How to ring the refrigerator thermal relay: complete instructions

A sudden stop of a refrigeration unit often causes panic, especially if there are perishable foods inside. However, the cause of a breakdown is not always an expensive compressor or a freon leak. In most cases, equipment failure is associated with the failure of a small but critical part - a thermal relay. This component is responsible for safe engine starting and overcurrent protection.

Diagnostics of this part does not require complex equipment and deep knowledge of electrical engineering. It is enough to have a basic set of tools at hand and multimeter. Understanding the operating principle of this unit will allow you to independently determine its performance and avoid unnecessary costs of calling a technician. In this article we will analyze the verification process in detail.

Before taking active steps, it is necessary to ensure safety. Working with electrical appliances requires complete disconnection from the network. Never measure resistance or circuit integrity while the refrigerator power cord is plugged into the outlet. This can lead not only to damage to the measuring device, but also to electric shock.

Principle of operation and protective device mechanism

A thermal relay, often called a start-up relay, is an electromechanical device that controls the supply of voltage to the windings of an electric motor. Its main task is to briefly apply current to the starting winding to create torque, and then turn it off, leaving only the main winding to work. In parallel with this, it performs a protection function: if the current in the circuit exceeds permissible limits, the device opens the contacts, preventing the motor from burning out.

There are two key systems inside the case. The first is a trigger device, often based on heating a bimetallic strip or using a posistor. The second is a current protection mechanism that responds to the amount of current passing through the coil. When overloaded, the plate heats up, bends and physically opens the power circuit. After cooling, the contacts should close again, allowing the compressor to start.

There are various types of such devices, for example, series RT, RTP or foreign analogues from Danfoss. Despite the design differences, the principle of their diagnosis remains similar. It is important to understand that the malfunction can lie either in sticking contacts, or in a break in the heating element or deformation of the bimetal.

What is the difference between a posistor and a bimetallic relay?

A posistor is a semiconductor element, which changes its resistance when heated. When cold, it passes current, and when heated, it becomes a dielectric, breaking the circuit. A bimetallic relay works by physically bending two welded metals with different expansion coefficients. PTC resistors are considered more reliable and durable.

Preparation for diagnostics and necessary tools

To carry out a quality test, you will need a minimum set. The main tool will be multimeter (tester), which can be purchased at any electrical goods store. Make sure that the device is in working order and its battery is charged, as the readings may be incorrect if the battery is dead. You will also need a screwdriver to remove the back panel of the refrigerator and dismantle the compressor casing.

Before starting work, the refrigerator must be completely defrosted and disconnected from the power supply for at least 2-3 hours. This is necessary so that the insides of the compressor and the protective mechanism itself cool down to room temperature. If you ring a hot relay, the readings may be incorrect due to thermal expansion of the metals inside the case.

Prepare your work area. You will need good lighting, as connectors and contacts may be hidden in the shadows of the case. It would be a good idea to have a camera or smartphone on hand to take a picture of the wiring diagram before disconnecting them. This will help to avoid errors during assembly.

Removing the relay from the refrigerator compressor

The process of removing the part requires care. First, remove the rear refrigerator cover that covers the engine compartment. It is usually secured with bolts or snaps. After removing the cover, you will see a black cylindrical or pear-shaped compressor housing, to which a bundle of wires fits.

A plastic casing is attached directly to the compressor terminals, inside of which the required relay is located. In some models, for example, in old refrigerators Biryusa or Atlant, this unit can be combined with a capacitor. Carefully bend the metal brackets or unscrew the screws securing the casing.

⚠️ Attention: When removing the relay, be extremely careful. The contacts inside can be oxidized, and the plastic can become brittle over time. A sudden movement can break the plastic clips or break the wires, which will complicate further repairs.

After releasing the fasteners, carefully pull the relay down, removing it from the three pins of the compressor. Do not use excessive force: if the part does not come off, check to see if you forgot to disconnect any additional terminals or wires going to the fan or sensors. Visually inspect the removed element for burning, melting or cracks.

If traces of soot are visible on the housing or a burning smell is felt, this is a sure sign that a short circuit or overheating has occurred inside. In this case, continuity testing may no longer be necessary - the part requires mandatory replacement. However, even with external well-being, the internal electrical circuit can be disrupted.

Visual inspection and search for external defects

Before picking up the tester, conduct a thorough visual analysis. Inspect the relay housing from all sides. The presence of even microscopic cracks may indicate that moisture or dust has gotten inside, causing the insulation to fail. For devices operating in vibration conditions, the integrity of the housing is critical.

Pay attention to the contacts. If you see a green coating of oxidation or black deposits on them, this will increase the resistance in the circuit and may cause the refrigerator to not operate correctly. In some cases, the contacts can be carefully cleaned, but if the plastic around them is melted, it is better not to risk it and purchase a new assembly.

Shake the relay next to your ear. If you hear a ringing or knocking noise from parts coming off inside, this means the internal components are destroyed. The whole relay should not make any sound when shaken. Also check the condition of the springs and bimetallic plates, if the design allows them to be seen without destroying the housing.

