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This article explains the "Top 5 Problems" that can occur on a contactor and how to troubleshoot them! The HVAC Contactor is a device that turns on the outdoor unit compressor and outdoor fan for air conditioners and heat pumps.
First, let me briefly explain what the contactor is and how it works. HVAC contactors typically come as single, double, or triple pole. This refers to the amount of poles it can switch on or off. The switching action occurs when powering the lower coil. The switch is in the normally open position and it closes when the lower coil is powered. The coil may be designed to be powered with 24-volts, 120-volts, or 240-volts. Most contactors installed on outdoor AC and Heat pump units use a 24v coil.
A contactor, regardless of the number of poles or coil voltage, operates in the same manner. They are usually, normally open switches that close when the correct voltage is applied to the lower coil. For example, a system may have a single pole contactor, with one permanently closed pole and one normally open switched pole. When 24 volts is applied to the coil, the normally open contact will close. Similarly, another example is a three-pole contactor with a 120-volt coil. When 120 volts is applied to the coil, the 3 contacts close simultaneously.
HVAC contactors have several ratings posted on them. These are the coil voltage, the FLA, the RES, and the voltage that can cross the contacts. FLA (full load amps) is the max current from an inductive load (such as a motor) that can cross the contacts. RES (resistive amps) is the max current from a resistive load (such as electric strip heaters) that can cross the contacts. The voltage rating of the contacts must be the same or higher than the actual voltage of the system. The coil voltage is the voltage that the coil can be powered with.
Make sure that when replacing a contactor, use the same number of poles (if a compressor crankcase heater is attached), the same FLA or higher , and the same coil voltage. The Max current of the outdoor unit posted on the outdoor unit rating plate must not exceed the FLA of the new contactor.
One method to test a contactor can be performed while the power is off at the system and with the power wires disconnected from the contacts of the contactor. A multimeter is used on its electrical resistance setting. Test if the contactor is operating properly by supplying power to the 24-volt coil with a 24-volt transformer (or power a 120v coil with 120 volts) and see if the contacts close. Connect your test probes, one to each of the contactor. Prior to powering the coil, the multimeter should read OL from one probe to the other across the normally open electrical contacts. After power is applied, the multimeter should read 0.0 Ohms of resistance. This means the contacts are closed and connected. This indicates that the contactor is operating correctly. If this is not the case, then troubleshooting the contactor will be necessary. Below I list five problems that can prevent a contactor from operating properly.
Problem #1:
The contactor is stuck in the closed position.
This can happen if the contacts are welded together due to high amperage melting the contacts together.
This can be tested with a multimeter. With the power off and the wires disconnected from the contactor coil and the contacts, attach the multimeter probes, one on each side of the contacts as shown above. In the example above, the reading is .2 Ohms across the contacts. This verifies it is in the closed position. Since there is no power to the coil, the contactor should not be in the closed position. The contactor will need to be replaced. In addition, take an amp draw on the compressor to see if there is a further problem, to make sure the unit is operating properly.
Problem #2
High electrical resistance is measured across the contacts when closed.
This can result due to pitting on the actual contacts. This is caused by arcing and partial melting at the contact face. This melting causes the contacts not to touch properly when the contacts are supposed to be in the closed position.
In the example above, the contactor is being powered with 24 volts. The reading is 3.2 mega Ohms. This indicates that although the switch is closed, the contacts are not touching. This results in an extremely high resistance reading. The contactor will need to be replaced. In addition, take an amp draw on the compressor to see if there is a further problem, to make sure the unit is operating properly.
Problem #3
Ants, other insect, and cobwebs inside the contractor
Some ants such as the Red Tawny Ant are attracted to the electricity at the contactor. Because of this, they could get stuck between the contacts. Likewise, a spider or a spider web may be present between the contacts. When the switch tries to close, it cannot because the contacts are not able to touch.
Look for cobwebs and other trapped insects like in the picture above. This can result in stopping the contacts from touching. This is a common occurrence, so it is good to be aware of this problem. Newer contactors are shielded (top and bottom), so there is less access to the contactor and the coil. Also an electronic contactor works well, in order to avoid the insects.
Problem #4
Coil burns out due to water or high amperage
If water is present at the contactor coil when it is powered, the coil will short out.
You can determine if the coil is intact or open by measuring on both sides of the coil with a multimeter. Make sure the power is off to the coil, the wires are removed and test measuring electrical resistance in ohms. If the multimeter is reading OL, this stands for "open line" and means that the coil is burnt out. If electrical resistance is measured, the coil is usually intact. Compare the electrical resistance reading to a known resistance on a new contactor of the same ratings and compare if the electrical resistance is too high.
The coil could burn apart due to water being present on the coil or due to high amperage. High amperage could occur if the contacts are only partially down due to an obstruction between the contacts when the coil is powered. If the coil measures OL, this contactor would need to be replaced.
Problem #5
Bad or loose electrical connection or improper voltage going into the contactor coil.
A corroded electrical connection at the spade terminal could result in low voltage on the contactor coil and the coil not closing the contacts. A burned out coil could be another outcome due to low voltage. The wire feeding from the indoor unit may have a bad connection and therefore low voltage at the contactor. Make sure to measure the voltage applied to the coil. This should be between 24 to 29.5 volts. If it is too low, this may result in a burnt out coil. In that case, replace the contactor and fix the electrical connections.
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Published: 06/30/2021 Author: Craig Migliaccio
About the Author: Craig is the owner of AC Service Tech LLC and the Author of the book “Refrigerant Charging and Service Procedures for Air Conditioning”. Craig is a licensed Teacher of HVACR, Sheet Metal, and Building Maintenance in the State of New Jersey of the USA. He is also an HVACR Contracting Business owner of 16 years and holds an NJ HVACR Master License. Craig creates educational HVACR articles and videos which are posted at https://www.acservicetech.com & https://www.youtube.com/acservicetechchannel & https://www.facebook.com/acservicetech/
