A sudden breakdown of a refrigerator is always stressful, because food supplies are at risk, and calling a repairman can be expensive. Often the reason for the lack of cold is a failure compressor, which is rightly called the heart of the refrigeration system. Understanding how to carry out an initial diagnosis of this unit will help you save time and accurately describe the problem to a specialist, or even fix the problem yourself if it is not too serious.
Before grabbing the tools, it is necessary to conduct an external inspection and an initial analysis of the situation. In most cases, motor-compressor does not start due to problems with the electrical circuit or thermostat, but the unit itself may be completely fine. It is important to distinguish the situation when the motor hums, but does not start, from complete silence or a characteristic click of the relay.
For accurate diagnosis, you will need a basic set of tools, including multimeter (tester), screwdrivers and, possibly, pliers. Do not ignore safety precautions: any work on electrical components should be carried out only after the device has been completely disconnected from the network. In this article, we will analyze an algorithm of actions that will allow us to determine the condition of the compressor with a high degree of probability.
⚠️ Attention: Before starting any diagnostic work, be sure to disconnect the refrigerator from the power supply. Working with current without the appropriate skills and protective equipment can lead to electric shock or short circuit.
Visual diagnostics and sound analysis
The first stage of testing is always a visual inspection and listening to the operating (or trying to operate) unit. Go to the refrigerator and listen carefully. If you hear a monotonous hum that ends abruptly with a click, this often indicates an attempt to start the engine. In such a situation start relay it may work, but the compressor does not gain speed due to jamming or problems with the windings.
Pay attention to the temperature of the compressor housing. If the motor is hot or even hot, but the refrigerator is not cooling, this is an alarming signal. Overheating may indicate an interturn short circuit or prolonged operation in emergency mode. However, remember that during the normal operating cycle the compressor also heats up, so you need to evaluate the temperature in conjunction with other signs.
Inspect the back wall of the refrigerator for mechanical damage to the tubes or traces of oil. The presence of oily spots around compressor fittings or on the floor under the refrigerator often indicates depressurization of the system, which makes further testing of the compressor pointless without refilling with freon. Also check the integrity of the wires going to the start relay.
Checking the serviceability of the starting relay
Before “climbing” into the electrical part of the motor itself, it is necessary to rule out a malfunction of the starting relay. It is this small block that provides the impulse to start the engine. Often, it is its combustion that owners mistakenly mistake for a compressor failure. The relay is located in a black casing at the back of the refrigerator, directly on the motor housing.
To check, carefully remove the protective casing and remove the relay. Visually assess its condition: there should be no cracks on the plastic case, and no traces of burning or melting inside. If the relay has a burning smell or appears blackened, replacing it may solve the problem without costly repairs to the compressor. In some models, the relay can be “ringed” with a tester, but the easiest way is to replace it with a known good one.
There is a simple but effective method of testing a relay by ear. Shake the removed relay next to your ear. If a clear rattling sound is heard inside, as if from a broken part, it means that the contacts or mechanical part are damaged. A working relay should be quiet or make a subtle noise from the slight movement of internal elements, but not ring like a rattle.
Why does the relay fail?
A frequent cause of burning out of the starting relay is voltage surges in the network or natural wear of the contacts during frequent on-off cycles. Also, the relay may not withstand starting currents if the compressor ages and it becomes more difficult for it to start.
Diagnostics of the electrical part with a multimeter
The most reliable way to check the technical condition of the compressor is to measure the resistance of its windings using a multimeter. To do this, you need to remove the start relay and the protective cover to gain access to the three contacts on the motor housing. They are usually located in a triangle and can be designated by the letters S (Start), R (Run) and C (Common).
Switch the multimeter to resistance measurement mode (Ohm). First check the continuity of the circuit to the housing. Press one probe to any contact, and the second to the metal body of the compressor or the stripped part of the tube. The device should show infinity (number 1 or OL). If you see any resistance values, it means that a breakdown of the winding has occurred on the housing, and operating such a compressor is strictly prohibited due to the risk of electric shock.
Next, we check the resistance between the winding contacts themselves. Normal values may vary depending on the engine power, but the proportions must be respected. The total resistance between the starting and operating windings must be equal to the sum of the resistances of each winding separately. Typical values for household refrigerators range from 10 to 30 Ohms for the working winding and slightly higher for the starting winding.
