The sudden stop of the refrigerator often becomes an unpleasant surprise, especially when there are perishable foods left inside. The first sign of a serious breakdown is usually silence: the unit stops humming, although the power indicators may be on and the light inside is on. In most cases, the root of the problem lies in compressor the heart of the refrigeration system, which is responsible for circulating the refrigerant. Before calling a technician or buying a new unit, it makes sense to carry out an initial diagnosis of the motor yourself.
For a quality check, you will need a minimum set of tools, the main one of which will be multimeter. This device allows you to measure the electrical resistance of the motor windings and detect the presence of breaks or short circuits. It would also be a good idea to prepare a screwdriver for removing the protective cover and, possibly, pliers. It is important to understand that diagnosing the electrical part is only the first stage; it does not always show mechanical jamming of the piston group, but it allows you to isolate faults in the electrical circuit.
Safety is the number one priority in any repair of electrical appliances. Before touching the internal components, you must fully de-energize the refrigerator by unplugging the plug from the socket. Even briefly touching live contacts can cause serious electric shock. In this article, we will analyze in detail an algorithm of actions that will help you determine the condition of the compressor using only basic knowledge of electrical engineering and a conventional tester.
Preparation for diagnostics and safety measures
Starting any work with household appliances requires proper preparation of the workplace. You will need to move the refrigerator away from the wall to allow easy access to the back wall. It is there, at the bottom of the unit, that the compressor is located, covered with a protective casing. Often this metal or plastic box is secured with bolts or latches that must be carefully removed. Do not use excessive force to avoid damaging the mounts or tubes.
After removing the casing, you will have a view of the motor itself and the start-up relay. Visually inspect the device for obvious damage: melted insulation, blackened wires or traces of burning. If you see that the wires are burnt away from the contacts, this is the first sign of a problem. However, even if everything looks intact on the outside, an internal malfunction of the windings could occur unnoticed.
⚠️ Attention: Before starting work, make sure that the capacitors (if they are in the circuit) are discharged and the compressor itself has cooled down. A hot motor may have altered resistance readings, which will lead to incorrect diagnosis. Let the equipment stand turned off for 30-40 minutes.
To take measurements, you will need a multimeter switched to resistance measurement mode (Ohm), usually indicated by the symbol Ω. If you have a pointer instrument, do not forget to calibrate the zero before starting measurements. Digital multimeters are more convenient in this regard - they are ready to work immediately after switching on. Make sure that the probes of the device are in good condition and have no damage to the insulation.
It is also important to consider that some models of refrigerators, especially modern inverter ones, have a complex electronic control system. Diagnostics of such compressors with a conventional multimeter can be limited only to checking the integrity of the windings, since the starting of the motor is controlled by the control board. In classic linear models, the verification algorithm is simpler and more understandable for the home craftsman.
Removing the start relay and accessing the contacts
To get to the terminals compressor windings, it is necessary to remove the start-up relay. This small block fits directly onto the motor terminals and is often secured with a bracket or clamp. Carefully disconnect the wire connectors going to the relay, first remembering or photographing their location. This will help you not to confuse anything during assembly.
After dismantling the relay, you will see three contacts coming out of the compressor housing. They are arranged in a triangle. On many models, the connection diagram is drawn directly on the metal casing of the motor or on the relay itself. If the markings are erased, knowledge of the standard will help you: the top contact is always grounding (body), and the two bottom ones are the winding terminals.
Remove the terminal block if it is installed on the terminals. Sometimes the wires are simply inserted into the relay sockets and need to be carefully removed. Your task is to provide direct access with the metal probe of the multimeter to the compressor terminals themselves. Do not allow the probes to touch each other or the metal parts of the case during measurements, so as not to burn the tester fuse.
☑️ Preparation for measurements
Pay attention to the condition of the terminals themselves. If they are heavily oxidized or coated with carbon deposits, the measurement result may be distorted. In this case, you can carefully clean the contacts with fine sandpaper or a file, being careful not to damage the threads or the metal structure. Clean contact is the key to accurate resistance measurement.
Searching and marking winding leads
The three pins protruding from the compressor housing form an isosceles triangle. To correctly diagnose, you need to understand which of them is responsible for what. Typically, the terminals are marked with Latin letters: C (Common) - common wire, R (Run) - working winding and S (Start) - starting winding. In some European models you can find the designations C, M (Main) and A (Auxiliary).
If the marking is missing or unreadable, it can be determined using a multimeter. The resistance between the common wire and the working winding is always less than between the common and the starting winding. The resistance between the starting and working windings will be equal to the sum of the two previous values. This is a physical law that works for all single-phase asynchronous motors.
To determine the common output, take measurements between each pair of contacts. Record the resulting values. The pair of contacts that gives the greatest resistance is not common. The third, remaining contact is the one you are looking for general conclusion (C). It makes it easier to determine the other two.
Why is it important not to mix up the windings?
If you apply voltage to the starting winding instead of the working winding, the engine will start to hum, overheat and can burn out in a matter of minutes. The starting winding is designed for short-term operation and has a thinner wire.
