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Refrigerator Troubleshooting: Defrost System


Maintaining and Troubleshooting Domestic Refrigerators: Chapter 3


Refrigerator Troubleshooting: Defrost System Part 1


In this module, we will discuss the common refrigerator problem associated with defrost system. We will also learn how to fix these problems. Skip to quiz!


Diagnosing malfunctions


The most common problems due to faulty defrost system in our refrigerator are:

  • The refrigerator/freezer does not defrost

  • The freezer is cool, but fridge stays warm


The reasons behind the refrigerator defrost problem are:

  • Defective defrost heater

  • Defective defrost thermistor

  • Defective thermostat

  • Faulty defrost timer

Let us take a look at how to troubleshoot each of them.


Defrost Heater


Recall that a defrost heater melts the frost developed over the evaporator coil. To troubleshoot the heater first, check if it turns on when in diagnostic test mode.


We should check whether the frost starts to melt in the test mode. If the frost melts in the test mode, the heater is working fine. Unplug the refrigerator to exit the test mode.


If the defrost heater doesn't work in the test mode, check for continuity through the defrost heater circuit. For checking the continuity of the circuit, the refrigerator temperature should be below 46℃


We need to remove the control panel down to access the electronic control board. Then, check the defrost heater circuit by putting multimeter leads on the wires connected to the control board.


The circuit has continuity if the multimeter reads about 10 to 150 ohms. If the circuit has continuity, then defrost heater is working fine.


If the defrost heater circuit does not have continuity, then defrost heater is faulty. We need to replace the faulty defrost heater.


We need to remove the mounting screws from the evaporator cover to access the defrost heater. Disconnect wiring harness connected to defrost heater. We should slide the defrost heater out and insert a new defrost heater back up under the evaporator.



Defrost Thermistor


Recall that the defrost thermistor sends a signal to the control board to control the defrost heater during the automatic defrost cycle. If the heater is working fine, there might be a problem with the thermistor or control board.


If the defrost thermistor fails, the control board won't control the defrost heating element correctly.


We need to remove the temperature sensor from the refrigerator. Grab the evaporator panel, pull it. Then, disconnect the wiring harness by depressing the locking clip and pulling it out. Next, remove the sensor from the refrigerator.


We need to visually inspect the temperature sensor for any cracks, breaks in the wires. For testing the temperature sensor, we will need a multimeter that can measure at least 20,000 ohms.


The refrigerator service manual contains a chart representing the relationship between temperature, resistance, and voltage. This gives exact resistance at a particular temperature for working sensor.


The temperature sensor is tested at two different temperature levels: One at room temperature and one in a glass of ice water. To test the temperature sensor, touch each of the two probes to each of the two wires of the temperature sensor.


Our room temperature is approximately 77℉. So, our multimeter reading should be in the range of 5000 ohms. If it is not in the range specified on the chart, then the sensor is faulty.


We should insert our temperature sensor into a glass of ice water to test it for other temperatures. With our cup of ice water just above 32℉, we should get a multimeter reading in the range of 13000 ohms. If it doesn't fall in the given range, we should replace it.



The most common problems due to faulty defrost system in our refrigerator are:

  • The refrigerator/freezer does not defrost

  • The freezer is cool, but fridge stays warm


The reasons behind the refrigerator defrost problem are:

  • Defective defrost heater

  • Faulty defrost thermistor

  • Defective thermostat

  • Faulty defrost timer

We will go over troubleshooting of thermostat and timer in the next topic.


 

Refrigerator Troubleshooting: Defrost System Part 2


In this module, we will discuss the common refrigerator problem associated with defrost system. We will also learn how to fix these problems. Skip to quiz!


Defrost Thermostat


Recall that defrost thermostat sends a signal to the control board during the automatic defrost cycle to terminate the defrost cycle. If we didn't get continuity through the heater circuit, there might be a defective bi-metal thermostat.


To test our defrost thermostat, we will need to unplug the refrigerator and disconnect the wires to the thermostat. If the thermostat has continuity, it will function properly. If there is no continuity means the electrical path is broken and the thermostat has failed.


We need to set the multimeter dial to the Ω setting. Since a temperature change activates the thermostat, we need to place it in a glass of ice water for one or two minutes.