☑️ Multimeter testing algorithm
Table of normal values resistance
To make it easier for you to navigate the device readings, we have compiled a table with approximate values. Remember that it is always better to look for accurate data in the technical documentation (datasheet) for a specific model motor-compressor, since different manufacturers (Danfoss, Secop, LG, Atlant) parameters may differ.
| Pair of contacts | Designation | Normal resistance (Ohm) | What does deviation mean |
|---|---|---|---|
| Operating - General | R — C | 10 - 25 Ohm | Open or short short circuit |
| Starting - General | S — C | 15 - 40 Ohm | Malfunction of the starting winding |
| Starting - Working | S — R | Sum (R+C) + (S+C) | Circuit integrity failure |
| Contact - Housing | Any - Metal | Infinity (∞) | Insulation breakdown (dangerous!) |
If the multimeter shows zero (0) on any pair of contacts, this indicates a short circuit of the turns inside the winding. In this case, the compressor will most likely burn out immediately when turned on, knocking out automatic plugs in the apartment. Repairing such windings at home is impossible; replacement of the unit is required.
⚠️ Attention: The resistance values may vary slightly depending on the temperature of the windings. A “cold” engine and an engine after operation will show different numbers. Always make allowances for heating when diagnosing.
Checking for interturn short circuit with current clamps
There are situations when the winding resistance is normal, there is no breakdown to the housing, the relay is working, but the compressor still does not work or overheats quickly. In this case, an interturn short circuit is suspected. It does not always change the total resistance enough for a conventional multimeter to notice, but it significantly affects the current consumption.
For such diagnostics you will need current clamps. They are put on one of the power wires of a running compressor at the time of startup. The starting current must be higher than the operating current, but not exceed the limit values indicated on the motor nameplate. If the current consumption is significantly higher than the nominal value, this is a sure sign of an interturn short circuit.
Symptoms of such a defect can also be: strong heating of the housing in a short time, humming, knocking out plugs in the panel a few seconds after switching on. You cannot operate a refrigerator with an interturn short circuit - this is a fire hazard. The motor must be replaced.
Mechanical check: pressure and jamming
The electrical part may be in perfect condition, but the mechanical part may be faulty. A compressor is a pump, and if its piston group is worn out or jammed, there will be no cold. Checking the mechanics is more difficult, since it requires connecting a pressure gauge station. The technician connects the hose to the service fitting and checks whether the motor creates pressure when starting.
If the compressor hums but does not start, and the electrics are working properly, there is a high probability of mechanical jamming. In older models, the “wedging” method sometimes helped by applying increased voltage (220V) directly to the working winding in short-term pulses, but this method is dangerous and requires experience. A modern compressor with a jammed mechanism is easier to replace.
It is also worth checking the condition of the suction and discharge tubes. If, when the compressor is operating, one tube is hot and the other is cold (or covered with frost/dew), this is a sign of normal operation of the freon circuit. If both tubes are at the same temperature or the compressor is running “idle,” the system may be leaking or the capillary is clogged.
Frequently asked questions (FAQ)
Can a burnt out compressor be repaired or is only replacement necessary?
In 99% of cases a complete replacement is required compressor. Rewinding the windings at home is impossible due to the need to evacuate the system and special soldering. The cost of repairing the windings is comparable to purchasing a new unit, and reliability will be low.
Why may a new compressor not start after replacement?
A common reason is installing the wrong start relay or not filling the system with freon. Also, some types of compressors (for example, inverter) will not start without a special control controller.
How long should the compressor operate normally?
The operating time depends on the load of the refrigerator, the room temperature and the thermostat settings. Typically, the work cycle is about 30-40% of the total time (for example, it works for 10 minutes, it costs 20). Constant operation indicates a freon leak or malfunction.
Is it dangerous to change the compressor yourself?
Yes, it is dangerous. The process requires working with an open flame (soldering tubes), using a vacuum pump and refrigerant charging equipment. Without experience, you can damage the system or get injured.
How to extend the life of the compressor?
Do not place the refrigerator close to the wall (a gap is needed for ventilation), defrost it regularly (if it is not No Frost), do not load hot food and keep the condenser clean at the back.