A sudden stop of the compressor or, conversely, its endless operation often indicates a failure in the temperature control system. Thermal relay (thermostat) is the main sensor that commands the motor to turn on or off depending on the degrees inside the chambers. If this element fails, the refrigerator ceases to perform its main function, which can lead to food spoilage or defrosting of the freezer.
Before calling a technician, it is worth carrying out an initial diagnosis, since in many cases the fault can be identified independently using a simple tool. Mechanical thermostats and electronic sensors have different principles of operation, but their general signs of failure are similar. Understanding exactly how the unit works will allow you to save time and money on diagnostics.
In this article we will analyze in detail the testing algorithm, the necessary tools and typical mistakes made during repairs. Safety when working with electrical appliances comes first, so all manipulations are carried out only after the device is completely disconnected from electrical networks. Carefully following the instructions will help you accurately determine whether the thermostat is to blame for the problem.
Main signs of a thermostat malfunction
The first step is always to observe the behavior of the refrigeration unit. If you notice that the equipment is not working correctly, you should pay attention to the characteristic symptoms indicating thermostat. Often a breakdown is disguised as other problems, so it is important to analyze the combination of symptoms.
The most obvious signal is the compressor refusing to start or its operation without stopping. In the first case, it becomes warm inside the chambers, in the second, the products may freeze to the back wall, and ice may appear on the floor due to frequent defrosting of the evaporator. Electronic modules controls may generate error codes associated with temperature sensors.
⚠️ Attention: If the compressor hums, but does not start, or starts for a few seconds and immediately turns off, the problem may not be in the thermal relay, but in the start relay or the motor itself.
It is also worth listening to the sounds. A characteristic click when the engine is turned off is the normal operation of the thermostat contacts. If there is no click, or it sounds too often (every 2-3 minutes), this is a sure sign that contact group it is stuck or oxidized. A visual inspection of the capillary tube for damage will also not hurt.
- 🔌 The compressor does not turn on at all, although there is voltage in the network.
- ❄️ The refrigerator works continuously, without turning off for days.
- 🔊 There is no characteristic click when the set temperature is reached.
- 💧 The appearance of ice in the refrigerator compartment or, conversely, complete defrosting.
Preparation for diagnostics and the necessary tools
For a high-quality check, you will need a minimum set of tools that you can find most home craftsmen. The main condition is a refrigerator. Even if you plan to simply remove the cover, the risk of electric shock due to faulty insulation remains high. complete blackout refrigerator. Even if you plan to simply remove the cover, the risk of electric shock from faulty insulation remains high.
The main diagnostic device will be a multimeter (tester). It is necessary for continuity testing and resistance measurement. If you don't have a digital multimeter, a dial multimeter will work, but it's more difficult to read accurate values. You may also need a screwdriver to remove the casing and, possibly, pliers.
☑️ Preparation for testing
It is important to determine in advance the type of your thermostat. Older models (for example Atlant, Biryusa, old Indesit) use mechanical devices with a capillary tube filled with freon. Modern units with a system No Frost and electronic control often have air sensors (thermistors), the testing principle of which is different.
Before starting work, be sure to photograph the location of the wires. When reassembling, mixed up contacts can lead to a short circuit or failure of the new thermostat. The color coding of wires is not always standard, so a photograph will be your insurance.
Removing the thermostat from the refrigerator
The process of removing the thermostat depends on the design of the refrigerator. On top-freezer models, the thermostat is often located inside the refrigerator compartment, hidden behind a plastic trim or directly under the temperature control knob. You will need to carefully remove the handle and unscrew the mounting screws.
If we are talking about two-compressor models or units like Side-by-Side, access may be difficult. Sometimes partial dismantling of the interior lining is required. Be careful with the capillary tube: it is very thin and can easily be pinched or burst if handled carelessly, which will lead to refrigerant leakage.
After removing the protective casing, you will see the thermal relay unit itself with the wires suitable for it. Usually there are three of them: one goes from the network, one to the compressor, and the third is grounding (casing). Before disconnecting the wires, make sure you remember or mark them with a marker.
⚠️ Attention: The capillary tube of the mechanical thermostat is filled with refrigerant under pressure. Do not bend or pierce it to avoid the release of gas and damage to the cooling system.
To remove the sensor in some models, it is necessary to remove the top cover of the refrigerator or dismantle the lampshade, since the tube may go into the foamed housing. Act slowly, loosening the tube so as not to damage the seals.
Checking a mechanical thermostat with a multimeter
The most reliable diagnostic method is to check the electrical circuit using a tester. Switch the multimeter to resistance (Ohm) or continuity measurement mode. If your device has an audio indication mode, this will greatly simplify the task.
