how to check voltage across contactor in ac unit


How to Check Voltage Across Contactor in AC Unit


The contactor is an essential component of an air conditioning (AC) unit that controls the flow of electricity to different parts of the system. Monitoring the voltage across the contactor is crucial for ensuring the efficient functioning of the AC unit. By regularly checking the voltage, you can identify any issues or malfunctions promptly, preventing potential damage to the system and ensuring optimal performance. In this article, we will walk you through the process of checking the voltage across the contactor in an AC unit, providing step-by-step instructions and valuable insights.

1. Understanding the Contactor in an AC Unit

The contactor is commonly located in the condenser unit of an AC system, which is usually positioned outside the building. It serves as the switch for the main electrical supply, controlling the flow of power to the compressor and condenser fan motor. A contactor consists of a coil, movable contacts, and stationary contacts. When the coil receives current, it creates an electromagnetic field that attracts the movable contacts, closing the circuit and allowing the electrical energy to flow.

To ascertain if there are any underlying issues with the contactor, or the voltage across it, you need to perform a few straightforward checks.

2. Ensuring Safety First

Safety should always be a priority when dealing with electrical components. Before proceeding with any inspections or tests, it is crucial to disconnect the AC unit from the power supply. Locate the electrical panel or circuit breaker and switch off the breaker associated with the AC system. This step ensures that no live current is present, minimizing the risk of electrical shock.

3. Gathering Required Tools

To perform voltage checks, you will need a few basic tools, such as a digital multimeter (DMM) or voltmeter, insulated screwdrivers, and safety gloves. Ensure that the DMM is capable of measuring AC voltage and set it to the appropriate range. This will allow you to obtain accurate readings during the tests. Safety gloves are crucial for protecting your hands during the inspection process.

4. Locating the Contactor

To begin the voltage check, locate the contactor within the condenser unit. The condenser unit is typically a large metal box with a removable panel. Remove the panel carefully, following the manufacturer's instructions if necessary. The contactor can usually be found near the bottom of the unit and is identifiable by its rectangular shape. It is best to inspect the contactor during a time when the AC unit is turned on and running.

5. Conducting the Voltage Check

With the contactor now accessible, you can proceed with the voltage check using the following steps:

Step 1: Put on safety gloves to protect yourself from any potential electrical hazards.

Step 2: Set the DMM to the appropriate AC voltage range. Common ranges are 250V or 500V, depending on the system's voltage.

Step 3: Locate the contactor's coil terminals. These can be found on the sides of the contactor, often marked as "C" for the common terminal, "NO" for normally open, and "NC" for normally closed.

Step 4: Place the voltmeter probes on the contactor's coil terminals. The red probe should be inserted into the terminal marked as "C," and the black probe into either "NO" or "NC." Ensure that the probes make good contact with the metal surfaces of the terminals.

Step 5: Turn on the AC unit using the thermostat or control panel.

Step 6: Observe the voltage reading on the DMM display. The typical voltage across the contactor should range between 24-28 volts for most residential AC systems. If the voltage reading falls within this range, the contactor is functioning correctly. If not, read below for further troubleshooting steps.


In conclusion, checking the voltage across the contactor in an AC unit is a vital maintenance task to ensure the proper functioning of the system. By understanding the components of a contactor, prioritizing safety, and following the step-by-step instructions provided in this article, you can accurately assess the voltage across the contactor. Regular inspections and voltage checks allow for the early identification and resolution of any potential issues, preventing further damage to the AC unit and optimizing its performance. Remember to always follow safety protocols and consult a professional if you encounter any complex problems or are uncertain about any part of the testing process.


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