When a household refrigerator stops maintaining the temperature correctly, the owner’s first reaction is often panic or calling a technician. However, in many cases, the reason lies in the failure of the thermostat, which in everyday life is often called rheostat, although technically it is thermostat or thermostat. It is this unit that is responsible for supplying voltage to the compressor depending on the temperature inside the chamber.
Understanding the operating principle of this element and the ability to check it is a key skill for self-repair. A malfunction of the regulator can manifest itself in different ways: from a complete refusal of the compressor to start to endless operation of the unit without turning off. In this article, we will analyze in detail the diagnostic methods, the necessary tools and the procedure for accurately determining the breakdown.
Before starting disassembly, it is important to make sure that the problem is really in the electronics, and not in a freon leak or damage to the door seal. If the compressor runs continuously, but there is no cold, the problem is most likely not in the thermostat, but in the refrigerant circulation system. However, if the unit is silent or turns on too often, diagnosing the regulator becomes a priority.
The operating principle and design of the thermostat
For effective repairs, it is necessary to understand what the device we are checking is. A classic mechanical thermostat is an electromechanical device that opens and closes an electrical circuit. It is based on a sealed bellows tube filled with a temperature-sensitive gas or liquid (freon, chlormethyl). When the temperature inside the refrigeration chamber changes, the pressure in the bellows changes, which leads to mechanical movement of the contacts.
The electrical connection circuit usually includes an input from the mains, an output to the compressor and, often, a separate output to a light lamp or fan. Contact group Inside the device is subject to wear due to constant switching of currents, especially if the starting current of the compressor is high. Over time, contacts can burn, stick or lose elasticity, which leads to malfunctions.
There are also electronic thermostats, where the role of the sensitive element is played by thermal sensor (thermistor). They are checked differently, most often by measuring the resistance at different temperatures. Mechanical models, such as Tam 133 or K-59, are more common in older and budget models of refrigerators and can be checked with a multimeter without complex equipment.
⚠️ Attention: Before starting any work on diagnosing electrical equipment, be sure to Disconnect the refrigerator from the electrical network. Working under voltage is deadly and can lead to a short circuit.
External signs of a regulator malfunction
Diagnostics often begin by observing the behavior of the unit. If you notice that the refrigerator is behaving uncharacteristically, this is the first signal to check the internal components. Symptoms can be obvious or hidden, so it is important to pay attention to any changes in the operating cycle.
Among the most common signs of breakdown are:
- 🔌 The compressor does not turn on at all, although the light inside is on and the network is working.
- ❄️ The refrigerator works without stopping, without turning off even when low temperatures are reached.
- 🔄 The unit turns on and off too often (clocks), unable to withstand normal downtime intervals.
- 🌡️ The temperature in the chambers does not correspond to the set values on the regulator.
It is important to differentiate these symptoms from problems with starting protection relay or the compressor itself. For example, if the engine hums but does not start, and after a few seconds you hear a clicking sound, the problem may be with the relay or motor windings, and not with the thermostat. However, if the silence is absolute, the probability of a malfunction of the control circuit (thermostat) is high.
Another indirect sign is the condition of the products. If everything in the refrigerator compartment is melting, but in the freezer, on the contrary, everything is frozen, this may indicate incorrect operation of the sensor or displacement of the sensing element tube. In such cases, a visual inspection and testing of contacts become necessary.
Necessary tools for diagnostics
To properly check the rheostat (thermostat) of the refrigerator, you will need a minimum set of tools. The main device is a multimeter (tester), capable of measuring resistance and checking the integrity of the circuit (testing mode). Without this device, accurate diagnostics is impossible, since a visual inspection of the contacts inside the case will not give a complete picture.
In addition to the multimeter, you will need:
- 🔧 A set of screwdrivers (phillips and flat) for removing panels and dismantling the assembly.
- 🔦 Flashlight for illuminating the interior refrigerator space and thermostat niche.
- 🧊 A container with water at room temperature and a container with hot water (about 50°C) to check the reaction of the bellows.
- 📝 Marker or stickers to mark the wires before disconnecting (so as not to get confused during assembly).
If you don’t have a multimeter, you can use a simple test (a light bulb with two wires), but this is less safe and does not allow you to measure precise resistance parameters. For electronic thermostats, you may need a table of thermistor resistances for a specific refrigerator model, since their ratings may differ.
Removing the thermostat with your own hands
The process of accessing the thermostat depends on the refrigerator model. In most modern units it is located inside the refrigeration chamber, often hidden by a plastic panel or located in the control unit. In older models, it may be located outside, on the back wall.
Algorithm for dismantling:
- De-energize the refrigerator by unplugging the plug from the socket.
- Remove the plastic cover covering the thermostat. It is usually held in place by latches or screws.
- Carefully remove the temperature control knob. It may fit tightly, sometimes a slight rocking is required.
- Unscrew the fastening of the thermostat housing itself.
- Remove the assembly, paying attention to the capillary tube. Capillary it is very thin and fragile, it should not be bent or damaged.
