Failure of a refrigeration unit is often accompanied by a complete lack of cold in the chambers, which creates a direct threat to the safety of products. If at the same time you hear that the engine is humming, but does not start, or, conversely, it runs endlessly without turning off, there is a high probability of problems with the tightness of the circuit or the motor itself. Before calling a technician or buying a new unit, experienced specialists recommend conducting initial diagnostics, which often comes down to measuring key parameters of the system.
The main unit that circulates the refrigerant is the compressor, and it is its performance that determines the efficiency of the entire cooling system. Checking the refrigerator compressor for pressure is the most informative method that allows you to accurately determine whether the unit is capable of creating the necessary pressure difference for freon to boil. Unlike a simple continuity check of the windings with a multimeter, a pressure test shows the real mechanical performance of the unit, revealing hidden defects in valves or piston groups that cannot be detected by electrical methods.
In this article we will analyze in detail the measurement technology, the necessary equipment and the interpretation of the data obtained. You will learn what values are considered normal for different types of refrigerants and how to distinguish a compressor malfunction from a leak or blockage in the system. Competent diagnostics will allow you to make an informed decision about the advisability of repairing or replacing an expensive unit, saving time and budget.
The principle of operation and the role of pressure in the circuit
To understand the importance of checking, it is necessary to briefly consider the physical cooling process. The compressor acts as the heart of the system, compressing the refrigerant gas and forcing it into the condenser. In this process, the critical parameter is pressure difference between the discharge zone (high pressure) and the suction zone (low pressure). It is this difference in temperature and pressure that allows freon to effectively transfer heat to the environment and absorb it inside the refrigerator chambers.
If the compressor is worn out, its valves can let gas back in, or the piston group loses compression due to exhaustion of the cylinders. In this case, the engine may hum, consume current, but not create the necessary hydraulic resistance. The result is insufficient pressure at the outlet, due to which freon does not boil at the required temperature, and cold is not produced, even if there are no leaks in the system.
⚠️ Attention: The refrigerator system is under pressure even when turned off. Before starting any work on depressurizing the circuit (cutting off pipes), you must make sure that the refrigerant is released in a safe place, away from open fire, since the mixture of oil and gas can be explosive.
Modern models, especially inverter ones, may have different operating parameters from classic ones, but the physical principle of creating compression remains unchanged for all types of units. Checking the pressure allows you to isolate mechanical breakdowns from problems with electronics or the start-protective relay, which are often confused during superficial diagnostics.
Necessary tools and preparation for diagnostics
To carry out high-quality diagnostics, you will need a specialized set of tools, without which accurate measurements are impossible. The main device is pressure gauge station (or at least one high pressure gauge), which allows you to visualize the force with which the compressor pushes out the gas. Standard household pressure gauges for measuring tire pressure are not suitable here because the scale range is too small.
In addition to the measuring device, you will need a set of plug tubes or a special fitting for connection to the process tube of the compressor. Often, craftsmen use a piece of copper tube with a diameter of 6 mm, which is tightly fitted onto the fitting and secured with a clamp or soldering, although for an express check a tight connection with a finger is sufficient (while observing safety precautions). It is also necessary multimeter to control the current consumed by the motor during the test.
Preparation of the workplace is a critical stage. You will need to gain access to the back of the refrigerator, remove the compressor guard and remove the start relay to gain access to the contacts and tubes. Before connecting the pressure gauge, make sure that there is at least residual freon pressure left in the system (usually 3-5 atmospheres), which will confirm the absence of complete depressurization before starting work.
☑️ Tools for checking
Step-by-step instructions: how to measure discharge pressure
The measurement process begins by disconnecting the refrigerator from the power supply. Find three tubes on the compressor: two copper (one thin - capillary/discharge, the second thick - suction) and one steel (blind, sealed at both ends - process). To check the discharge pressure, we need to connect a pressure gauge to a thin copper tube going from the compressor to the condenser (usually it is hot during operation) or, more correctly for diagnosing the motor itself, to the process fitting, if it is possible to switch.
However, the classic method with a “finger” or a pressure gauge on the output tube looks like this: carefully bite off the thin copper discharge tube at a distance 5-10 cm from the compressor housing. Place the pressure gauge hose onto the cut end. Now plug in the refrigerator. It is important to do this quickly and carefully monitor the readings of the device in the first seconds of operation.
Normal operating discharge pressure: 9–15 bar (depending on the type of freon)If you use the “finger” method (without a pressure gauge, only for a rough estimate), then with a working compressor, a stream of air should forcefully escape from the tube, and plug it will be almost impossible to use her thumb - the air will escape with a hiss and whistle. If you easily plug the hole with your finger, and the gas comes out in a sluggish stream, this is a sure sign compression loss. In this case, the mechanical part of the compressor is worn out, and repair is often not economically feasible.
Why can’t you keep the compressor open for a long time?
When operating without connecting to the condenser (to the atmosphere), the compressor operates in a light mode, but oil can be intensively discharged through the open tube. It is not recommended to keep the unit turned on for measurements for more than 30-40 seconds, so as not to overheat the windings and not lose too much oil.
