How to check the starting relay of the refrigerator compressor

A sudden stop of the refrigerator, the absence of the characteristic hum of the motor, or, conversely, its frantic attempts to start with a loud click - these are sure signs that a failure has occurred in the refrigerant circulation system. Often the cause of such symptoms is not the expensive compressor itself, but a small but critical element of the electrical circuit - the start relay. It is this unit that is responsible for supplying a starting pulse to the motor winding, allowing it to move from a rest state to the operating mode of shaft rotation.

Diagnostics of this part does not require complex laboratory equipment, but it does require basic skills in working with electrical devices and an understanding of safety principles. If you notice that your Indesit, Atlant or any other unit has stopped freezing, and the motor is either silent or hums for several seconds and turns off, you cannot ignore this. Long-term operation in the mode of constant starting attempts can lead to overheating of the windings and final failure of the compressor itself, the repair of which will cost much more than replacing a small plastic box.

Before you begin disassembling the unit, you need to prepare a workplace and tools. You will need a multimeter (tester) to measure resistance, a flathead and Phillips screwdriver, and possibly pliers. In this article, we will analyze in detail the design of the relay, the symptoms of its breakdown and a step-by-step algorithm for checking the integrity of the contacts and coil.

The principle of operation and the design of the trigger mechanism

To carry out a quality diagnosis, you need to understand exactly how this unit functions. The start relay is an electromechanical switch that controls the voltage supply to the start winding of the compressor motor. When the refrigerator is turned on, the motor rotor is motionless, and moving it from its place requires a significant force, which is created by the starting winding. As soon as the shaft reaches the required speed (usually about 75% of the rated speed), the relay should break the starting winding circuit, leaving only the main one working.

Modern household refrigerators most often use induction-type relays with a posistor or electromagnetic mechanisms. In electromagnetic models, the movable core is drawn in under the influence of current, closing the contacts, and when the motor accelerates, the current drops, and the core, under the weight of its own body, opens the circuit. The posistor options work by changing the resistance of the ceramic element when heated: a cold posistor conducts current, starting the motor, heats up from the passing current and practically breaks the circuit, putting the compressor into operating mode.

Structurally, the device is enclosed in a plastic case, which is attached directly to the terminals of the compressor or to the motor frame. Inside there are moving and fixed contacts, an inductor or a posistor, and sometimes an integrated thermal protective relay (thermopart). It is the thermal relay that often acts as a fuse, opening the circuit when the compressor is critically overheated or the rotor is jammed, protecting the windings from combustion.

⚠️ Attention: Inside the relay housing there may be capacitors or other elements that can retain a residual electrical charge. Even after disconnecting from the network, do not touch live parts with your bare hands and avoid short-circuiting the terminals with each other until a visual inspection is carried out.

Understanding the differences between types of relays is important, since the methods for testing them may differ slightly. For example, in posistor models, the key element is the resistance of the ceramic disk, while in electromagnetic models it is the mobility of the core and the state of the contacts. Incorrect interpretation of the tester readings can lead to an erroneous conclusion about the serviceability of the part.

Typical symptoms of a faulty start relay

You can determine that the problem lies in the starting mechanism by a number of characteristic signs that appear during operation of the refrigeration unit. The most common symptom is the so-called “idling”. The compressor begins to hum, trying to start, a loud click is heard, after which the engine goes quiet. After some time, the cycle repeats. This indicates that the starting pulse is either not supplied, or is supplied, but the contacts do not close properly to start the rotor.

Another clear sign is the complete lack of response of the refrigerator when plugged into the network, while the light inside the chamber may light, indicating the presence of voltage. In this case, the thermal relay built into the starting unit is often to blame, which has burned out and broken the power circuit completely. If at the same time the compressor housing remains cold and does not heat up, it means that no current is supplied to the windings at all.

• 🔊 Loud clicks every few seconds or minutes, accompanied by a short hum of the motor, indicate attempts to start without successfully turning the shaft.
• 🌡️ Overheating of the compressor: if the motor is running, but the relay housing or the compressor itself becomes dangerously hot to the touch, this may indicate sticking contacts or a faulty posistor that does not disconnect the starting winding.
• 💡 Operation of automatic machines: in rare cases, a short circuit inside the relay (for example, melting of the contacts) can cause knockout of plugs or circuit breakers in the electrical panel apartments.

It is also worth paying attention to the appearance of the part after removal. If the plastic case has turned black, melted or emits an unpleasant burning smell, then further checking of the electrical parameters may no longer be necessary - the element requires unconditional replacement. A visual inspection often allows you to immediately identify 30% of all possible faults associated with overheating and short circuits.

⚠️ Attention: If the compressor hums but does not start, do not leave the refrigerator plugged in for a long time. Constant starting currents that do not go into operating mode can burn out the motor windings in a short time, which will lead to the need to replace the entire unit.

Dismantling and visual inspection of the part

The inspection process begins with gaining access to the compressor, which is usually located in the lower rear part of the refrigerator. To do this, you need to disconnect the device from the power supply, move the refrigerator away from the wall and remove the rear protective panel (crankcase) covering the engine compartment. In some models, for example, old ZIL or Biryusa, access may be limited by the design of the case, but the principle remains the same: you need to get to the black cylindrical compressor tank.

On On the side or bottom outlet of the compressor (depending on the model), you will see a plastic box placed over three contacts. This is the required relay. It can be secured to a bracket or simply held in place by a tight fit. Carefully remove the clips or bend the bracket, then pull the relay down. It is important not to pull sharply so as not to damage the suitable wires, which may be short.

