Sudden cessation of operation of a refrigeration unit often occurs by surprise, causing people to panic about food spoilage. However, the cause of a breakdown is not always an expensive compressor or a freon leak. Quite often, a small protective element fails, which costs a penny, but performs a critical function - thermal relay. It is this component that protects the engine from overheating and voltage surges, opening the circuit at critical levels.
Understanding how this unit works and how to diagnose it will allow you to save a significant amount on calling a technician. In most cases, the test takes no more than 15–20 minutes and requires only a basic set of tools. If you are ready to approach the matter carefully and follow safety precautions, you will be able to independently determine the malfunction.
In this article we will analyze in detail the device of the start-up relay, consider the symptoms of its failure and provide a step-by-step testing algorithm using a multimeter. You will learn to distinguish a burnt winding from mechanical sticking of contacts and understand when it is really necessary to replace a part.
Operating principle and device of the PZR
Thermal relay, often called a start-up protection relay (PZR), is an electromechanical device installed directly on the compressor housing. Its main task is to control the current flowing through the motor windings. Inside the case there is a bimetallic plate that bends under the influence of heat generated by the passing current. If the load exceeds the permissible limits, the plate is deformed and opens the electrical circuit, stopping the motor.
In addition to thermal protection, the device is often combined with a trigger mechanism. At the moment of startup, the compressor requires a powerful impulse to break the rotor from its place. The starting relay (often electromagnetic) closes the additional winding for only a split second. After the engine has picked up speed, the current drops and the contacts open, transferring the unit to normal operation. Modern models can have a solid-state structure, where the role of a switch is performed by a posistor.
Structurally, the device looks like a black plastic box with three contacts for connecting to the compressor and two for supplying voltage from the network. Inside there are moving and fixed contacts, springs and that same sensitive plate. The tightness of the housing is usually low, which makes the unit vulnerable to moisture and corrosion, especially if the refrigerator was in a damp room.
⚠️ Attention: There may be a mercury sensor inside the relay (in very old models) or simply dangerous oxides. Do not break the device body with a hammer and do not try to solder the contacts without completely dismantling the internals, as the plastic may melt.
Main signs of a relay malfunction
Diagnostics should begin long before disassembly, simply by listening to the operation of the refrigerator. A characteristic click, followed by a humming noise and a subsequent knocking sound that fades away, often indicates that the engine is trying to start, but the protection is instantly activated. This phenomenon is called "cyclic starting". The compressor hums for a few seconds, then a shutdown click is heard, and there is silence until the next cycle.
Another obvious symptom is the strong heating of the compressor housing in the lower part, where the relay is attached. If you touch a metal tube or the motor body itself (be careful, you can get burned!) and feel that it is hot to 80–90 degrees, but the refrigerator is not cooling, most likely the current is not passing to the working windings due to burnt out relay contacts.
It is also worth paying attention to the external condition of the part. If during a visual inspection (after removing the casing) you find traces of soot, melted plastic, or smell a persistent burning smell, then checking with a multimeter may no longer be necessary - the device has burned out. Sometimes the contacts inside simply oxidize, which leads to high resistance and constant heating.
- 🔊 Characteristic clicks: periodically turning the motor on and off at intervals of 1-3 minutes.
- 🔥 Overheating: the compressor is hot, but cold does not accumulate in the chambers.
- 💡 Lack of reaction: the refrigerator is silent, the light is on, but the engine does not start.
- 💨 Smell: the appearance of the smell of burnt wiring in the area of the engine compartment.
Necessary tools for diagnostics
To conduct a high-quality test, you do not need complex industrial equipment. A standard DIY kit is sufficient. The main tool will be multimeter (tester). It will allow you to measure the resistance of the windings and check the integrity of the circuit. Without this device, diagnostics will only be tentative.
You will also need screwdrivers (usually Phillips and flathead) to remove the protective casing and dismantle the relay itself. Some models, such as Indesit or Atlant, may require pliers to carefully remove clamps or terminals. Do not forget to prepare a dry rag to clean the contacts from dust and possible oxides.
It is important to ensure good lighting of the work area, since the engine compartment is often dark. If you have a flashlight or the ability to use your phone to illuminate it, this will make it much easier to inspect the contacts for carbon deposits. Before starting any work, make sure that the refrigerator is unplugged from the power supply.
☑️ Preparation for testing
Step-by-step instructions for checking with a multimeter
The diagnostic process begins with safe access to the assembly. Remove the back cover of the refrigerator (if there is one) and locate the compressor. Carefully remove the plastic casing covering the wire connection point. You will see a box placed on three pins of the compressor. This is what we need to extract. Usually it is fixed with a metal bracket or simply sits tightly on the contacts.
After removing the relay, remove it from the compressor housing. To do this, you may need to slightly loosen it or press out the locking tabs. Carefully inspect the device: if the plastic is blackened or there is a burning smell coming from the contact hole, the probability of a malfunction is close to 100%. Next, switch the multimeter to resistance measurement mode (Ohm) to a limit of 200 Ohm.
Now you need to ring the contacts. Relays usually have input contacts (from the power cord) and output contacts (to the compressor). Close the tester probes to the input contacts - the device should show zero or a value very close to it, which indicates the integrity of the circuit. If the screen shows “1” or “OL” (open), it means that the thermal protection has burned out or the contacts have oxidized so much that they do not conduct current.
