Checking the refrigerator compressor with a multimeter: complete instructions

A sudden stop of the refrigerator is a situation that requires immediate intervention, since all products are at risk of spoilage. Most often, owners of units are faced with the fact that the engine hums, but does not start, or turns on only for a few seconds and goes silent again. In such cases, the first suspicion is a malfunction of the electrical part of the engine, namely the stator windings. Multimeter is the main tool that allows you to quickly and accurately determine the condition of the motor without an expensive call to a technician.

Before embarking on complex measurements, you need to understand that the compressor is the heart of the cooling system, and its failure can be fatal for the entire device. However, do not rush to buy a new unit at the first sign of unstable operation. Often the problem lies in the start relay or thermostat, but the motor itself is working. Competent diagnostics using a tester allows you to weed out unnecessary options and focus on the real culprit of the malfunction.

To conduct a quality test, you will need not only the measuring device itself, but also basic skills in working with electrical circuits. Digital multimeters are now available to everyone, and their functionality is more than enough for home diagnostics. It is important to observe safety precautions, since the work will be carried out with high voltage, which is dangerous to life. In this article, we will analyze in detail the algorithm of actions that will help you determine the condition of the windings.

Preparation for diagnostics and safety measures

Any work related to the electrical part of the refrigerator requires strict adherence to electrical safety rules. The first and mandatory step is to completely disconnect the device from the power supply. Unplug the power plug and make sure that access is blocked so that no one accidentally turns on the power during the diagnostic process. Even momentary voltage may result in electric shock or damage to the meter.

To access the compressor, you must remove the rear panel of the refrigerator cabinet. It is usually secured with screws or plastic clips. After removing the cover, you will see a black barrel-shaped element - this is the motor-compressor. An electrical cable, as well as copper pipes of the cooling system, are suitable for its body. Attention: Do not bend or damage the copper pipes under any circumstances, as this will lead to freon leakage and will require complex repairs with refilling.

The next stage is dismantling the starter relay and protective thermal relay. These elements are put on the contact group of the compressor and covered with a plastic casing. Carefully remove the clips and disconnect the block with wires. Three pins arranged in a triangle should protrude onto the contacts of the compressor itself. These are the ones we will call. Before starting measurements, wipe the contacts with a dry cloth to remove oxides and dirt that may distort the readings.

⚠️ Attention: If traces of oil, soot or melted plastic in the contact area are visible on the compressor housing, this may indicate an interturn short circuit or insulation breakdown. In such cases, a visual inspection already gives 90% of the answer to the malfunction.

Make sure that your multimeter is working and its battery is not discharged. Switch the meter to resistance (ohms) mode, usually indicated by the symbol Ω. For an initial check of circuit integrity, you can use the continuity test mode with an audible signal. If you are using a pointer analog tester, first calibrate the zero by shorting the probes together.

Identification of the compressor winding terminals

Before interpreting the instrument readings, it is necessary to accurately identify the winding terminals. A standard single-phase motor has three terminals forming the vertices of a triangle. In the diagrams they are designated as Common (common), Run (working winding) and Start (starting winding). On the body of the compressor itself, markings may be absent or erased, so the determination of contacts is often carried out empirically.

The logic of the determination is based on the difference in winding resistance. The starting winding is made of thinner wire and has a larger number of turns, so its resistance is always higher. The working winding is wound with a thicker wire and has less resistance. The common wire is the connection point between these two windings inside the motor. The resistance between the common wire and the working wire is always less than between the common and the starting wire.

First, measure the resistance between each pair of contacts. Record the three values ​​obtained. The highest value will correspond to the sum of the resistances of the starting and working windings (measurement between the starting and working contacts). The smallest value will indicate a “general-worker” pair. The average value is for the “general - trigger” pair. The intersection point of these pairs is the desired common contact.

The location of the contacts on different compressor models may differ. Some manufacturers, such as Atlant or Indesit, have a standardized pin layout, but it is better to double-check the data with a multimeter every time. An error in identification can lead to incorrect connection of the start relay and re-combustion of the engine.

Color coding of wires

Often the start relay wires are color coded, which helps in diagnosis. The red or orange wire usually goes to the starting winding, blue or black to the working winding, and white or yellow-green to the common contact. However, you cannot rely only on color, since the previous master could have changed the wiring.

Algorithm for checking windings for open circuits and short circuits

The most common electrical malfunction is an open circuit. This means that the wire inside the winding has burned out and current cannot flow. When measuring the resistance between the terminals of a working winding, the device will show a specific value in Ohms. If the unit on the left (infinity) lights up on the multimeter screen or the device does not emit a sound signal in the pronunciation mode, then the circuit is broken.

Checking for a short circuit (short circuit) of the turns inside the winding requires care. If the resistance of one of the windings is significantly lower than the rated values ​​(for example, 5 Ohms instead of 20 Ohms), this indicates an interturn short circuit. In this state, the motor will consume enormous current, hum, and heat up quickly until the thermal protection trips or the wiring burns out. Normal resistance values ​​for household refrigerators vary widely, but the starting winding is always more powerful than the working winding.

Below is a table with approximate resistance values ​​for various types of compressors. Remember that the exact data depends on the model and manufacturer.

