How to check the motor on a refrigerator: complete instructions for diagnosing a compressor
The situation when the refrigerator suddenly It stops cooling, and there is silence in the cell, which always causes panic. The first thing that unit owners blame is the thermostat or the start relay, but often the compressor is the culprit of the silence. This is the heart of the refrigeration system, and if it stops beating, food quickly begins to spoil. Before calling a technician or buying new equipment, it is worth conducting independent diagnostics to accurately determine the nature of the malfunction.
Checking motor-compressor is a process that requires care and compliance with safety precautions, but is quite accessible to a person with basic skills in handling tools. It is important to understand that a motor is a complex electromechanical unit, consisting of an electrical part (winding) and a mechanical part (piston group). The malfunction may be hidden in any of them. In this article we will analyze in detail how to identify the problem without having complex laboratory equipment at hand.
The most important rule that you need to remember before starting any work: Diagnostics of the electrical part of the compressor can only be carried out with the refrigerator completely disconnected from the network. Any manipulation with live wires is deadly. Also, if you plan to open the compressor casing, make sure that the refrigerant is completely vented, although this is not required for the initial check with a multimeter. Let's figure out what signs can be used to suspect an engine breakdown.
Primary diagnostics without instruments and visual inspection
Before grabbing a multimeter, it is necessary to conduct a thorough visual inspection and auditory analysis of the unit's operation. Often compressor malfunction gives itself away long before it stops completely. If the refrigerator hums unusually loudly, makes metallic knocks, or, conversely, works absolutely silently with the light on, these are the first signs. Plug in the refrigerator and listen: if you hear a quiet hum, which ends abruptly after a few seconds with a click, most likely the thermal protection relay is triggered.
Inspect the back wall of the refrigerator in the area of the compressor. If you see melted wires, blackened insulating material on the terminals, or traces of soot on the unit itself, this is a clear sign of an electrical breakdown. Also pay attention to the temperature of the motor housing. During normal operation, it heats up, but if it is impossible to touch it due to the heat, then interturn closure has already happened. In some cases, you can notice oil leaks, which indicates depressurization of the housing.
🔇 Complete absence of sound from the motor when the light bulb is on.
🔊 Intermittent operation: buzzing for 3-5 seconds, then a click and silence.
🔥 Excessive heating of the compressor housing or the appearance of a burning smell.
💡 Flashing light when trying to start the motor (voltage drop).
It is also important to check the condition of the starting relay, since its breakdown often simulates combustion of the windings motor. If the relay is stuck or burnt out, the current simply does not reach the engine, or it cannot reach operating mode. Remove the plastic box from the back of the compressor and inspect the contacts for oxidation. If everything is visually clean, we move on to more accurate testing methods.
⚠️ Attention: If, when you turn on the refrigerator, you hear a loud bang and see sparks, immediately unplug the device. Further operation is impossible and there is a fire hazard.
📊 How does your refrigerator behave when you try to start?
It hums and clicks, but does not start
It works, but does not get cold
It turns on for a couple of seconds and turns off
Completely silent, the light is on
Instrumental check of the windings with a multimeter
The most accurate way to diagnose the electrical part is winding continuity using a multimeter. To do this, you will need to switch the device to resistance measurement mode (Ohms) to the limit of 200 Ohms or 2 kOhms. First you need to gain access to the compressor contact group by removing the start-up relay. Usually there are three pins: two upper and one lower (or three in a row, depending on the model). Contacts can be labeled as S (Start), R (Run) and C (Common).
The measurement process requires consistency. First, we measure the resistance between pairs of contacts. Normal resistance values for a working household compressor are usually in the range from 10 to 30 Ohms for the starting winding and from 15 to 45 Ohms for the working winding. The exact numbers depend on the engine power and manufacturer, for example, for models Danfoss or Secop they may differ from their Chinese counterparts. If the device shows one (infinity), it means there is an open circuit and the motor has burned out.
Particular attention should be paid to checking for breakdown on the housing. This is a critical step to ensure your safety. Switch the multimeter to diode test mode or to the maximum resistance limit. With one probe, touch any of the three motor contacts, and with the other, touch a cleaned area on the metal body of the compressor (you can lightly scratch the paint for contact). The device should show infinity.
☑️ Dial algorithm with a multimeter
Done: 0 / 4
If the device needle deviates or specific resistance values (even high ones) appear on the screen, this means that winding insulation it is broken and the current flows to the case. It is strictly forbidden to operate such a refrigerator, as this will lead to electric shock when touching the metal parts of the refrigerator body. The table below shows approximate resistance values for different types of windings.
Measurement type
Normal reading
What does deviation mean
Starting winding (S-R)
20 - 50 Ohm
Open (∞) or short circuit (0 Ohm)
Working winding (C-R)
10 - 25 Ohm
Interturn closure
Total (S-C)
Sum of S-R and C-R
Incorrect measurements
On the body (any contact)
Infinity (1)
Insulation breakdown (dangerous!)
What to do if the resistance values are very different from the norm?
If the resistance of one of the windings is significantly lower than the norm (for example, 2-3 Ohms instead of 20), this indicates an interturn short circuit. The motor will get very hot and knock out the circuit breaker. Repair by rewinding at home is impossible; the compressor needs to be replaced.
