Modern No Frost and drip defrosting systems operate according to a clear algorithm, the central element of which is the defrost sensor. This small component, often called a defrost thermostat or defrost thermostat, is responsible for controlling the temperature of the evaporator during the defrost cycle. Its main task is to turn off the heating element of the heater when the ice on the evaporator has completely melted, preventing overheating of the system and failure of plastic parts.
If you notice that the refrigerator has stopped freezing, and a “coat” of ice has formed on the back wall of the freezer, most likely the defrost circuit is broken. In 80% of cases, the culprit is precisely defrost thermostat, which stopped signaling the completion of defrosting or did not start it at all. Understanding the physics of the process will help you diagnose the problem yourself, without resorting to an expensive call to a technician.
The operating principle of the device is based on a simple physical phenomenon - a change in the volume of a bimetallic plate or the state of the gas inside the flask when the temperature changes. When the evaporator temperature drops below a predetermined threshold (usually around -10...-15°C), the contacts close, allowing current to pass to the heating element. As soon as the ice melts and the temperature rises to +5...+10°C, the mechanism opens the circuit, stopping the heating. This cyclicity ensures energy efficiency and durability of the compressor.
Design features and types of thermostats
External sensor defrost is a compact cylinder or disk made of copper, brass or plastic, from which two wires extend. Inside the case there is a sensitive element that responds to temperature fluctuations. Depending on the refrigerator model (be it Indesit, LG or Atlant), the design may differ slightly, but the functionality remains the same.
There are two main types of devices used in climate control technology. The first type is mechanical thermostats with a bimetallic plate. They are highly reliable and independent of electronics, but have a limited lifespan of compression-expansion cycles. The second type is electronic sensors (thermistors), which change their resistance depending on the temperature and transmit data to the main control module.
It is important to note that mechanical sensor is often called a “tablet” because of the characteristic flat shape in some models. It works autonomously: even if the refrigerator control board burns out, this element will continue to physically open the circuit when a critical temperature is reached, acting as a fuse. Electronic analogues require a constant supply of voltage to read readings.
Algorithm of system operation defrosting
The defrosting process does not start randomly, but according to a timer built into the control module or a mechanical defrosting timer. Typically the heater cycles every 8, 12 or 16 hours of compressor operation. The duration of the defrosting process itself is not strictly fixed in time, it directly depends on the amount of frozen ice and the efficiency of the defrost temperature sensor defrost temperature sensor.
As soon as the timer gives the command to defrost, the compressor and fan stop. At this moment, current flows to the heating element of the evaporator. The sensor is in a closed state at this time, since the evaporator temperature is low. The heater begins to melt the ice, turning it into water, which flows into the drainage tray.
The critical moment comes when all the ice has melted. The temperature on the surface of the evaporator begins to rise sharply. At this moment bimetallic plate the inside of the sensor bends and opens the contacts. The supply of electricity to the heating element stops. If this did not happen, the heater would continue to operate, melting the plastic air ducts and potentially causing a fire.
⚠️ Attention: If the defrost sensor is “stuck” in the open state, the heating element will never turn on, and the evaporator will become overgrown with ice, blocking air circulation. If it is “stuck” in a closed circuit, the heating element will heat constantly, which is dangerous due to overheating.
Symptoms of a thermostat malfunction
Diagnostics of a breakdown begins with observing the behavior of the refrigeration chamber. Symptoms may not be obvious, so it is important to pay attention to the combination of signs. Most often, users notice a problem when food begins to deteriorate or the freezer becomes warmer than usual.
The main sign of a malfunction is the formation of a snow “coat” on the back wall of the freezer or on the evaporator (if the panel is removed). This indicates that the defrost cycle does not start or is interrupted prematurely. You may also experience constant operation of the compressor without shutting down, since the temperature sensor in the chamber cannot detect the achievement of the set values due to the heat-insulating layer of ice.
Sometimes the refrigerator can behave strangely: it freezes and then stops. This is typical when the contacts inside the sensor burn or oxidize, creating an unstable connection. In such cases, the system may spontaneously switch to defrost mode or, conversely, block it.
- ❄️ There is a thick layer of ice or snow on the evaporator, which interferes with the closing of the door or the movement of the drawers.
- 🔊 The compressor hums continuously, trying to compensate for the lack of cold, but the temperature in the chamber is not falls.
- 💧 Water in the tray under the refrigerator may appear more often than usual if the sensor allows excess heat and the ice melts too intensely.
- 🌡️ The temperature in the freezer rises above -6°C, which leads to softening of the ice cream.
Hidden symptom
Crackling and clicking: Sometimes, before failure, the sensor may emit quiet clicks more often than usual, which indicates that the bimetallic plate is triggered due to unstable temperature conditions.
