after the power is turned off. Utilize a screwdriver with a plastic handle to short the terminals of the capacitor and safely discharge any stored energy. This step is essential for your safety during the testing process.

4. Testing for Voltage

One of the initial methods to evaluate a bad AC contactor is by testing for voltage using a multimeter. Follow these steps to perform voltage testing:

- Remove the access panel: Use a screwdriver to remove the access panel from the air conditioning unit. This will expose the contactor and other electrical components.

- Set the multimeter: Switch the multimeter to the voltage setting (VAC) on the appropriate range. This setting allows you to measure alternating current (AC) voltage.

- Locate the contactor and terminals: Identify the contactor, which is usually a rectangular or cube-shaped component with multiple wires connected to it. Locate the terminals labeled "L1," "L2," and "C" on the contactor.

- Test for voltage: Place the red probe of the multimeter on terminal "L1" and the black probe on terminal "C." Ensure the probes make good contact with the metal terminals. Record the voltage reading displayed on the multimeter.

- Repeat the test: After noting the voltage reading, remove the probes from terminals "L1" and "C." Repeat the process by testing for voltage across terminals "L2" and "C" using the same procedure. Record the voltage reading for this test as well.

5. Checking for Continuity

Continuity testing is another effective method for inspecting the integrity of an AC contactor. Continuity refers to the electrical connection between two points in a circuit. By checking for continuity, you can determine if the contactor is providing an unbroken path for electrical current. To perform continuity testing:

- Set the multimeter: Switch the multimeter to the continuity or resistance setting (the symbol resembles sound waves). This setting enables the multimeter to measure the electrical continuity between two points.

- Locate the contactor and terminals: Identify the contactor in the air conditioning unit and find the terminals labeled "L1" and "C."

- Test for continuity: Place the red probe of the multimeter on terminal "L1" and the black probe on terminal "C." Ensure the probes make good contact with the metal terminals. The multimeter should emit a continuous beep or display a value close to zero ohms if continuity is present.

- Repeat the test: After completing the first continuity test, remove the probes from terminals "L1" and "C." Repeat the process by testing for continuity between terminals "L2" and "C" using the same procedure.


Regularly testing an AC contactor for functionality is essential to ensure optimal performance and prevent potential issues. By examining the signs of a bad contactor, acquiring the necessary tools, and performing voltage and continuity tests, you can assess the condition of your contactor and take the appropriate steps for repair or replacement.

Remember to prioritize safety by turning off power, discharging the capacitor, and using insulated gloves and safety goggles. If you are uncomfortable conducting these tests yourself, consider contacting a professional HVAC technician to inspect and repair your AC contactor. By taking proactive measures, you can extend the lifespan of your air conditioning system, enhance energy efficiency, and ultimately enjoy a comfortable indoor environment.


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