The situation when the refrigerator stops freezing, and only a quiet hum or complete silence is heard inside, often confuses the owner. The first suspicion is a failure motor-compressor, which is the heart of the entire cooling system. However, it is too early to rush to call a technician or buy new equipment, because often the problem lies in the electrical circuit, which you can diagnose yourself.
To accurately determine the malfunction, you will need a simple one digital multimeterthat is capable of measuring resistance and checking the integrity of the circuit. The skill ring the compressor allows you to eliminate expensive repairs if the problem turns out to be a burnt winding, or, conversely, save on diagnostics by finding only a broken wire. In this article, we will analyze in detail the verification process, safety measures and interpretation of the readings obtained.
Before you get started, make sure that you have basic skills in handling electrical appliances. Any diagnostic electrical circuits requires strict adherence to electrical safety rules, since the voltage in the network can be deadly. We will consider all stages from preparing the tool to the final conclusions about the condition of the unit.
Preparation for diagnostics and safety measures
Any work related to opening the technical compartment and contact with electrical components requires complete isolation of the device from the electrical network. It is strictly forbidden to measure resistance while the power cord is plugged into the outlet, since this is guaranteed to lead to failure of the multimeter and can cause a short circuit.
First, move the refrigerator away from the wall to provide free access to the back wall. You need to locate the start relay housing, which is usually located at the bottom of the unit, next to the black cylindrical compressor tank. Remove the protective cover by unscrewing the mounting screws or loosening the plastic latches, and carefully remove the relay unit itself along with the thermostat.
⚠️ Attention: Before starting work, be sure to wait until the compressor has cooled down if it has recently been running. Hot windings may have altered resistance, which will lead to incorrect readings from the device, and the risk of hand burns remains high.
After dismantling the relay, you will see a contact group with three terminals included in the compressor housing. It is these contacts that we have to explore. Visually inspect the wires for melted insulation, blackening or traces of sparking - this may indicate overload or breakdown.
Setting up the multimeter and searching for contacts
To carry out high-quality diagnostics, it is necessary to correctly configure the measuring device. Switch multimeter to resistance measurement mode (denoted as Ω or Ohm). If your device has an automatic limit selection function, simply set the “Ohm” mode; if manual, select a limit of 200 or 2000 Ohms, since the winding resistance is usually low.
The contacts on the compressor are arranged in a triangle. In order not to guess where the output is, you need to understand the logic of their work. Typically, the two lower contacts are responsible for working winding, which has less resistance, and the top or side contacts are responsible for the starting contact. The third contact, often located separately or marked, goes to ground (housing).
- 🔍 Visual marking: look closely, often the letters R (Run - working), S (Start - starting) or C (Common - common) are pressed next to the terminals.
- 📏 Measurement logic: the resistance between the working and common contacts is always less than between the starting and common contacts.
- ⚡ Insulation check: one probe is always applied to the cleaned place on the compressor body (where there is no paint), and the second to the contacts.
It is important to ensure reliable contact of the probes with the terminals. Oxidized metal can give false high resistance readings. If the contacts look dirty, carefully clean them with fine sandpaper or a blade before starting measurements.
Algorithm for checking windings for open and short circuit
The most common defect that can be detected with a multimeter is a broken winding. Attach the probes to a pair of contacts (for example, working and starting). If the device displays a unit (or the infinity sign “OL”), this means that the circuit is open and no current is passing.
The normal resistance of a working household compressor varies in the range from 10 to 30 Ohms for the starting winding and from 15 to 45 Ohms for the working winding. The total resistance between the extreme contacts must be equal to the sum of the resistances of the two separate windings. If you see values of several hundred Ohms or, conversely, close to zero (less than 1 Ohm), this is a sign of a malfunction.
☑️ Checklist for checking the windings
The case of an interturn short circuit is more difficult to diagnose, since the resistance may remain within the normal range, but the compressor will hum and knock out the machine. However, strong heating of the housing during operation and low resistance (below 5-7 Ohms) often indicate a turn short circuit inside the coil.
Diagnostics of breakdown to the housing (grounding)
One of the most dangerous fault is a breakdown of the insulation of the windings on the compressor housing. In this case, voltage may appear on the metal body of the refrigerator, which creates a direct threat to human life. This condition must be checked in the continuity mode or resistance measurement at the maximum limit (2 MOhm).
