How to check the compressor of an old refrigerator: fault diagnosis

The failure of a refrigeration unit always becomes an unpleasant surprise, especially when it comes to old but reliable models that have served faithfully for decades. The first sign of a serious breakdown is often silence: the usual hum of the motor disappears, and the chamber becomes warm, despite the fact that the thermostat is set to maximum. In such a situation, the owner is faced with a logical question: is it worth spending resources on restoring equipment or is it easier to purchase a new one?

However, you should not rush to conclusions, since compressor is the heart of the refrigerator, and its failure does not always mean a complete loss of the unit. Often the problem lies in adjacent elements, such as the start relay or thermostat, while the motor-compressor itself remains operational. Competent primary diagnostics allows you to save significant money and avoid the purchase of new equipment if the old device is still subject to resuscitation.

In this article we will analyze in detail the algorithm for checking the performance of the compressor, using available tools and methods that even a person without deep knowledge in electrical engineering can apply. You will learn to distinguish the symptoms of burnt out windings, mechanical jamming and malfunction of the starting mechanism, which will become the basis for making an informed decision on further repairs.

Primary visual diagnostics and preparation for work

Before taking up measuring instruments, it is necessary to conduct a thorough external inspection of the back of the refrigerator, where the motor-compressor. In older models, this unit is often hidden behind a metal casing, which should be carefully removed after first disconnecting the device from the power supply. Safety is the main priority, so any manipulations with electrical components are carried out only with the plug unplugged.

Carefully inspect the compressor housing for swelling, burnt marks or melted plastic in the area of ​​the electrical connector. If you notice black carbon deposits or smell a strong smell of burnt insulation, this is a sure sign that starting relay or the wiring has been overloaded. Also check the integrity of the copper tubes: the presence of oily stains or traces of freon may indicate a violation of the tightness of the system, which makes checking the motor itself a secondary task.

Pay special attention to the contact group where the relay connector is inserted. The contacts must not be oxidized or blackened. If a green coating of oxidation is visible on the copper terminals, they must be cleaned, as this may interfere with normal current exchange.

⚠️ Attention: Before starting any work, make sure that the capacitor (if it is in the circuit of your model) is discharged, and the refrigerator itself is disconnected from the network for at least 10-15 minutes, so that the residual voltage in the control circuits has time to dissipate.

Often owners of old equipment are faced with the fact that the compressor hums, but does not start. This may indicate that the engine is trying to turn over due to a jammed piston or a faulty starting mechanism. It is impossible to visually determine jamming, but an indirect sign is a characteristic metallic clang or vibration of the housing when turned on for a short time.

Checking the serviceability of the start relay

One ​​of the most common reasons why the compressor does not start is the failure of the start protection relay. This small block is responsible for supplying a starting pulse to the starting winding and protecting the motor from overcurrent. In old refrigerators, most often there are relays with a posistor or electromagnetic models, which lose their properties over time.

To check the relay, it must be removed from its seat, disconnected from the compressor contacts. Gently shake the device next to your ear: if a characteristic ringing or knocking sound is heard inside, it means that the posistor or mechanical part is damaged and the relay must be replaced. External inspection is also important: a melted body or traces of soot inside indicate that a short circuit current passed through the device.

If there are no visual defects, you can carry out a simple check for circuit integrity, but this will require a multimeter. Switch the device to resistance measurement mode (Ohm) and ring the contacts. Depending on the type of relay, resistance must either be absent (break) or be within certain limits specified in the technical documentation. However, the easiest way is to replace it with a known-good device with similar parameters.

• 🔌 Remove the relay from the compressor casing, having first photographed the contact connection diagram.
• 👀 Inspect the insides for charring, cracks or melted plastic.
• 🔊 Shake relay: the presence of rattling inside indicates destruction of the posistor.
• 📏 Test the contacts with a multimeter to check the integrity of the circuit (if you know the type of relay).