Take two probes and touch them to the contacts where the power cable and the wire going to the compressor were connected. At room temperature and the control knob position is “on” (or minimal cold), the circuit should be closed. The multimeter will show a value close to zero or emit a beep.
Nuances of checking contacts
If, when making a call, the multimeter shows one (infinity), then the circuit is broken. Try lightly tapping the thermostat housing or turning the knob. If the readings have not changed, the thermostat is faulty and requires replacement. Also check the contact with the case (grounding): there should be no resistance between it and other contacts.
Next you need to check the reaction to cold. To do this, place the sensitive part of the thermostat (capillary tube or bellows) in a container with ice water or place it in frost (if it is winter outside, but not extreme). After 2-3 minutes, take the measurement again. The circuit should break, and the multimeter should show a lack of conductivity.
If the contacts are “stuck” and continue to ring even in the cold, or, conversely, do not close at room temperature, thermostat is considered faulty. Such units, as a rule, cannot be restored and require replacement with a similar model.
| Condition | Temperature conditions | Multimeter readings | Output |
|---|---|---|---|
| Normal | Room (+20°C) | 0 Ohm (Sound) | Contacts closed |
| Normal | Cooling (-5...-10°C) | ∞ (No sound) | Contacts open |
| Fault | Room (+20°C) | ∞ (No sound) | Open circuit, motor will not start |
| Fault | Cooling (-10°C) | 0 Ohm (Sound) | Contacts are stuck, the motor will not turn off |
Diagnostics of the electronic sensor (thermistor)
Modern refrigerators use instead of mechanical levers thermistors. These are semiconductor elements whose resistance changes depending on temperature. It is impossible to check them “by ear” or visually; you only need a multimeter.
First measure the resistance of the sensor at room temperature. Record the resulting values. For most refrigerator models (LG, Samsung, Bosch), the normal resistance at +20°C is from 2 to 5 kOhm, but the exact data depends on the specific model.
Then heat the sensor by holding it in your hand, or cool it (for example, by blowing cold air or applying an ice pack). The resistance should change smoothly. If the values remain unchanged or jump chaotically, the sensor is faulty.
It is important to know that for different manufacturers the dependence of resistance on temperature may be direct or inverse. For some, when heated, the resistance increases, for others, it decreases. The main thing here is the very fact of changing the readings. Lack of response indicates failure of the element.
Replacement and calibration of a new thermostat
If diagnostics confirm a breakdown, you must purchase a new thermostat. Look for the markings indicated on the body of the old device. Even if you find an analogue with similar characteristics, the length of the capillary tube should be equal to or greater than the original (the excess can be carefully twisted, but not cut!).
Installation is carried out in the reverse order of dismantling. The sensitive element (cone) must be fixed in a regular place, usually this is a hole in the evaporator or a special clip. Poor contact with the cooled surface will lead to incorrect operation of the refrigerator.
⚠️ Attention: After replacing the thermostat, the refrigerator may operate abnormally for the first few hours. Do not load the chambers with products right away, let the equipment reach the temperature and turn off on its own.
Calibration of mechanical thermostats at home is impossible; they are supplied configured from the factory. If your new thermostat clicks too early or too late, it may be a defective product or the wrong model. In electronic systems, after replacement, you may need to reset errors through the service menu.
Frequently asked questions (FAQ)
Is it possible to temporarily close the contacts of the thermal relay to make the refrigerator work?
Yes, this is possible for a short-term test of the compressor or to save food. However, leaving the refrigerator in this condition is dangerous: it will work continuously, which will lead to freezing of the evaporator, overload of the motor and a possible fire. Use this method only as a temporary measure.
Why does the new thermostat not fit the length of the tube?
The capillary tube must be long enough to go around the evaporator. If it is shorter than the original, the sensing element will not read the temperature correctly. If it is longer, the excess can be carefully laid in rings in the free space, observing the bend radius, but do not cut it off under any circumstances.
How to distinguish a thermostat failure from a freon leak?
If there is a freon leak, the compressor can work for a long time, but the cold will not be created effectively, and the condenser (grid at the back) will barely work warm or cold. If the thermostat breaks down, the compressor either remains silent or drives the cold all the way (everything freezes). The easiest way is to check the condenser tubes: if they are hot along the entire length, but there is no cold, most likely the problem is in the gas, and not in the relay.
How much does it cost to replace a thermal relay?
The cost of the mechanical thermostat itself varies from 800 to 2500 rubles, depending on the model. Electronic sensors may cost more. The work of a replacement technician is usually assessed additionally, but replacing it yourself allows you to save on service costs.