How to Test Defrost Thermostat: A Step-by-Step Guide

Common Problems With The Defrost System

One of the most common issues that homeowners face with their domestic refrigerators is a faulty defrost system. Two main problems are associated with a malfunctioning defrost system: the refrigerator/freezer not defrosting and the freezer being cool but the fridge staying warm. When the defrost system fails, ice builds up on the evaporator coils, preventing proper cooling and airflow. This can lead to spoiled food and an inefficient refrigerator. The potential culprits behind these issues include a defective defrost heater, defective defrost thermistor, defective thermostat, or faulty defrost timer.

Troubleshooting The Defrost Heater

To troubleshoot the defrost heater, follow these steps:

1. Put your refrigerator into diagnostic test mode.
2. Check if the heater turns on and if the frost on the coils starts to melt.
3. If the defrost heater fails to activate, there may be a problem with the continuity of the circuit.
4. Make sure the refrigerator’s temperature is below 46℃.
5. Place the multimeter leads on the wires connected to the control board.
6. If there is continuity, the defrost heater is functioning properly.
7. However, if there is no continuity, this indicates a faulty defrost heater that needs to be replaced.

Note: It is important to troubleshoot the defrost heater in diagnostic test mode to ensure accurate results.

• Troubleshoot the defrost heater in diagnostic test mode
• Check if the heater turns on and if the frost starts to melt
• Ensure refrigerator temperature is below 46℃
• Test continuity using a multimeter on the wires connected to the control board

“If there is continuity, the defrost heater is functioning properly. However, if there is no continuity, this indicates a faulty defrost heater that needs to be replaced.”

Testing The Defrost Thermistor

The defrost thermistor is responsible for detecting the temperature inside the refrigerator and sending signals to the control board to initiate the defrost cycle.

To check if the defrost thermistor is working correctly, follow these steps:

1. Remove the temperature sensor from the refrigerator.
2. Inspect it visually for any cracks or breaks.
3. Use a multimeter that can measure at least 20,000 ohms to test the temperature sensor.

The refrigerator service manual provides a chart that gives the exact resistance at a particular temperature for a working sensor.

Test the temperature sensor at:

• Room temperature (around 77℉) – It should read around 5000 ohms.
• In a glass of ice water (just above 32℉) – It should read around 13000 ohms.

If the readings are significantly different or there is no reading at all, the defrost thermistor is most likely defective and should be replaced.

“The defrost thermistor plays a crucial role in the refrigerator by detecting the temperature and initiating the defrost cycle. To ensure its proper functioning:

1. Remove the temperature sensor for inspection.
2. visually check for any cracks or breaks.
3. Use a multimeter that can measure at least 20,000 ohms to test the temperature sensor.

According to the refrigerator service manual, it is important to conduct the test at both room temperature (around 77℉) and in a glass of ice water (just above 32℉). At room temperature, the temperature sensor should read around 5000 ohms, while in the ice water, it should read around 13000 ohms. Any significant deviation from these values or the absence of a reading suggests a defective defrost thermistor that needs replacement.”

Steps To Test The Defrost Thermostat

To test the defrost thermostat, follow these steps:

1. Unplug the refrigerator and disconnect the wires from the thermostat.
2. Use a multimeter to test for continuity.
3. Place the multimeter probes on the thermostat’s terminals.
4. Submerge the thermostat in a glass of ice water.
5. A functioning thermostat should display zero ohms of resistance, showing continuity.
6. If the multimeter reading shows OL or 1, it means there is no continuity.
7. This indicates a failed thermostat that needs to be replaced.

Remember, a working defrost thermostat should show zero ohms of resistance when submerged in ice water.

Note: This test is crucial to determine if the defrost thermostat is functioning properly.

• Unplug refrigerator
• Disconnect thermostat wires
• Test for continuity with a multimeter
• Place probes on thermostat terminals
• Submerge thermostat in ice water
• Zero ohms resistance indicates a working thermostat
• OL or 1 reading on multimeter means no continuity, thermostat has failed.

Replacing The Defrost Thermostat

If testing reveals that the defrost thermostat is faulty, you will need to replace it to restore proper functioning of the defrost system. Here are the steps to replace the defrost thermostat:

1. Remove the mounting screws from the evaporator cover located in the freezer.
2. Gently pull the cover out, being careful not to damage any surrounding components.
3. Unplug the wires connected to the defrost heater.
4. Release the defrost bi-metal thermostat from the evaporator coils.
5. Connect the new thermostat to the supply wires.
6. Attach the new thermostat securely to the evaporator coils.