It is worth noting that even a working relay does not guarantee the compressor will start if the problem lies deeper. However, excluding this element from the list of suspects significantly narrows the scope of troubleshooting. If, after replacing the relay, the motor is still silent or hums and clicks, we move on to a deeper diagnosis of the electrical part of the engine.

Diagnostics of compressor windings with a multimeter

The most reliable way to check the condition of the electrical part of the motor is to measure the resistance of the windings using a multimeter. At the end of the compressor there are usually three contacts forming a triangle: two upper and one lower (or three in a row, depending on the model). These pins correspond to the starting winding, the working winding and the common wire.

To carry out diagnostics, switch the multimeter to resistance measurement mode to the 2 kOhm limit. First you need to determine which pin does what. Typically, the resistance between the common wire and the working winding is 10-30 Ohms, and between the common and starting windings it is 20-60 Ohms. The resistance between the starting and working windings should be the sum of these two values.

If the device shows infinity (one on the left side of the screen or the OL symbol) on any of the measurements, this indicates a winding break. In this case, it is almost impossible to restore the compressor and requires its replacement. If the resistance is close to zero, this indicates an interturn short circuit, which is also a fatal malfunction for the engine.

It is important to understand that resistance values may vary depending on the power and model motor-compressor. For example, in industrial refrigerators or high-power units, the resistance may be lower. The main rule: there should not be a complete absence of resistance (short circuit) or its infinite value (break).

Insulation breakdown test (case test)

One ​​of the most dangerous malfunctions is a breakdown of the insulation of the windings on the compressor housing. In this case, an electrical potential may appear on the metal casing of the refrigerator, creating a real risk of electric shock to the user. You can check for breakdown using the same multimeter or a special device - a megometer.

For diagnostics, set the multimeter to the resistance measurement mode at the maximum limit (usually 2 MOhm or 20 MOhm). Press one probe to any of the three contacts on the compressor, and the second to a clean, unpainted part of the metal body of the unit or to a copper tube. The device should show infinity.

If you see any resistance values, even very large ones (for example, several megaohms), this indicates an insulation failure. In conditions of high humidity inside the refrigeration chamber, such a breakdown may intensify, leading to the operation of an RCD or an electric shock when touching the case.

⚠️ Attention: If a breakdown is detected on the case, operation of the refrigerator is strictly prohibited until the malfunction is eliminated. An attempt to “isolate” the breakdown from the outside is impossible, since the breakdown is located inside the sealed motor.

Sometimes breakdown appears only under load or when heated. In such cases, cold testing may not reveal the defect. If you have suspicions, but the multimeter is silent, you can carefully apply voltage to the compressor through a grounded extension cord, turning it on briefly, and check the leakage current, but this requires high qualifications.

Mechanical check: jamming of the piston group

Electrical serviceability of the windings does not guarantee that the compressor will be able to start. The mechanical part of the unit is subject to wear, especially in older models where the oil may have lost its properties or thickened. Jamming of the piston group is a common reason why an electrically serviceable motor hums but does not rotate.

You can identify jamming by a characteristic sound: when power is applied, a dull hum is heard, lasting several seconds, after which the thermal relay clicks and turns off the motor. If you try to start such a compressor immediately again, it will hum again and turn off. In some cases, the so-called “kick method” helps - a short-term application of increased voltage or pulse, but this method is risky and can finish off the unit.

A more gentle test method is to try to turn the compressor shaft manually. To do this, you need to remove the start relay and, using a thin screwdriver or a special wrench, carefully try to turn the motor shaft. In good condition, the shaft should rotate with effort, but evenly. If the shaft stands tightly or turns jerkily with a metallic grinding noise, the compressor is jammed.

• 🔇 Listen to the sound: a steady hum followed by a click is a sign of an attempt to start due to a mechanical obstacle.
• 🛑 Check the thermal release: if it works instantly, the starting current may be too large due to jamming.
• 🛠️ Try turning the shaft through the technological hole (if the design allows) or remove the back cover for access.
• 🌡️ Consider the temperature: in a cold room the oil thickens, which can simulate jamming at the first start after inactivity.