The table below shows approximate resistance values for medium-power compressors, which will help you navigate the data obtained:
| Winding type | Designation | Approximate resistance (Ohm) | Characteristics |
|---|---|---|---|
| Working | R / M | 10 - 25 Ohm | Thicker wire, fewer turns |
| Starting | S / A | 25 - 45 Ohm | Thin wire, more turns |
| Common point | C | - | Sum of resistances R and S |
| Case | - | Infinity | The insulation must be perfect |
Measuring winding resistance with a multimeter
Now we move on to the most important stage - direct measurement. Set the measurement limit on the multimeter 200 Ohm or 2 kOhm. Attach one probe to the common terminal, and the second to the terminal of the working winding. Record your readings. Then, without lifting the first probe from the common terminal, move the second one to the starting winding. Write down the result again.
The state is considered normal when the device shows specific numbers corresponding to the data in the table above. If the display shows one (or OL, 1.), this means an open circuit. The break may be inside the winding or at the soldering point of the lead. In both cases, the compressor requires replacement or complex professional rewinding, which is impractical in everyday life.
If the device shows zero or a value close to zero, this indicates interturn short circuit. In this case, the current flows along the path of least resistance, bypassing some of the turns. An engine with such a malfunction will get very hot, hum and knock out the circuit breaker. It is impossible to operate such a refrigerator.
Special attention should be paid to the stability of the readings. If the numbers on the multimeter screen start to fluctuate erratically while you are holding the test leads, there may be a poor connection somewhere or internal insulation damage that is showing up due to vibration. In this case, try to slightly move the wires at the base of the terminals.
Checking for insulation breakdown (short circuit to the housing)
One of the most dangerous faults is a breakdown of winding insulation on the metal body of the compressor. This can lead to the entire body of the refrigerator being energized, which is fatal to humans. To check this malfunction, the multimeter is switched to continuity mode (with a speaker icon) or measuring resistance to the maximum limit (megaohms).
One probe is applied to any of the three terminals of the compressor, and the second is applied to the place on the motor housing that has been cleared of paint. If the device is silent and shows infinity (or one in the left digit), then the insulation is intact. The appearance of any sound signal or digital values indicates a breakdown.
⚠️ Attention: Even a weak “leakage” of current to the body, which the multimeter may not show (due to low battery voltage), will be fatal in a 220V network. If there is the slightest suspicion of a breakdown or you feel a slight tingling current when you touch the body of the refrigerator when it is turned on, immediately stop using it and call an electrician.
Sometimes a breakdown occurs only when the engine heats up. When cold, the insulation can hold, and at operating temperature it can expand and short-circuit. Therefore, if scorch marks on the body or a burning smell are visually visible, you should not rely only on cold measurements. In such cases, it is better to replace the unit.
Additional diagnostics: starting relay and starting current
If the compressor windings ring normally and there is no breakdown on the housing, but the motor still does not start, the problem may be hidden in the starting relay or thermal release. These elements often fail before the motor itself. The relay can be checked for coil integrity and core mobility. The thermal relay “rings” in a cold state - it must conduct current.
There is also a “direct” test method when they try to start the compressor bypassing the relay. This is done by briefly applying voltage to the working and starting windings. However, this method requires extreme caution and experience, since you can easily get an electric shock or damage a working motor with incorrect phasing.
For deeper diagnostics, professionals use current clamps. They allow you to measure the actual current consumption of the engine at the moment of starting. If the current significantly exceeds the rated current (indicated on the tag), it means that the motor has an interturn short circuit or mechanical jamming, even if the winding resistance is formally normal.
If after all the checks it turns out that The electrical part of the compressor is working, but it does not work; perhaps there is a problem in the control system (thermostat, circuit board) or a mechanical failure has occurred (the piston is jammed). In the latter case, the motor will hum, but not rotate, consuming a huge current.
Is it possible to check the compressor without a multimeter?
Indirectly - yes. If, when plugged in, the motor hums and after a few seconds the relay clicks but does not start, this often indicates a problem. However, it is almost impossible to distinguish a winding break from a jammed or malfunctioning relay without a device. A visual inspection can only reveal obvious signs of combustion.
What to do if the winding resistance differs from the norm by 10-15%?
A small deviation may be caused by instrument error or temperature. However, if the difference between the working and starting windings does not correspond to logic (the starting winding always has greater resistance), this is a reason to be wary. A deviation of more than 20% from the passport data or from the symmetry of twin engines is considered critical.
Is it dangerous to open the compressor casing yourself?
Yes, this is strictly prohibited. Inside the casing there is freon under pressure and oil. Opening it will result in the release of refrigerant and will require a complete recharge of the system, which is impossible to do at home. In addition, the tightness is broken, and the compressor becomes unusable.
Why does the multimeter show a break, although the wires are intact?
The break may be located inside the motor winding, which has burned out due to a power surge or old age. A break is also possible at the place where the lead is soldered to the copper wire of the winding, hidden inside the housing. In both cases, repairs require replacing the compressor.