We should connect the multimeter probe to one of the thermostat terminals and the second probe to another terminal. If the multimeter reading shows zero ohms of resistance, the thermostat has continuity.


There is no continuity if the multimeter reading shows OL or 1 in the far left digit. This means the thermostat has failed and will need to be replaced.


We need to remove the mounting screws from the evaporator cover. Then, we need to pull the cover out of the freezer. Unplug the wires from the defrost heater and release the defrost bi-metal thermostat from the evaporator coils.


Then, we should connect the new bi-metal thermostat to the supply wires using the wire connectors. We then attach the defrost bi-metal thermostat to the evaporator. Put the evaporator cover in position and reinstall the mounting screws.



Defrost Timer


Recall that defrost timer has four pins labeled from 1 to 4. The main input power is attached to pin 1. When pin 1 connects to pin 2, the timer turns off the compressor, and defrost cycle begins.


When the timer is not in defrost mode, pin 1 connects to pin 4, which activates the compressor and fan. Pin 3 is our ground. For troubleshooting the timer, we should check continuity across these pins.


For the cooling cycle, we should get continuity between pin 1 and pin 4. This is because when the timer is not in defrost mode, pin 1 connects to pin 4. If we do not get continuity, there may be a problem with defrost timer.


We need to rotate the manual dial on the defrost timer to put it into the defrost mode. Then put the multimeter lead on pin 1 and pin 2 to check continuity.


If we have continuity between pin 1 and 2, it means the timer activates defrost cycle. This is because in the defrost mode, pins 1 and 2 get connected. If we do not get the continuity, we need to replace the timer.


Remove the screws that secure the timer to the refrigerator. Remove the wires connected to the timer and take out the old-timer.


Then, we need to reattach the wiring to the time. We then secure the timer back to the refrigerator with screws.



When pin 1 connects to pin 2, the timer turns off the compressor, and defrost cycle begins. When the timer is not in defrost mode, pin 1 connects to pin 4, which activates the compressor and fan. We check continuity across these pins in both the mode to troubleshoot the timer.


If the multimeter reading shows zero ohms of resistance, the thermostat has continuity. If there is no continuity if the multimeter reading shows OL or 1 in the far left digit and the thermostat is defective.


 

Question #1: What is the reason behind the faulty defrost system of the refrigerator? (select all that apply)

  1. Defective thermostat

  2. Defective ice bucket auger

  3. Defective defrost heater

  4. Defective door sensor

Scroll down for the answer...

















Answer: Defective thermostat

Defective defrost heater

The reasons behind the refrigerator defrost problem are:

  • Faulty defrost timer

  • Defective defrost heater

  • Defective thermostat


Question #2: How do we troubleshoot defrost heater of the refrigerator?

  1. By measuring heating time

  2. By checking continuity of the defrost circuit

  3. By checking heater color

  4. By checking heater temperature

Scroll down for the answer...
















Answer: By checking continuity of the defrost circuit

We check the continuity of the defrost circuit to troubleshoot the defrost heater. If the circuit has continuity, then defrost heater is working fine.


Question #3: What resistance value should the multimeter display for the functioning thermistor of the refrigerator?

  1. Less than 100 ohms

  2. A value corresponding to the temperature resistance chart

  3. Infinite

  4. Less than 10 ohms

Scroll down for the answer...
















Answer: A value corresponding to the temperature resistance chart

We should get a resistance value corresponding to the temperature resistance chart on the multimeter for a particular temperature.


Question #4: How do we know there is no continuity in the defrost thermostat of the refrigerator?

  1. Multimeter reading shows more than 100 ohms

  2. Multimeter reading shows 10 ohms to 100 ohms

  3. Multimeter reading shows OL or 1

  4. Multimeter reading shows 10 ohms

Scroll down for the answer...


















Answer: Multimeter reading shows OL or 1

If the multimeter reading shows OL or 1 in the far left digit, then there is no continuity in the defrost thermostat of the refrigerator.


Question #5: The defrost timer should show continuity between pins 1 and 2 for a cooling mode of the refrigerator.

  1. True

  2. False

Scroll down for the answer...
















Answer: False

False. The defrost timer should show continuity between pins 1 and 2 for a defrost mode of the refrigerator.


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