Testing technology with a multimeter: step-by-step algorithm

We move on to the most important stage - electrical testing. Switch the multimeter to resistance measurement mode (indicated as Ω or 200 ohms). If your device has a “dialer” mode with a sound signal, use it - it’s more convenient. First, connect the tester probes to each other to make sure it is working: the device should show zero or make a squeak.

Identify the contacts on your relay. Usually there are three or four. Two pins are responsible for the main power circuit (passing through the bimetallic strip), one for the starting winding, and sometimes there is a fourth pin for ground or a capacitor. To accurately determine the contacts, it is better to find a diagram of a specific model (model) on the Internet, but the general principle is the same.

Attach the probes to the input and output contacts of the main circuit. In a cold state (and the relay must be cold!) the circuit must be closed. The multimeter will show a resistance close to zero (0–5 ohms) or beep. If the device shows one (infinity), it means that open circuitoccurred, and the relay is faulty - current will not flow to the motor.

Next, check the contacts of the starting circuit. Here the situation may be different. In some relays (with a posistor), the circuit is closed in the cold state, in others it is open. However, the key point is that there is no short circuit. If the tester shows 0 Ohm where the load should be, or the relay body rings, this is an insulation breakdown.

Pay special attention to checking for “breakdown” on the housing. Place one probe on any contact and the other on the metal part of the relay housing or on the compressor pins (without touching the contacts). The device should not respond. Any resistance reading indicates a violation of the insulation, which is deadly during operation.

Analysis of results and table of faults

Interpretation of the received data is the final diagnostic step. If the multimeter shows an open in the main circuit, the compressor simply will not start, the refrigerator will not cool, and the light bulb inside may not even light up (if the circuit breaks before it). This is the most common breakdown caused by a burnt-out bimetallic plate.

If the circuit rings, but the refrigerator still does not work or hums and clicks without starting, the problem may not be in the relay itself, but in the compressor (turn-to-turn short circuit of the windings) or in the starting capacitor. In this case, the relay may be working properly, but it cannot perform its function due to a motor malfunction.

A common mistake is to ignore the state of the contacts. Even if the relay rings, the burnt contacts create a high contact resistance. This leads to heating, sparking and repeated failure of the new part. Therefore, if the contacts look suspicious, it is better to replace the entire assembly.

Let's look at the main symptoms and their probable causes in the table below. This will help compare the behavior of your refrigerator with the measurement results.

It is worth noting that the relay parameters must strictly correspond to the power of your compressor. Installing a relay with a lower current rating will cause it to be permanently disabled. Installation with a large current can lead to burnout of the motor windings, since the protection will not operate in time. Always look for markings, for example 132-137 or 100N, and select an analogue.

⚠️ Attention: The rated current of the relay must match the current indicated on the compressor nameplate, or be within the permissible range. Using “universal” Chinese relays without current selection often leads to repeated failure after a month of operation.

Frequent errors during diagnostics and operation

One ​​of the common mistakes is an attempt to “reanimate” the relay by stripping the contacts or bending the plates. In modern sealed enclosures this is almost impossible to do efficiently. In addition, a violation of the calibration of the bimetallic plate will result in the protection not working at the right time, and the compressor will burn out.

Another mistake is ignoring the reasons that caused the relay to break down. If the relay burns out, it means there was an overload. By simply replacing a part, you risk repeating the situation. Check whether the refrigerator's condenser is dirty, whether the fan (if any) is working, and whether the refrigerator is too close to the wall, which interferes with heat transfer.

Also, users often forget that after replacing the relay, the refrigerator needs time to stabilize. You shouldn't load it with products right away. Let the unit idle for 4-6 hours to ensure the correct on and off cycles.

In conclusion, regular maintenance and careful attention to the first signs of malfunction (frequent switching on, extraneous sounds) will extend the life of your refrigerator. Self-diagnosis using a multimeter is a useful skill that saves time and money, but requires compliance with safety rules and attention to detail.

Is it possible to temporarily close the relay contacts so that the refrigerator can be completed before the technician arrives?

It is strictly not recommended. Closing the contacts eliminates motor protection. The compressor will start, but if it is overloaded or jammed, it will burn out irrevocably. The cost of a new compressor and its replacement is several times higher than the cost of the relay and calling a technician.

Why does a new relay burn out a week after replacement?

There may be several reasons: the rated current is incorrectly selected (too large or small), the compressor itself is faulty (an interturn short circuit requiring its replacement), or problems with the voltage in the network. It is also possible that the refrigerator condenser is clogged with dust and does not give off heat.

Where can I find the markings for selecting an analog relay?

The marking is applied to the side or top edge of the plastic relay case. Look for numbers, for example, "RTK-1-3", "100N", "117N". The parameters can also be found on the nameplate of the compressor itself (Amp or LRA columns). Based on these data, an analogue is selected in spare parts catalogues.

You can hear a crackling sound inside the relay during operation, but the refrigerator gets cold. Is this normal?

A slight click when turning on and off is normal (bimetal operation). Constant crackling, buzzing or sparking indicates poor contact, internal structure failure, or vibration of a loose component. It is better to replace such a relay before it completely fails.