After removing the device from the engine compartment, perform a thorough visual inspection. Shake the relay next to your ear: if you hear the ringing of falling parts inside, it means that the mechanical part is destroyed and the device must be replaced. Inspect the contacts for oxidation, carbon deposits, or signs of plastic melting. If cracks or melting are visible on the body, this is a sure sign that processes accompanied by high temperature and sparking took place inside.

Pay attention to the markings. The case is usually marked with a model number or manufacturer code (for example PTM-R, RS-4 etc.). Write down this number, since the relays are selected strictly according to the compressor power and the type of refrigerant. Installing a relay that is not suitable for its specifications can lead to repeated breakdowns or incorrect operation of the refrigerator.

Instrumental test with a multimeter

The most reliable diagnostic method is to “test” the contacts using a multimeter switched to resistance measurement mode (Ohm). Before starting the test, make sure that the device is completely disconnected from the network and the wires going to the relay are disconnected to avoid other refrigerator circuits influencing the device readings.

The test begins with determining the type of relay and the corresponding terminals. Typically the relay has three main contacts connected to the compressor: common (C), start (S) and run (R). On the relay itself they can be marked with corresponding letters or numbers. You need to find a pair of contacts responsible for the starting winding circuit (usually this is the power input and the output to the compressor starting contact).

Turn on the multimeter and close the probes to make sure it is working (the device should show zero or make a beep). Then apply the probes to the relay contacts. If the device is working properly, when cold, the resistance of the starting circuit (for posistor relays) should be low, usually in the range of 3 to 50 ohms, depending on the model. If the device shows “infinity” (one on the screen or no reaction), it means that the circuit is broken and the relay does not work.

For electromagnetic relays, it is important to check the mobility of the contacts. Ring the contacts that should close the circuit when starting. In a state of rest (the relay lies horizontally), they can be open or closed depending on the design. Try to carefully turn the relay over or lightly tap on it - in a working mechanism, the contacts should respond to a change in position, which will be visible by a change in readings on the multimeter screen (the appearance or disappearance of resistance).

Pay special attention to checking the thermal relay if it is built into the starting housing. It should “ring” short (resistance is close to zero). If the thermal relay circuit is open, the compressor will not receive power at all. Sometimes the thermal relay can be temporarily bypassed (closed) for testing, but operating the refrigerator without thermal protection is strictly prohibited.

Alternative diagnostic methods without instruments

In situations where you do not have a multimeter at hand, you can resort to indirect testing methods, although they are less accurate. One way is to visually assess the condition of the contacts through transparent holes (if any) or after carefully opening the case (which is not possible on all models). If the contacts are covered with black soot or oxides, you can try to clean them with fine sandpaper or a file, but this is a temporary measure.

There is a method of checking “by ear” and by the reaction of the system, but it requires caution. You can connect the relay to the compressor, turn on the refrigerator and carefully monitor the startup process. If a click is heard, the motor hums for 3-5 seconds and turns off, and the relay heats up noticeably - there is a high probability of its malfunction. However, this method is risky, since if the contacts stick, you can burn the motor winding.

Another option is to check the posistor by heating it. If you have a low voltage DC source (such as a battery and a light bulb in series), you can apply current to the relay contacts. A working posistor should first light up (close the circuit), and then, as it heats up, go out or reduce its brightness (open the circuit). If the glow does not change or is absent altogether, the element is defective.

⚠️ Attention: Experiments with applying voltage to the removed relay should be carried out only by experienced craftsmen. Incorrect connection may result in electric shock or damage to the item being tested. It is safer to use a multimeter.

It is worth noting that the lack of specialized tools should not become a reason for refusing high-quality diagnostics. Even a simple visual inspection and “testing” a battery with a light bulb can provide more information than completely ignoring the problem. However, for the final verdict, the use of a professional measuring instrument remains the most reliable solution.

Replacement and selection of a new starting relay

If diagnostics have confirmed a malfunction, the only correct solution is to replace the element. Starting relays cannot be repaired at home, since their tightness and calibration accuracy are compromised when opened. When purchasing a new part, it is critical that the characteristics match the original. The main parameters are the compressor power (in watts or horsepower) and the diameter of the mounting hole.

Universal relays often come complete with adapters for different types of compressors (Danfoss, Secop, LG, Atlant). When installing, pay close attention to the orientation: the housing usually has a “UP” marking or an arrow pointing upward. Incorrect installation (upside down) will result in the electromagnetic core not being able to open the contacts under the influence of gravity, and the compressor will burn out.

• 📏 Diameter rod: measure the diameter of the compressor terminal, as the relay mounting holes may vary (usually 3 mm, but there are variations).
• ⚡ Power: make sure that the rated power of the new relay matches or exceeds the power of your compressor (indicated on the motor nameplate).
• 🔌 Contact configuration: the location of the contacts (triangle, line) must match, or use the adapter blocks from the kit.

The installation process is the reverse of dismantling. Push the new relay onto the compressor contacts as far as it will go, making sure that it is seated tightly and without distortion. Connect the wires in accordance with the previously made diagram or photograph. Attach the fixing bracket. After assembly, do not rush to close the back panel - turn on the refrigerator and listen. The start-up should be soft, without unnecessary clicks, and after 10-15 minutes of operation, the compressor housing should begin to heat up evenly.

Frequently asked questions (FAQ)