Table of normal indicators and interpretation of anomalies
Interpretation of pressure gauge readings requires taking into account the type of refrigerant for which the system is designed. Different gases (R134a, R600a, R404a) have different physical properties and operating pressure at the same temperature. Below is a table of approximate values that a working compressor should produce when tested “for a burst” (into the atmosphere) or in static conditions.
| Refrigerant type | Normal discharge pressure (bar) | Characteristic symptoms of a malfunction | Probable cause |
|---|---|---|---|
| R134a | 9 – 12 | Pressure below 6 bar | Wear of the piston group, stuck rings |
| R600a (Isobutane) | 7 – 10 | Pressure drops during operation | Valve malfunction, gasket breakdown |
| R404a / R507 | 15 – 20+ | Pressure too high | Clogging in the system (capillary), air in the circuit |
| Any type | 0 (no pressure) | Humming without result | Compressor wedge or broken piston rod |
It is important to understand that the static pressure (when the refrigerator is turned off for several hours) must be equal to the saturated vapor pressure of the refrigerant at the current ambient temperature. For R134a at +25°C this is about 6.5 bar. If the pressure gauge shows 0, then the system is completely depressurized, and there is no point in checking compression - first you need to find a leak.
If, when turned on, the pressure gauge needle twitches, but does not rise above 2-3 bar, and the compressor consumes low current, this indicates that valve group does not hold pressure. Gas simply flows from a high-pressure zone to a low-pressure zone inside the motor housing, without doing any useful work. In such cases, replacing the compressor is the only effective solution.
⚠️ Attention: Pressure gauge readings may vary depending on the ambient temperature. A cold compressor will show lower pressure than a warm compressor. Always correct for room temperature when diagnosing.
Diagnosing suction capacity and checking for leaks
In addition to the discharge pressure, it is critical to check how the compressor copes with gas intake. To do this, the test is carried out on the suction pipe (thick tube). When operating properly, a noticeable vacuum should be created at the inlet. If you bring your finger to the hole in the suction tube of a running compressor, it should literally be “sucked in”, and it will be difficult to tear off your finger.
The absence of vacuum at the inlet with seemingly normal blowing at the outlet may indicate clogged capillary tube or a filter drier. In this case, the compressor tries to pump, but cannot push the gas through the system, which creates a vacuum at the inlet and may cause unstable pressure at the outlet. This may also be a sign of an “oil wedge” when the oil has clogged the channels.
Checking the compressor itself for leaks is also important. Inspect the places where the tubes exit from the motor housing. If you see traces of oil, “fogging” or hear a characteristic hissing sound, it means that the tightness of the welds is broken. Vibration during operation gradually destroys the metal, and gas begins to escape. Repairing such places is possible only with the help of argon welding, but it is often easier to replace the entire assembly.
Typical mistakes when checking independently
Beginners often make mistakes that lead to incorrect conclusions. One of the most common is an attempt to measure pressure immediately after switching on, without allowing the system to enter mode, or, conversely, measurement after prolonged operation, when the compressor has already overheated. Thermal expansion gases and oils can distort the readings.
Another mistake is ignoring the current consumed. The pressure may be normal, but if the motor draws more current than the rated current (indicated on the label), it means it is overloaded. This may be caused by inter-turn shorting of the windings or mechanical friction. Ignoring this parameter will result in the new compressor burning out in a few days due to problems with the wiring or relay.
Do not forget about the cleanliness of the system. When the circuit is depressurized (the tube is cut off), moisture and air instantly enters. Moisture combined with oil and freon forms an acid that corrodes the windings from the inside. Therefore, all work should be carried out as quickly as possible, and it is advisable to plug the tubes immediately after cutting.
Sometimes users confuse a compressor malfunction with a breakdown of the start relay. If the motor hums for 3-5 seconds and clicks when it turns off, it is most often a relay or wedge, and not a lack of pressure. In this case, checking the pressure is impossible until the cause blocking the start is eliminated.
Frequently asked questions (FAQ)
Is it possible to check the compressor without a pressure gauge, only by sound and vibration?
It is impossible to accurately determine the pressure by ear. The sound of a running compressor can be smooth even if there is no compression at all (for example, if the rod inside is broken). Vibration is also not a reliable indicator. Only a pressure gauge or a clear feeling of a powerful stream of air at the outlet provides objective information about the mechanical health of the unit.
What pressure should be in the system when the refrigerator is turned off?
When the refrigerator is turned off and warmed up to room temperature, the pressure in the system is equalized. For freon R134a it is about 6-8 bar, for R600a it is about 3-5 bar. If the pressure gauge shows zero, then all the gas has escaped through the leak. If the pressure is much higher than normal for a given temperature, there may be air in the system.
Why is the compressor hot, but does not create pressure?
This is a classic symptom of a breakdown of the valve group or piston wear. The engine works, compresses the gas, but due to the loose fit of the valves, the gas flows back without creating the required pressure. All energy goes into heating the case. Also, the cause may be a blockage of the system, but then the discharge pressure will be abnormally high if the tube is not completely blocked.
Is it possible to repair a compressor if it does not pump?
In domestic conditions, repairing hermetic compressors (replacing valves, rings) is practically impossible and economically impractical. The body is welded, and to access the insides it needs to be sawed and then re-sealed properly. It is easier and more reliable to buy a new or refurbished unit of the same power and refrigerant.
Does the amount of oil in the compressor affect the pressure created?
Yes, the oil level is critical. A lack of oil will lead to overheating and jamming, and an excess will lead to water hammer and the release of oil into the system, which will clog the capillary. In both cases, the compressor will not be able to work normally and create pressure. The oil should cover the lower part of the rotor/stator, but not splash in the upper part.