Diagnostics of the refrigeration system often begins by listening to the operation of the motor, but the most accurate data on the condition compressor can only be obtained by measuring the pressure in the circuit. When the refrigerator stops freezing and the compressor hums or, conversely, is silent, checking the pressure becomes a key step in determining the malfunction. It is this parameter that allows you to understand whether there is a freon leak, whether the capillary pipeline is clogged, or the unit itself has lost its performance.
Many self-taught craftsmen mistakenly believe that if the compressor pumps air “by ear” or your finger feels suction, then it is working. However, the real picture opens only when you connect a professional pressure manifold. The discharge and suction pressure will tell you about the tightness of the system and the ability of the motor to create the necessary difference for the circulation of the refrigerant. Without this data, repairs turn into guesswork.
In this guide we will examine in detail the process of connecting equipment, interpreting pressure gauge readings and identifying typical problems. You will learn what values are normal for different types of refrigerants and how to distinguish a faulty compressor from a blockage in the system. Correct diagnostics will save you time and money, allowing you to avoid purchasing unnecessary spare parts.
Necessary tools and workplace preparation
Before you start inserting into the system, you need to prepare a specialized tool. The main device here is pressure gauge station (manifold), consisting of two pressure gauges (blue for low pressure and red for high) and a set of hoses. For domestic refrigerators, a blue pressure gauge and hose are most often sufficient, since the test is often carried out on the suction side, but for a complete diagnosis you need a complete set.
⚠️ Attention: Before starting any work on the refrigeration circuit, make sure that the refrigerant is completely removed from the system. The release of freon into the atmosphere is prohibited by environmental regulations, and working under pressure without bleeding can lead to injury.
You will also need tools for inserting the Schrader valve (or using a piercing valve), a pipe cutter, a roller (if you plan to replace sections of the tube) and an electronic leak detector or soap solution. Do not forget about personal protective equipment: glasses and gloves, since a sudden release of gas can burn your skin or eyes.
Preparation of the workplace includes ensuring good ventilation, since freon is heavier than air and can displace oxygen in the lower part of the room. In addition, make sure that you have access to a grounded electrical outlet, as the compressor will have to be started briefly for measurements.
☑️ Preparing for diagnostics
Valve insertion and connection technology pressure gauges
To connect pressure gauges to the sealed circuit of the refrigerator, it is necessary to install a service valve. Most often this is done on the compressor process fitting (filling tube) on the suction side. If the factory fitting is missing or cut off, a brass solder mortise valve or piercing valve is used (although the latter is considered a temporary solution and is not recommended for long-term use).
The mortise process requires care. The tube is cleaned of oxides, a valve is put on it, and soldering is done with soft solder. It is important not to overheat the tube, so as not to change the properties of the metal and burn a hole. After cooling, the blue hose of the pressure gauge station is connected to the valve. Before connecting, it is recommended to remove air from the hoses by briefly opening the valves on the manifold.
After connecting the hoses, but before starting the compressor, it is necessary to check the static pressure in the system. If there is still freon left in the circuit, the pressure gauge will show a value corresponding to the ambient temperature. If the system has been evacuated or completely empty, the device will show zero. The presence of positive pressure when the compressor is turned off indicates that the seal may not be broken and the refrigerant has not escaped anywhere.
Pressure standards for various types of refrigerants
Understanding pressure standards is critical for correct diagnosis. The pressure in the system directly depends on the type of refrigerant used and the ambient temperature. In household refrigerators, they are most often found R134a, R600a (isobutane) and, in older models, R12. Each of them has its own physical properties and operating pressure.
For example, isobutane (R600a) operates at a lower pressure compared to freon R134a. An attempt to assess the serviceability of a compressor operating on isobutane using tables for freon will lead to erroneous conclusions. Below is a table of approximate suction pressure values at an ambient temperature of about +25°C.
| Refrigerant type | Suction pressure (Low Side), Bar | Discharge pressure (High Side), Bar | Features |
|---|---|---|---|
| R134a | 0.8 – 1.2 | 8.0 – 12.0 | Works with mineral or synthetic oil |
| R600a | 0.4 – 0.6 | 5.0 – 8.0 | Explosive, low operating pressure |
| R12 | 1.0 – 1.5 | 9.0 – 11.0 | Outdated, found in old equipment |
| R404A | 3.5 – 4.5 | 15.0 – 18.0 | Used in chest freezers and display cases |
It is important to understand that the numbers in the table are approximate. The actual static pressure (when the compressor is off) depends on the room temperature. There is a direct correlation: the higher the temperature, the higher the saturated vapor pressure of the refrigerant. Therefore, if you carry out diagnostics in a cold garage in winter, the indicators will be significantly lower.
