Has the refrigerator stopped freezing, but there is deathly silence inside? This is a sure sign that the heart of the unit - compressor - has stopped working. Before calling a technician or buying new equipment, it is worth conducting an initial diagnosis. Often the problem lies in the starting relay or motor windings, and this can be identified using a conventional tester.
In this article we will look in detail at how to test a refrigerator compressor with a multimeter to understand whether it can be restored or requires replacement. You will learn to determine open circuit, turn-to-turn short circuit and breakdown to the frame using available tools. This is a basic skill that will allow you to save significant money on calling a specialist.
To carry out the work you will need a minimum set of tools: screwdrivers for dismantling the casing, gloves to protect your hands and, of course, the multimeter itself. Disconnect the refrigerator from the power supply before starting any work.
Diagnostics of the motor takes no more than 15-20 minutes, but gives a clear understanding of the condition of the unit. If the tester shows normal resistance values, there may be a problem with the thermostat or refrigerant. If the device is “silent” or shows one, the reason is precisely in the electrical part of the engine.
Preparation for diagnostics and safety precautions
The first step to successful repair is proper preparation of the workplace and the unit itself. You need to ensure good lighting and easy access to the back of the refrigerator. Do not forget to remove the protective cover covering the compressor compartment to gain access to the electrical contacts.
The main diagnostic tool will be multimeter. Switch it to resistance (ohms) measurement mode, usually a range of up to 200 or 2000 ohms. Before starting measurements, make sure that the probes are in good condition by connecting them to each other - the device should show zero or a value close to zero.
Carefully inspect the compressor for external damage. Look for burn marks, melted plastic, or blisters on the body. If you notice black oily liquid around the motor, this may indicate a gasket failure or serious internal failure.
⚠️ Attention: It is strictly forbidden to touch electrical contacts with wet hands. The capacitor in the circuit can retain a charge even after being disconnected from the network, so let the equipment stand turned off for 5-10 minutes before starting work.
After a visual inspection, you need to get to the contact group. To do this, carefully remove the terminal block from the three terminals of the compressor. It is usually secured with a clamp or screw. Clean the contacts from possible oxide or carbon deposits to ensure a reliable connection with the tester probes.
If there are traces of rust or mechanical dents on the compressor housing, this does not always indicate an electrical malfunction, but requires a more thorough check of the system's tightness. However, our task now is to check the electrical circuit of the windings.
Connection diagram and contact identification
Before poking with probes at random, you need to understand the markings of the terminals. On most modern and older compressors, the winding terminals have standard markings on the block or housing next to the contacts. Usually Latin letters or numerical designations are used.
There are three main contacts that interest us: starting winding (Start), working winding (Run) and common wire** (Common). On European models they may be designated as S, R and C respectively. The starting winding is responsible for starting the engine, the working winding is responsible for its constant operation.
In some models, for example, Atlant or Indesit, markings can be made in the form of dots or color marks on the terminal block itself. A red dot often indicates the starting winding, a blue dot indicates the run winding, and a white or black dot indicates the total ground. It is better to look for the exact diagram in the technical documentation for a specific model.
If the markings are erased or missing, you can determine the contacts using the resistance exclusion method, which we will talk about in the next section. The main thing is not to confuse the terminals, since incorrect connections can lead to instant combustion of the engine when turned on.
⚠️ Attention: Never try to start the compressor directly, bypassing the start relay and thermal protection, without special knowledge. This can lead to a short circuit and fire.
For ease of understanding, let's look at the main symbols in the table below. Memorizing these symbols will simplify the diagnostic process.
| Designation | Winding type | Function | Wire color (often) |
|---|---|---|---|
| S (Start) | Starting | Starting the rotor | Red / White |
| R (Run) | Working | Maintaining operation | Blue / Green |
| C (Common) | General | Common zero/phase | Black / Brown |
| Ω | - | Grounding (housing) | Yellow-green |
Checking windings for open circuits and short circuits
Now let's move on to the most important part - measurements. We need to call pairs of contacts between each other. Set the multimeter to resistance measurement mode (2 kOhm limit). First, measure the resistance between contacts S and R (start and run windings).
In a working motor you should get a certain resistance value, usually in the range from 30 to 80 Ohms, depending on the power of the motor. If the device shows one (infinity), it means that the circuit is in break. This indicates that the conductor inside the winding has burned out, and it is almost impossible to restore such a compressor.
Next, check the resistance between contact C (common) and contact R (working winding). Here the resistance should be the least, since the working winding is wound with a thicker wire. Then measure the pair C and S. The sum of the resistances of the pairs C-R and C-S should be approximately equal to the resistance of the pair S-R.
If the multimeter shows zero or a value very close to zero (less than 1-2 ohms) on any of the pairs, this indicates short circuit turns. In this case, the current passes along the path of least resistance, bypassing the turns, which leads to overheating and tripping of the protection or knocking out the plugs.
Normal resistance values may vary depending on the manufacturer. For example, for compressors Danfoss the resistance of the working winding can be about 15-20 Ohms, and the starting winding - 40-50 Ohms. Old Soviet models may have higher resistances.
⚠️ Attention: The winding resistance may vary slightly depending on the engine temperature. A cold engine will show less resistance than a hot one. Take this into account when comparing with tabular data.