7. Reposition the evaporator cover and secure it in place with the mounting screws.

8. Ensure the defrost thermostat is fully disconnected and powered off before starting the replacement process.

9. Take note of the placement and position of each wire and component to simplify reassembly.
10. Carefully handle the evaporator cover to prevent any unwanted damages to the surrounding components.
11. Properly secure the new thermostat to ensure it remains in place during operation.

Note: It is crucial to follow the manufacturer’s instructions and guidelines for your specific appliance when replacing the defrost thermostat.

Understanding The Defrost Timer And Its Functions

The defrost timer is an essential component of the defrost system. It is responsible for controlling the timing of the defrost cycles to prevent excessive ice buildup on the evaporator coils. The defrost timer has four pins labeled 1 to 4. Pin 1 is the main input power and, when connected to pin 2, it turns off the compressor and starts the defrost cycle. Conversely, when the timer is not in defrost mode, pin 1 connects to pin 4, activating the compressor and fan for cooling.

To test the defrost timer, you can use a multimeter. When the refrigerator is in defrost mode, there should be continuity between pins 1 and 2. However, in cooling mode, there should be no continuity. If the multimeter reading shows OL or 1 instead of continuity, it indicates that the defrost thermostat has no continuity and may need to be replaced.

In summary, troubleshooting and testing the components of the defrost system are crucial to avoid issues with your domestic refrigerator. By following these step-by-step procedures, you can identify and fix problems related to the defrost heater, defrost thermistor, defrost thermostat, and defrost timer. Regular maintenance and effective troubleshooting techniques will ensure optimal performance of your refrigerator for years to come.

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Frequently Asked Questions

How many ohms should a defrost thermostat read?

To determine the optimal reading for a defrost thermostat, it is crucial to consider the continuity of the circuit. Typically, a multimeter reading between 10 to 150 ohms indicates that the circuit has continuity. This signifies that the defrost heater is functioning adequately. Conversely, if there is no continuity detected in the defrost heater circuit, it suggests a malfunction in the heater. Therefore, the desired reading for a defrost thermostat lies within the range of 10 to 150 ohms, ensuring the proper functioning of the defrost heater.

1. What are the different methods to test a defrost thermostat in a refrigerator?

There are a few different methods to test a defrost thermostat in a refrigerator. One method is to use a multimeter to check for continuity. This involves disconnecting the thermostat from the refrigerator and setting the multimeter to the resistance or ohms setting. The multimeter probes are then placed on the terminals of the thermostat, and if there is continuity (the meter reads zero or close to zero resistance), then the thermostat is functioning properly.

Another method is to manually bypass the thermostat. This involves disconnecting the power to the refrigerator and removing the thermostat. The two wires that were connected to the thermostat are then connected together using a wire or paperclip. If the refrigerator now starts defrosting, it indicates that the thermostat was faulty and needs to be replaced. However, it’s important to note that bypassing the thermostat in this way is only a temporary solution and should not be left like that for long-term use.

2. What are the signs or symptoms that indicate a faulty defrost thermostat, and how can it be tested to confirm the issue?

Signs or symptoms that indicate a faulty defrost thermostat include ice buildup in the freezer, inconsistent cooling, and reduced efficiency of the refrigerator. When the defrost thermostat is not functioning properly, it can cause the freezer to become excessively cold or fail to defrost, leading to the formation of ice on the evaporator coil and inside the freezer. This can result in inadequate cooling in the refrigerator section and cause food to spoil.

To test the defrost thermostat and confirm if it is the issue, you can start by manually advancing the defrost timer into the defrost cycle. This can be done by using a screwdriver to turn the timer clockwise until it clicks into defrost mode. If the defrost heater does not turn on, it may indicate a faulty defrost thermostat. In this case, the defrost thermostat can be tested using a multimeter by checking for continuity. If there is no continuity, it suggests the thermostat is faulty and needs to be replaced.

3. Are there any special tools or equipment required to test a defrost thermostat, and what steps should be followed to accurately perform the test?

To test a defrost thermostat, you may need a few tools like a multimeter, a screwdriver, and possibly a hairdryer. First, ensure the power to the appliance is disconnected to avoid any electrical hazards. Locate the defrost thermostat, typically found behind the rear panel of the freezer or refrigerator. Remove the screws holding the panel using a screwdriver and access the thermostat. Using a multimeter, set it to the ohms resistance setting and test the thermostat by touching its terminals with the multimeter probes. If the reading shows infinite resistance, the thermostat may be faulty and needs replacement. To check the accuracy further, heat the thermostat with a hairdryer and observe if the resistance decreases. If the resistance doesn’t change, it signifies a faulty thermostat that needs to be replaced.

References: 1, 2, 3, 4