how much low voltage at the contactor on ac unit



Low voltage at the contactor in an AC unit can be a common issue that many homeowners encounter. When the contactor, a vital component responsible for activating the compressor and condenser fan motor, does not receive sufficient voltage, it can lead to malfunctions and inadequate cooling. Understanding the causes and troubleshooting methods related to low voltage at the contactor is crucial in maintaining the efficiency and performance of your AC unit. In this article, we will delve into the various factors that contribute to low voltage at the contactor and provide insightful solutions to rectify the problem.

Common Causes of Low Voltage at the Contactor:

1. Electrical Supply Issues:

Electrical supply problems often contribute to low voltage at the contactor. If the voltage from the electrical panel is below the nominal rating required for the AC unit, it can result in reduced voltage at the contactor. This can be due to inadequate wiring, loose connections, or insufficient power supply from the utility provider. It is crucial to troubleshoot the electrical supply system to identify any potential issues that may be causing low voltage.

One common method to diagnose electrical supply problems is to use a multimeter to measure the voltage at the contactor terminals. If the voltage reading is significantly lower than the rated voltage, the electrical supply system should be thoroughly inspected by a qualified electrician. Additionally, checking the circuit breaker and fuses connected to the AC unit is essential to ensure they are not damaged or tripped, causing low voltage issues.

2. Faulty Contactor:

A faulty or worn-out contactor itself can also be a potential cause of low voltage. Over time, contactors can experience wear and tear due to continuous usage, resulting in reduced voltage transfer. If the contactor's points become burnt or pitted, it can obstruct the flow of voltage, leading to low voltage at the contactor.

To determine if the contactor is faulty, visually inspect the contactor for any signs of damage or wear. If the contactor points appear burnt, blackened, or corroded, they may need to be cleaned or replaced. It is advisable to consult an HVAC professional to safely handle the inspection and replacement of the contactor if required.

3. Defective Capacitor:

Capacitors play a crucial role in providing a boost of electrical energy to start the compressor and fan motor. When a capacitor fails or becomes defective, it can inhibit the contactor's ability to receive the necessary voltage. A faulty capacitor can lead to low voltage at the contactor and subsequently affect the overall operation of the AC unit.

Testing the capacitor with a capacitance meter can help determine if it is functioning properly. If the capacitance value is significantly lower or if the capacitor shows signs of physical damage, it is necessary to replace the defective component. Capacitor replacement should be carried out by a qualified technician to ensure safety and proper installation.

4. Wiring Problems:

Issues related to the wiring within the AC unit can contribute to low voltage at the contactor. Loose connections, damaged wires, or improper wiring can impede the flow of voltage, resulting in reduced voltage levels reaching the contactor. It is crucial to inspect the wiring connections and ensure they are tightly secured and free from any damage.

Check the wiring for any signs of wear, corrosion, or loose connections. If any issues are identified, they should be promptly addressed. It is recommended to hire a licensed HVAC technician to handle any complex wiring problems in order to avoid safety hazards and ensure the correct installation of new wiring if required.

5. Thermostat Malfunctions:

The thermostat serves as the control center for the AC unit's operation. If the thermostat malfunctions, it may not send the proper signals to the contactor, resulting in low voltage. Thermostat issues can include faulty wiring, temperature sensor problems, or incorrect settings.

Start by checking the thermostat display for any error codes or abnormal readings. Verify that the thermostat settings are appropriate for the desired cooling operation. If the thermostat appears to be functioning incorrectly or shows signs of physical damage, it may require professional assessment and potential replacement.

How to Troubleshoot and Solve Low Voltage Issues:

1. Check Voltage at the Contactor:

To troubleshoot low voltage at the contactor, begin by turning off power to the AC unit at the electrical panel. Then, using a multimeter, set it to measure voltage and carefully measure the voltage at the contactor terminals. Compare the reading with the rated voltage specified on the AC unit's label. If the voltage reading is significantly lower than the specified rating, the issue lies within the electrical supply system.

2. Evaluate the Electrical Supply System:

If the voltage reading at the contactor is low, it is necessary to inspect and troubleshoot the electrical supply system. Start by examining the circuit breaker connected to the AC unit. Ensure it is not tripped or showing signs of damage. If the circuit breaker has tripped, reset it to see if the low voltage issue resolves. If the breaker continues to trip, there may be an underlying electrical problem that requires professional assistance.

Examine the wiring connections at the electrical panel and the AC unit, ensuring they are secure and free from any corrosion or damage. Loose or damaged wiring can result in voltage drop and lead to low voltage at the contactor. Tighten any loose connections and replace any damaged wiring as necessary.

If all electrical connections appear to be in good condition, it is advisable to contact a licensed electrician to inspect the electrical supply system further. They have the expertise to diagnose any underlying electrical issues and make the necessary repairs or replacements.

3. Inspect and Replace the Contactor:

If the voltage at the contactor is within the specified range, the contactor itself may be faulty and contributing to the low voltage issue. Visual inspection of the contactor can provide valuable information about its condition. If the contactor points are burnt, pitted, or show signs of damage, it is recommended to replace the contactor.

Replacing a contactor involves turning off the AC unit's power, removing the wires connected to the contactor, and unscrewing it from its mounting bracket. Take note of the wiring connections before removing them to ensure correct reinstallation of the new contactor. Install the new contactor and reconnect the wires according to the labeled positions. It is advisable to consult the AC unit's manual or seek professional assistance if unsure about the replacement process.

4. Test and Replace the Capacitor:

A faulty capacitor can also lead to low voltage at the contactor. To troubleshoot the capacitor, discharge it by using an insulated screwdriver to short the terminals. Then, using a capacitance meter, test the capacitor's capacitance value. If the measured capacitance is significantly lower than the rated value, the capacitor should be replaced.

Replacing the capacitor involves turning off power to the AC unit, discharging the capacitor, and removing the wires connected to it. Take note of the wiring connections before removing them. Install the new capacitor, ensuring it is connected correctly, and tighten the wires securely. Again, if unsure about the replacement process, consult a professional.

5. Verify and Reset the Thermostat:

If low voltage issues persist, it is necessary to inspect the thermostat for potential malfunctions. Start by checking the thermostat display for any error codes or abnormal readings. Ensure the thermostat is set to the correct cooling mode and the desired temperature is programmed accurately.

Verify that the thermostat is correctly wired and that all connections are secure. If any wiring issues or thermostat malfunctions are identified, it is advisable to consult an HVAC technician to assess the problem further and potentially replace the thermostat.


Low voltage at the contactor on an AC unit can be caused by various factors such as electrical supply issues, faulty contactors, defective capacitors, wiring problems, and thermostat malfunctions. Troubleshooting these issues requires careful inspection, testing, and, if necessary, replacement of the faulty components. It is important to remember that dealing with electrical components carries inherent risks, and it is always recommended to seek professional assistance when unsure or uncomfortable with DIY troubleshooting. By addressing low voltage issues promptly, homeowners can ensure their AC units operate efficiently and maintain optimal cooling performance. Regular maintenance and periodic inspections by qualified technicians can help prevent low voltage issues and increase the longevity of the AC unit.


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