Checking the starting and thermal relay
Often the motor itself is working, but the faulty starting relay prevents it from starting. This small block is responsible for supplying an impulse to the starting winding at the moment of start. If the relay is stuck in the closed state, the windings may burn out from overload. If it is open, the engine will hum, but will not start. To check the relay, it must be carefully removed from the compressor contacts. It is usually held on by a bracket or latches.
Visually inspect the inside of the relay. If it is a start relay with a posistor (black pill inside), check it for cracks. When heated, the posistor should increase its resistance, opening the starting winding circuit. The thermal relay (bimetallic plate) is checked for an open circuit: in a cold state it should ring, and when heated (you can gently warm it up with a hairdryer without overheating!) it should open. However, heating it with an open fire is prohibited.
🔍 Shake the relay: if it rattles inside, it means the mechanism is destroyed.
🔌 Check the contacts for carbon deposits or oxidation.
🌡️ Make sure that the relay marking matches the power of your compressor.
If the relay is working properly, but the engine still does not start, the problem may lie in the wiring or thermostat. However, if the motor starts when you apply voltage directly (which only professionals should do), then the control circuit is faulty. Replacement of the relay is an inexpensive repair that often brings the refrigerator back to life. The main thing is to buy an analogue with exactly the same parameters indicated on the body of the old relay.
Diagnostics of the mechanical part: jamming of the piston
There are times when the electrical part of the compressor rings perfectly, the relay is working properly, the voltage comes, the motor hums, but does not start. This is a sure sign that a piston group has occurred. The compressor could run out of oil inside, score on the cylinder, or jam the valves. In this state, the motor shaft cannot turn, and the current in the windings increases to a maximum until the thermal protection is triggered. mechanical wedge piston group. The compressor could run out of oil inside, score on the cylinder, or jam the valves. In this state, the motor shaft cannot turn, and the current in the windings increases to a maximum until the thermal protection is triggered.
There is a popular method of “wedging,” which helps in rare cases if the jamming is not critical. The essence of the method is to briefly apply increased voltage (about 220-230V) directly to the working winding at the time of start, sometimes using a large capacitor. However, this method is extremely dangerous and can completely kill the compressor or lead to a refrigerant explosion if there is a spark in the system.
If the compressor is tightly jammed, no amount of dancing with a tambourine will help. Mechanical failure inside the sealed casing cannot be repaired. The only way out is replacing a compressor entirely. This is a complex process that requires a vacuum pump, a torch, and skills in soldering copper pipes, as well as charging the system with freon. It is impossible to carry out such a replacement independently without special equipment.
⚠️ Attention: Attempts to drill a hole in the compressor housing to “revive” or check the oil level will lead to an instant release of freon and moisture entering the system. After this, the compressor can only be thrown away.
Causes of failure and prevention
Understanding the causes of failure will help extend the life of the new compressor. Most often burning of the windings occurs due to overheating. This may be caused by dust and animal hair contaminating the condenser (the grill at the back of the refrigerator). When heat transfer is disrupted, the compressor works with increased load, trying to gain temperature, and eventually overheats.
Another common reason is unstable voltage in the network. Voltage surges destroy the insulation of the windings and damage the electronics. If lights flicker frequently in your area, be sure to use a voltage stabilizer. Frequent switching on and off also has a negative impact, for example, if the thermostat is faulty or the refrigerator door does not fit tightly and the motor runs non-stop.
❄️ Defrost the refrigerator regularly (if it is not No Frost).
🧹 Clean the rear grill of dust at least once every six months.
🔌 Use surge protectors or stabilizers to protect against surges.
🚪 Monitor the tightness of the door and the integrity of the seal.
The service life of a modern compressor is on average 10-15 years. However, with proper use and favorable conditions, it can last longer. If your refrigerator is more than 15 years old and its motor has burned out, sometimes it is more advisable to think about buying new equipment, since the cost of replacing the compressor with work and refilling can be up to 70% of the price of a new budget refrigerator.
FAQ: Frequently Asked Questions
Is it possible to replace the compressor on a refrigerator yourself?
Theoretically, it is possible if you have the skills to solder copper tubes, a vacuum pump and a pressure gauge station. However, without these tools it is impossible to qualitatively replace the compressor and fill the system with freon. Unprofessional replacement will lead to rapid failure of the new motor.
Why does the new compressor get hotter than the old one?
Modern compressors often operate at higher housing temperatures for energy efficiency purposes. If the temperature does not exceed 80-90 degrees and the refrigerator is cold, this is normal. However, if it is red-hot or knocks out the machine, it means there is a problem with the cooling system or freon filling.
You hear the sound of flowing liquid after turning off the engine - is this normal?
Yes, this is absolutely normal. After the compressor stops, the refrigerant in the system continues to move and flow under the influence of pressure differences and gravity. This sound can last from a few seconds to a couple of minutes and does not indicate a malfunction.
How to determine that it is the compressor that has burned out, and not the relay?
You need to ring the windings with a multimeter. If the resistance is normal and there is no breakdown to the housing, but the motor does not start (hums and clicks), most likely the problem is in the relay or mechanical wedge. If the multimeter shows a break or short circuit in the windings, the motor has burned out.