The measurement process must be consistent. First, we measure the resistance between the first and second contacts, then between the second and third, and finally between the third and first. Record all results. If the device shows a break in at least one pair of contacts, and normal resistance in the other two, then the break occurred in one of the windings common to these pairs.

Diagnostics of breakdown to the housing (grounding)

The most dangerous defect is the breakdown of winding insulation on the metal body of the compressor. In this case, the phase falls to ground, which creates a real risk of electric shock for the user when touching the refrigerator body. You can also check for breakdown using a multimeter by switching it to the resistance measurement mode at the maximum limit (megaohms) or to the continuity mode.

Press one probe of the multimeter to any of the three contacts on the compressor, and the second to a place on the metal body of the engine that has been cleared of paint. The ideal option is to completely strip a small area of ​​metal next to the contact group. If the device shows “infinity” or a resistance of tens and hundreds of megaohms, the insulation is intact. Any specific resistance value, even a few megaohms, or a sound signal indicates a violation of the insulation.

Important: even a weak breakdown that does not knock out the circuit breaker in the panel makes the operation of the refrigerator deadly. The body of the device may be under network potential, and if touched with hands (especially with wet hands), a person will receive an electric shock. Such compressors must be immediately replaced or professionally rewound, which is impractical in domestic conditions.

Sometimes the breakdown can be “floating”. This means that in a cold state the insulation holds, but when heated or vibration contact appears. Therefore, if in doubt, it is better to play it safe. It is also worth checking the resistance between the case and each of the three contacts separately in order to localize the problematic winding.

⚠️ Attention: If the multimeter shows a breakdown on the case, do not under any circumstances try to operate the refrigerator by additionally grounding it or using an RCD as a panacea. These are temporary measures that do not guarantee safety.

Analysis of results and indirect signs of malfunction

After taking all measurements, it is necessary to analyze the data obtained as a whole. If all three resistances between the contacts are normal, but the engine does not start, the problem may not be in the windings, but in the mechanical part or starting equipment. However, a multimeter also helps to identify more subtle defects, such as changes in resistance when heated.

There is the concept of “thermal breakdown”. The engine may run normally when cold, but when it warms up to operating temperature, the resistance drops or a short circuit appears. To check this phenomenon, sometimes you have to start the compressor for a short time and quickly (with caution!) measure the parameters, but this is a job for experienced specialists. At home, it is better to focus on the stability of the readings in a cold state.

An indirect sign of problems with the windings may be a burning smell or a specific sweetish smell of burnt insulation emanating from the compressor. Also pay attention to the color of the oil if you decide to check its condition (although this already requires depressurizing the system). If the oil is black and has a sour smell, this is a sure sign that insulation was burning inside the engine.

If the multimeter shows that the electrical part of the engine is working properly (no breaks, short circuits or breakdowns), but the compressor does not start or is humming, the reason lies in the starting relay, capacitor (if there is one) or mechanical jamming of the piston group. In case of jamming (“wedge”), the motor will hum and consume high current, but the shaft will not turn. Attempts to start such a motor can lead to rapid burnout of the windings.

Frequent diagnostic errors and expert advice

When checking the compressor themselves, beginners often make mistakes that lead to false conclusions. One of the most common is measuring resistance with dirty or wet hands, as well as touching the metal parts of the probes. This introduces error into the measurements, especially at high limits. Always hold the probes only by the insulated handles.

Another mistake is ignoring the state of the contacts on the relay itself. Often it is they that oxidize, and not the terminals on the compressor. Before taking measurements, clean the relay contacts with fine sandpaper or a file. Also, do not forget that some modern inverter compressors have a three-phase connection circuit and built-in electronics, which cannot be fully checked with a conventional multimeter.

You should not immediately reject the compressor if the resistance is slightly different from the table values. A variation of up to 10-15% of the norm is allowed. The main thing is the absence of short circuits and breakdowns to the housing. If you doubt the readings of your device, try testing it on a known-good load, for example, an incandescent lamp or other household appliance.

Remember that diagnostics is only the first stage. Even if you find a malfunction, replacing the compressor requires evacuation of the system and charging with refrigerant, which cannot be done without special tools. Therefore, if the diagnosis of a “burnt out motor” is confirmed, it is more rational to contact a service center for a comprehensive repair.

Is it possible to check the compressor without a multimeter?

Indirectly - yes. You can apply 220V voltage directly to the working and starting windings (following the diagram), but this is dangerous and requires experience. If the engine starts and runs, it is alive. However, this method will not show the magnitude of the current and will not reveal hidden insulation defects, and the risk of getting an electric shock or starting a fire is very high.

What to do if the winding resistance is normal, but the refrigerator does not freeze?

This indicates that the electrical part is working, but there are problems with the mechanics (the compressor does not pump) or the circulation system (clogged, freon leak). In this case, the multimeter is powerless, you need to check the pressure in the system and listen to the operation of the motor.

Why does the starting relay burn out after replacing the compressor?

A ​​common reason is the use of the wrong type of relay or a malfunction of the new compressor itself (hard start). Also, the relay can burn out due to power surges in the network or problems with the thermostat, which turns off the power at the wrong time.

Which multimeter is better to choose for repairing refrigerators?

For household use, any digital multimeter with a continuity function and a resistance measurement range of up to 2000 kOhm (2 MOhm) is suitable. The presence of a function for checking the capacitor capacity will be a useful bonus for diagnosing starting equipment.