Diagnostics and testing with a multimeter
To accurately determine the malfunction, a visual inspection is not enough. It is necessary to test the defrost sensor with a multimeter in resistance measurement mode (Ohm). Before starting work, the refrigerator must be disconnected from the power supply and defrosted for 24 hours so that the evaporator warms up to room temperature.
The inspection process is as follows: first you need to gain access to the evaporator by removing the back panel in the freezer compartment. The sensor is usually mounted on the fins of the evaporator or inserted into a special hole. After disconnecting the terminals, connect the multimeter probes to the sensor contacts. At room temperature (above +5°C), a working mechanical sensor should show infinite resistance (open circuit).
Then it is necessary to cool the sensor. You can use a quick defrost spray or simply place it in the freezer for 15-20 minutes. After cooling, the resistance should drop to zero (or close to zero), which means the contacts are closed. If the readings do not change when the temperature changes, defrost thermostat needs to be replaced.
| Sensor condition | Environment temperature | Multimeter readings (Ohm) | Diagnosis |
|---|---|---|---|
| Normal | Room (> +5°C) | ∞ (Infinity) | Good (open) |
| Normal | Frost (< -5°C) | 0 (Zero) | Good (closed) |
| Fault | Any | ∞ (Always) | Open circuit (heating element does not heat) |
| Fault | Any | 0 (Always) | Short circuit (heating element heats constantly) |
☑️ Check before replacement
Technology for replacing the defrost sensor
Replacing a defective element is a procedure available to a home craftsman with basic skills. The main rule: the new sensor must have identical temperature characteristics. They are marked on the body (for example, K59-L1102 or DM-6C). Installing a sensor with other parameters will lead to either underheating or overheating of the system.
First remove the old sensor. In some models it is simply inserted into the evaporator tube, in others it is screwed with a bracket. Carefully bite off the old sensor wires closer to the body, but leave enough length for connection. It is better to connect the new sensor through a terminal block or by twisting followed by heat shrink insulation, since soldering can damage the sensitive element due to overheating.
After installing the new component, reassemble the refrigerator in the reverse order. Turn on the equipment and let it run for several hours. It is important to check the first defrost cycle: after 8-12 hours of operation, you should hear the sound of dripping water or hissing, which indicates the ice is melting and the No Frost system is working correctly. Attention: Never install the sensor “by eye” or a universal one without checking the temperature threshold. A difference of even 5 degrees can disrupt the entire operating algorithm of the refrigerator, causing frequent starts of the compressor. No Frost systems.
⚠️ Attention: Never install a sensor “by eye” or a universal one without checking the temperature threshold. A difference of even 5 degrees can disrupt the entire operation algorithm of the refrigerator, causing the compressor to turn on frequently.
Frequent errors during repair and operation
One of A common mistake is to try to “revive” an old sensor by tapping or heating. Mechanical action can temporarily restore contact, but the reliability of such repairs is extremely low. Soon, oxidation or fatigue of the metal will lead to a second breakdown, but at the most inopportune moment.
Another mistake is ignoring the condition of the wire insulation. When replacing a sensor, many people forget to check the wires going to the heating element and timer. If the insulation is melted or the wires are broken, the new sensor will also quickly fail or will not work correctly. Always check the integrity of the entire defrost circuit.
Also, users often confuse the defrost sensor with the air temperature sensor in the chamber. They may look similar, but perform different functions. The first controls the temperature of the evaporator, the second controls the temperature inside the refrigerator. By mixing them up during installation, you will disrupt the thermoregulation settings, and the refrigerator will either stop freezing or will work non-stop.
- 🚫 Using unsuitable materials for insulation (regular tape melts at low temperatures).
- 🚫 Forced defrosting with a knife, which can damage the evaporator tubes next to sensor.
- 🚫 Ignoring cleaning the drainage hole, which creates a false impression of a sensor malfunction.
Is it possible to temporarily remove the defrost sensor and short-circuit the wires?
Theoretically, if you close the wires, the heating element will heat constantly every time the defrost timer is turned on. This will cause the refrigerator to overheat, and the heating element can quickly burn out or melt the plastic. This is an emergency measure that should not be used constantly.
Why does the new sensor not start defrosting?
The reasons may be a malfunction of the defrost timer itself, an open circuit of the heating element, or problems with the control module. It is also possible that a sensor with an incorrect temperature response threshold is installed.
How often do you need to change a defrost sensor?
The service life of a high-quality sensor is 5-10 years. However, if there are frequent power surges in the network or poor quality assembly of the refrigerator, it may fail earlier. Scheduled replacement is not required, only in the event of a malfunction.
Does the type of refrigerant affect the operation of the sensor?
No, the defrost sensor only reacts to the surface temperature of the evaporator. The type of freon (R600a or R134a) affects the pressure in the system and the operation of the compressor, but not the algorithm of the defrost thermostat.