After removing the device, you need to gain access to the contact group. To do this, remove the protective casing (if any) and disconnect the wires. Extremely important take a photo of the connection diagram or sketch the location of the wires before disconnecting. An error during assembly can lead to a short circuit or failure of the compressor.
☑️ Dismantling procedure
Methods of checking with a multimeter
The most reliable way to check is to use a multimeter in resistance (Ohm) or continuity measurement mode. The essence of the method is to check the closure and opening of contacts depending on the temperature of the sensitive element.
Consider step-by-step instructions for a mechanical thermostat:
- Set the multimeter to continuity or resistance measurement mode.
- Connect the probes to the input and output contacts thermostat (usually these are contacts coming from the network and to the compressor).
- At room temperature, the contacts should be closed (resistance is close to 0, there is a sound signal).
- Immerse the sensitive tube (bellows) in hot water (about 45-50°C). After some time, a click should be heard and the circuit will open (the resistance is infinite, there is no signal).
- Let it cool or immerse it in the cold. The contacts should close again.
If, when the temperature changes, the contacts do not open or close, or the resistance remains constantly high, the thermostat is faulty and requires replacement. It is also worth checking the breakdown on the housing: one probe on the contact, the other on the metal body of the device. The multimeter should not show resistance.
⚠️ Attention: Do not heat the thermostat with an open fire or a hairdryer at maximum power. Sudden overheating can deform the bellows or damage the insulation, making a serviceable part unusable.
Table of typical values and states
For ease of diagnosis, you can use a summary table describing the typical states of the thermostat and the corresponding instrument readings. This will help to quickly interpret the measurement results.
| Thermostat status | Environment temperature | Multimeter readings | Action |
|---|---|---|---|
| Good (normal) | Room (~20°C) | 0 Ohm (Closed) | Leave |
| Good (reaction) | Heated (>45°C) | ∞ Ohm (Open) | Leave |
| Faulty (break) | Any | ∞ Ohm (Always) | Replace |
| Faulty (stuck) | Any | 0 Ohm (Always) | Replace |
| Breakage to the body | Any | There is resistance | Replace urgently |
It is worth noting that for electronic sensors (thermistors) the logic is reverse or depends on the type of resistance (NTC or PTC). Negative temperature coefficient (NTC) thermistors have a drop in resistance as they heat up. For them, you need to check the manufacturer's technical documentation.
Can the thermostat be repaired?
In most cases, mechanical thermostats are non-separable or their repair is not economically feasible. Attempts to clean the contacts or bend the springs have a temporary effect and may disrupt the calibration, which will lead to food spoilage. A complete replacement of the unit is recommended.
Replacement and adjustment of a new regulator
If diagnostics confirm a malfunction, the only correct solution is to replace the part. When purchasing a new thermostat, pay attention to the labeling. The numbers and letters in the code (for example, TAM 133, K-59 L1133) indicate the length of the capillary tube, temperature range and type of fastening. Installing an unsuitable model may lead to incorrect operation of the refrigerator.
Installation process:
Install the new thermostat into the seat, carefully straightening the capillary tube along the standard path. Sensing element tube It must fit tightly to the evaporator or fit into a special socket, otherwise the temperature readings will be incorrect. Connect the wires according to the previously made diagram or photograph.
After assembly and connection to the network, adjustment may be required. Mechanical models sometimes have an adjustment screw under the seal or knob. Turning this screw changes the contact opening temperature. However, the factory setting is, as a rule, optimal, and it is worth interfering with it only if you have a special thermometer and an understanding of the processes.
⚠️ Attention: The characteristics and markings of thermostats may differ from one manufacturer to another. Before purchasing, be sure to check the model of your equipment with a spare parts catalog or a consultant, since the length of the capillary and the type of fastening are critical.
Frequently asked questions (FAQ)
Is it possible to temporarily close the thermostat to make the refrigerator work?
Technically, you can close the contacts by applying voltage directly to the compressor. However, this will cause the refrigerator to run continuously until you manually turn it off. This is dangerous due to overheating of the motor and defrosting of food in the refrigerator compartment. Use this method only for a short-term check of the compressor operation.
Why does the new thermostat not fit the length of the tube?
The length of the capillary tube is a critical parameter. If it is too long, you can carefully twist it into a ring (without bending it!), but you cannot shorten it. If the tube is too short, the sensing element will not reach the measurement area and the adjustment will be incorrect. In this case, you need to look for a model with a suitable length or use an extension casing, if the design allows it.
How to distinguish a thermostat from a defrost sensor?
The thermostat controls the compressor and usually has an adjustment knob inside the chamber. The defrost sensor is located in the freezer compartment, often hidden in the foam layer or on the evaporator, and has no user controls. They perform different functions and are not interchangeable.
What to do if, after replacement, the refrigerator begins to freeze more?
Perhaps a thermostat with a different temperature range is installed. Also check that the bellows tube is installed correctly - if it does not fit snugly against the evaporator, the sensor will “think” the chamber is warmer than it actually is and will cause the compressor to run longer. Try reducing the value on the regulator.