Attach one multimeter probe to any contact of the compressor block, and press the second firmly to the area of the copper tube that has been stripped of paint or to the motor housing itself. The device should show infinity (open circuit). If you see any resistance values or hear a continuity squeak, it means that the insulation is broken.
⚠️ Attention: Operating a refrigerator with a breakdown in the body is prohibited! Even if it continues to freeze, the risk of electric shock when touching the metal parts of the unit is extremely high.
Sometimes a breakdown only appears under load or when heated. If the device shows the norm “in cold conditions”, but knocks out the RCD or circuit breaker when turned on, the insulation is probably destroyed due to thermal expansion of the windings. In this case, professional troubleshooting or replacement of the unit is required.
Why does the RCD knock out?
The residual current device (RCD) reacts to current leakage. If there is no breakdown when checking, but the RCD is triggered, it is possible that current is flowing through condensation, dust inside the shield, or the power wire going to the socket is damaged, and not the compressor itself.
Checking the starting and protective relay
Often the reason that the compressor does not start is not the motor itself, but attachments. The starting relay is responsible for briefly supplying voltage to the starting winding, and the protective (thermal) relay turns off the power in case of current overload. They also need to be checked.
The thermal relay is checked for circuit integrity: in a cold state it should “ring” (show zero ohms). A starting relay (often a posistor type) has a more complex logic: the resistance between the contacts must increase as it warms up or be determined in a cold state. To accurately check a posistor, you often have to heat it with a hairdryer, observing the change in resistance.
| Relay type | Normal state (cold) | Malfunction sign | Action |
|---|---|---|---|
| Thermal | Circuit closed (0 Ohm) | Open circuit (infinity) | Replacement |
| Starting (posistor) | Resistance 10-50 Ohm | Short circuit or break | Replacement |
| Starting (induction) | Contact closure | Contact sticking | Cleaning/Replacement |
| Combined | Check according to the diagram | No click or breakdown | Replacing the unit |
If the relay is faulty, The compressor may hum, trying to start, but the rotor will not budge. After 5-10 seconds, the thermal protection is triggered, and the click of the relay breaks the circuit. The cycle is repeated until the thermocouple cools down.
Interpretation of results and further actions
After all measurements have been carried out, it is necessary to analyze the data obtained. If the windings are intact, there is no breakdown to the housing, and the relay is working properly, but the compressor does not start, the problem may lie in the electronic control module or in mechanical jamming of the piston group.
The mechanical wedge is often accompanied by a characteristic hum and an attempt of the shaft to rotate, followed by a click of protection. It is sometimes possible to “unwedge” such a motor using a special device (“pusher”) that supplies increased voltage for a short time, but this is a risky procedure, accessible only to specialists.
In the case where the electrical part is working, but there is no cold, it is worth checking refrigerant circulation system. The capillary tube may be clogged or there may be a freon leak. These malfunctions require a vacuum pump and a pressure gauge station for diagnosis and elimination.
Frequently asked questions (FAQ)
What resistance is considered normal for a compressor?
The normal resistance is considered to be the resistance of the working winding in the range of 15-45 Ohms, and the starting winding - 10-30 Ohm. The main rule: the sum of the resistances of the two windings should be approximately equal to the resistance between the extreme contacts. The exact numbers depend on the power and model of the motor.
Is it possible to check the compressor without a multimeter?
Indirectly - yes. You can apply 220V voltage directly to the working winding (through a capacitor or starting relay). If the engine starts and hums evenly, creating a vacuum at the intake, it is alive. However, this method is dangerous and does not provide information about the state of the insulation.
Why does the compressor hum but does not start?
This can be caused by three reasons: jamming of the mechanical part (wedge), a malfunction of the starting relay (does not give a start impulse) or an interturn short circuit due to which there is not enough starting torque.
What to do if a breakdown is detected in the housing?
Operating such a refrigerator is deadly. It is necessary to either replace the compressor, or (temporarily and with great care) use the device only through an isolation transformer or RCD with a leakage current of 10 mA, although a complete replacement is the only correct solution.