Write the measurement results in a notebook so as not to get confused. Compare the figures obtained with the reference values for your model, if they are available in the instructions. Any strong deviation from the norm (up or down) is a sign of a malfunction.
Diagnostics of a breakdown on the housing (interturn short circuit)
The most dangerous type of malfunction is a breakdown of the insulation of the windings on the compressor housing. In this case, the refrigerator can work, but its body will “shock” with electric current, which is fatal to humans. You can check for breakdown by measuring the resistance between any of the three contacts and the metal motor housing.
To do this, clean a small section of the copper tube or metal part of the compressor housing until it shines to remove paint and oxides. Apply one multimeter probe to the cleaned area, and the second to each of the three contacts of the block in turn.
On a working device, the multimeter should show infinity (one in the most significant digit). This means that the insulation between the copper winding wire and the steel body is intact. If you see any numbers, even a few kilo-ohms, it means the insulation is broken.
A breakdown in the housing often occurs due to overheating, aging of the varnish on the wires, or moisture getting inside the casing. Operating such a refrigerator is strictly prohibited without replacing the compressor, since the risk of electric shock is extremely high.
Sometimes the breakdown can be “floating”. This means that when the engine is cold, the insulation holds, but when it heats up, a leakage current appears. Therefore, it is recommended to carry out diagnostics both on a cold and slightly warmed-up (after a short-term switching on) engine, using extreme caution.
What to do if a breakdown is detected?
If the multimeter shows resistance between the winding and the housing, the compressor can only be disposed of or professionally rewinded, which is not economically feasible. Repair at home is not possible, as it requires dismantling the sealed casing and special soldering in an inert gas environment. The only way out is to replace the unit with a new one or a used one in good condition.
Checking the starting relay and thermal protection
Often the cause of compressor failure is not the motor itself, but the attachments: the starting relay and thermal protection relay. These small devices are mounted directly on the compressor housing and are responsible for starting the engine and protecting it from overloads.
Remove the relay from the compressor (after photographing the position of the contacts). Shake it near your ear. If you hear a rattling noise, like a rattle, the core or contacts inside are broken and the relay needs to be replaced. This is a common reason why the refrigerator hums but does not start.
Ring the relay contacts with a multimeter in dialing mode. In normal condition, the contacts should either ring or not ring, depending on the type of relay (electromagnetic or posistor). For posistor relays, the resistance should increase when heated, but “when cold” it should be small.
The thermal relay is also checked for circuit integrity. It should “ring” briefly. If the circuit is broken, it means that the thermal protection has burned out or simply does not close. In some cases, it can be carefully disassembled and the contacts cleaned, but it is safer to replace it.
☑️ Relay diagnostics
If the relay is working properly, but the compressor does not start or the machine knocks out, the problem is most likely in the interturn short circuit of the windings of the motor itself. In this case, the relay simply performs its protection function, breaking the circuit when the current surges.
Interpretation of results and table of faults
After all measurements have been carried out, it is necessary to systematize the data. Multimeter readings are not always unambiguous, so it is important to understand the physical meaning of the numbers obtained. Below is a summary table that will help you make a “diagnosis” of your refrigerator.
| Multimeter readings | Between the windings | On the body | Probable cause |
|---|---|---|---|
| ∞ (Unit) | Yes | No | Winding break (wire burnt out) |
| 0 - 2 Ohm | Yes | No | Short circuit of turns |
| Any number | No | Yes | Insulation breakdown on the body |
| Normal | No | No | The compressor is OK (look elsewhere) |
If you get a “Compressor OK” result based on electrical parameters, but it still doesn’t work, the problem may be mechanical (piston jammed) or in the refrigerant circulation system (clogging, freon leak). A mechanical wedge is often accompanied by a characteristic hum and attempts to start.
In the case of a mechanical jam, the “swinging” method by applying increased voltage sometimes helps, but this is a risky procedure that requires special equipment. For a home mechanic, it is safer to replace the entire unit.
Do not forget that even an electrically serviceable motor may not start due to low voltage in the network. If your light often “flashes”, the compressor may not gain the required speed and goes into protection.
Frequently asked questions and answers (FAQ)
Is it possible to repair a burnt compressor winding yourself?
Theoretically, you can rewind the motor, but in practice this requires complex equipment to wind the coils, dry the varnish, and evacuate the system. The cost of such work in a workshop often exceeds the cost of a new compressor, so independent repair of the windings is impractical.
Why does the compressor heat up, although according to the tester it is working?
Heating can be caused by several factors: a lack or excess of freon, a blockage in the capillary tube, a malfunction of the thermostat (the refrigerator does not turn off) or Poor ventilation around the condenser at the back. Also, the motor can heat up when operating at maximum loads.
Which multimeter is better to choose for diagnostics?
Any entry-level digital multimeter (for example, DT-838 or analogs) is suitable for household needs. The main requirement is the presence of a dialing mode and resistance measurement up to 20 kOhm. Professional models with automatic selection of the measurement limit are more convenient, but more expensive.
A click is heard, but the compressor does not start. What is this?
A click is usually made by a starting relay or thermal protection. If the click is followed by a buzzing sound and silence, most likely the start relay has burned out, the compressor has jammed, or an inter-turn short circuit has occurred, due to which the protection is triggered.