The starting part is checked separately. Turn the relay over: in the correct position (arrow up) the circuit should be open. If you turn it over (arrow down), the contacts should close with a click, and the tester will show zero. If this does not happen, the mechanism is jammed. Also check the resistance between the case and the contacts - it must be infinite, otherwise it will break through to ground.
Nuances of checking posistor relays
In starting relays with a posistor (solid-state), checking with a multimeter can be uninformative in a cold state, since they show low resistance, and increase it when heated. It is better to check them by replacing them with a known-good analogue or by applying short-term voltage through a fuse.
⚠️ Attention: Do not measure resistance with a refrigerator plugged in! This will lead to instant failure of the multimeter and may cause an electric shock.
Table of typical values and faults
For ease of diagnosis, you can compare the results obtained with typical values. Remember that resistance may vary slightly depending on the ambient temperature and the specific engine model Danfoss or Secop. However, the order of the numbers must be preserved.
| Test type | Normal reading | Malfunction symptom | Probable cause |
|---|---|---|---|
| Thermal contact resistance | 0 – 3 Ohm | Infinity (break) | Bimetallic plate burned out |
| Starting contact resistance | 0 Ohm (closed) | Infinity or high resistance | Scale on contacts, jamming |
| Check for breakdown (body-contact) | Infinity (1) | Any numerical value | Breakdown insulation, moisture inside |
| Resistance of the compressor windings | Starting > Working | Equal or open | Malfunction of the compressor itself |
If your measurements match with the column "Symptom of malfunction", then the part requires replacement. Please note that even if the resistance is normal, if the relay does not click when turned over, its mechanical part is broken. In such cases, operation is impossible.
Causes of failure and prevention
Why does a seemingly serviceable device stop working? The most common reason is power surges in the network. A sharp surge of current causes instant heating of the bimetallic plate, and it is deformed irrevocably. That is why voltage stabilizer is a mandatory element for protecting expensive equipment.
The second reason is the natural wear of contacts. Every time you turn on the refrigerator, a spark jumps between the contacts. Over time, carbon deposits form on the surface, which increases resistance. The contacts begin to heat up even at normal current, which leads to false protection activations or, conversely, to sticking.
The third reason lies in the compressor itself. If the motor is hard to spin (wear of bearings, thickened oil, wedge), it consumes a current higher than the rated one. The relay honestly tries to protect the circuit and burns out. In this case, replacing the relay will only give a temporary effect - the new one will burn out in a few days or weeks.
- 📉 Low voltage: forces the motor to work with overload, heating the relay.
- 💧 Moisture: water ingress during floor washing or leakage causes corrosion of the contacts.
- 🕸️ Dust: a thick layer of dust on the motor impairs heat transfer, the relay heats up from external heat.
- 🔌 Bad contact: loose terminals at the input heat up and melt the relay body.
Replacement of the part and first start-up
If the check confirms a malfunction, you must purchase an analogue. There are many universal relays on the market, but it is better to look for the original by the catalog number stamped on the body. Installation is carried out in the reverse order of removal. It is important to orient the part correctly: there is always an arrow on the body indicating the “UP” (up) direction. If you place the relay upside down, it will not work.
Push the relay onto the compressor pins until it stops. Make sure the contacts are tight. Connect the power wires to the terminals, observing polarity (although for alternating current, phase and neutral are often not critical, grounding must be connected). Install the protective cover and secure it.
Plug in the refrigerator. You should hear a characteristic start-up click, then the hum of the motor, and after a while - a second click when the starting winding is turned off (if the relay is electromagnetic). The compressor should operate smoothly, without vibrations or extraneous sounds. During the first hour of operation, periodically check the temperature of the relay - it may be warm, but should not become hot.
⚠️ Attention: Technical characteristics of compressors and requirements for the relay may vary depending on the year of manufacture and manufacturer. Always check the marking of the new part with the data on the nameplate of your refrigerator before installation.
Is it possible to repair the thermal relay, or only replace it?
In 95% of cases, the relay must only be replaced. This is a low cost consumable. Attempts to clean the contacts or straighten the plate have a short-term effect, since the calibration of the thermal protection is disrupted. It is not advisable to risk a burned out compressor to save on a relay.
Why does a new relay burn out after a few days?
This indicates a problem not in the relay, but in the system. Either there are constant voltage surges in the network, or, worse, the compressor itself has an interturn short circuit or mechanical wear, consuming increased current. Also, the cause may be incorrect installation (not in the direction of the arrow).
Where can I find the markings for purchasing an analogue?
The markings are usually applied to the side or top edge of the black plastic case. Look for codes like RT, R, 3012 and amperage numbers (eg 1.2A, 1.4A). Also, the part number can be found in the technical documentation of the refrigerator or on specialized forums for the model.
Is it dangerous to operate a refrigerator with a faulty relay?
Yes, it is dangerous. If the relay does not perform a protective function, the compressor winding may burn out during overload, which will lead to expensive repairs or the need to replace the entire unit. In addition, sparking contacts can cause a wiring fire.
Is a relay from another brand of refrigerator suitable?
Yes, if the basic parameters match: rated current, maximum starting current, type of fastening and location of contacts. Many manufacturers use universal components from third-party suppliers, such as Danfoss or Secop.