It is worth noting that in some cases, jamming is temporary and is caused by oil thickening during prolonged inactivity in a cold room. If the refrigerator was not turned on at a low temperature, before diagnosing it should be kept warm for several hours.

Analysis of operation under load and current consumption

The most accurate picture of the condition of the compressor is provided by measuring the current consumption during operation. To do this, you will need a current clamp, which allows you to measure current without breaking the circuit. The normal operating current of a household refrigerator compressor is usually from 0.3 to 0.8 Amperes, depending on the power and model.

To take measurements, turn on the refrigerator and wait until the compressor starts. Cover one of the power wires (phase or neutral) with pliers while the engine is running. If the current significantly exceeds the rated current (indicated on the compressor tag, for example, 1.5 A instead of 0.5 A), this indicates an overload, a possible inter-turn short circuit or mechanical resistance.

If the current is too low or zero, although the motor is humming, it means that the motor does not reach the operating mode and operates in the rotor or starting winding short circuit mode. Long-term operation in this mode will lead to overheating and final failure.

Normal operating current = 0.3 - 0.8 A (depending on the model)
Starting current (short-term) = up to 2.0 - 3.0 A
Critical current (sign of malfunction) = > 1.5 A in operating mode

It is also important to pay attention to the temperature of the housing of the operating compressor. Normally it should be hot, but not scalding. If one part of the housing is hot and the other is cold, or if heating occurs unevenly, this may indicate problems with the circulation of refrigerant or oil within the system.

Frequent errors when checking independently

When diagnosing refrigeration equipment, beginners often make mistakes that can lead to false conclusions or damage to serviceable components. One of the most common mistakes is ignoring the status of the thermostat. If the thermostat is faulty and does not close the contacts, the compressor simply will not receive a command to start, and the owner will look for a problem in the motor, although it lies in the temperature sensor.

Another mistake is checking the compressor directly from the network without a start relay. Although it is technically possible to start the motor by applying voltage directly to the working and starting windings, doing this without experience is dangerous. Incorrect connection or overexposure of the starting pulse can burn windings that were previously intact.

Also, do not forget about the No Frost system, if it is present in the old model. Clogged air exhaust channels or a malfunction of the fan can create the illusion of a non-working compressor, although it can properly cool the evaporator, but not supply cold to the chamber.

⚠️ Attention: Characteristics and connection diagrams may vary depending on the manufacturer and year of manufacture. Always check the technical documentation or markings on the compressor itself before carrying out work.

Do not attempt to repair the sealed compressor housing itself, open it or change valves at home. This requires special equipment for vacuuming and charging with freon. Your task is only to diagnose the condition of the unit and make a decision on replacement.

FAQ: Frequently asked questions

Is it possible to replace an old compressor with a more modern analogue?

Yes, this is possible, but it requires selecting a model with similar cooling capacity and mounting dimensions. It may also be necessary to replace the start relay and thermostat, since their parameters must correspond to the new motor.

Why does the compressor run constantly and does not turn off?

This may indicate a freon leak, wear on the door seals, a malfunction of the thermostat, or loss of performance of the compressor itself. If the motor is old and works non-stop, its resource may be close to exhaustion.

How much does it cost to replace a compressor in an old refrigerator?

The cost consists of the price of the new unit and the work of the technician. Often, the cost of a repair with replacement of a compressor can be 50-70% of the cost of a new refrigerator, which makes repairs economically feasible only for premium appliances or if there is a free compressor.

Is it dangerous to touch a running compressor?

The compressor itself should not give an electric shock if the insulation is intact. However, due to vibration and heating, microcracks in the wiring are possible. It is recommended to carry out all manipulations with rubber gloves and dry shoes, and also to use tools with insulated handles.

Can an old refrigerator work without a compressor?

No, the compressor is the only engine that circulates the refrigerant. Without it, the refrigeration cycle is impossible. There are absorption refrigerators that operate silently, but they also have a heating element that acts as a compressor, and if it burns out, no cold is produced.