Algorithm for checking the compressor under load
The most informative stage is checking the pressure over time, that is, with the compressor running. After connecting the pressure gauge to the service port, turn on the refrigerator. In the first seconds, the pressure gauge needle will begin to sharply creep down (if we check the suction side), creating a vacuum. This is a normal reaction of a working unit, which begins to pump gas out of the evaporator.
If the compressor is working properly and the system is sealed, the pressure on the suction side will drop to values close to vacuum (negative values, if the scale allows, or close to 0 bar absolute pressure), and then begin to slowly increase as the evaporator thaws and freon vapor enters. However, if the arrow immediately shows a deep vacuum and the compressor operates “in tension”, this may indicate a blockage of the capillary tube.
On the other hand, if when the compressor is turned on, the pressure does not drop at all or drops very slowly, remaining high, this is a sure sign of loss of performance of the piston group or the presence of breakdown of the valves. In this case, they say that the compressor “does not hold compression.” It is also worth checking the current consumption: if the current is low and the pressure is high, the compressor is not pumping; if the current is high and the pressure is high, there may be a problem with the condenser or the presence of air in the system.
⚠️ Attention: Never leave a running compressor connected to the atmosphere (without a condenser or with the outlet open) for more than 1-2 minutes. This can lead to overheating of the motor windings or damage to the valves due to operation at a vacuum without cooling.
Diagnostics of leaks and checking for tightness
One of the main tasks of a pressure test is to search for leaks. If, after refueling the system and checking the compressor, the pressure drops over time (in static conditions, with the unit turned off), it means that the seal is broken. To find a leak, the system is pressurized (filled) with dry nitrogen at a pressure of about 10-12 bar.
The use of nitrogen is preferable, since it is inert and does not contain moisture that could freeze in the capillary. After creating pressure, all connections, seams and tubes are generously lubricated with soapy water. The appearance of a “soap bubble” will indicate where the gas is escaping. Pay special attention to the insertion point of the process fitting and the evaporator, if it is foamed in the refrigerator wall.
Sometimes the leak can be microscopic, and the soap solution does not immediately show the result. In such cases, the pressure gauge is left under observation for several hours or even days. A drop of the needle even by a fraction of a division indicates a violation of the tightness. Also, to find hard-to-reach leaks, you can use an electronic leak detector that reacts to refrigerant vapor.
What to do if the leak is in a foamed evaporator?
If it turns out that the evaporator, which is located inside the polyurethane foam “coat” of the housing, is poisoning, repairs become difficult. Often you have to open the back wall or side of the refrigerator, cut out the evaporator and install an external (crying) evaporator inside the chamber. This is a labor-intensive process that requires restoration of thermal insulation.
Interpretation of abnormal pressure gauge readings
The ability to read pressure gauge readings is a skill that comes with experience, but every repairman should know the basic anomalies. Let's consider several scenarios. If on the discharge side (red pressure gauge) the pressure increases very quickly and reaches high values, and on the suction side (blue) a deep vacuum is created, this is a classic sign clogging of the capillary tube or filter drier.
In the opposite situation, when there is pressure at both the inlet and outlet level out and become low, and the compressor runs quietly, it can be diagnosed freon leak. If the pressures are high on both sides, and the temperature of the discharge tube is not very high, this may indicate the presence of air in the system or a malfunction of the condenser blower fan (in No Frost models).
The situation when the compressor hums, but does not start, and the pressure in the system does not change deserves special attention. Here the problem may not be in the hydraulics, but in the electrics: the starting relay is faulty, the rotor is jammed, or the winding is burned out. Checking the pressure in this case is secondary, the primary thing is checking the electrical parameters and trying to start through the converter.
Frequent errors in diagnosing pressure
Beginners often make mistakes that lead to false diagnoses. One of the most common is checking the pressure immediately after turning off the refrigerator. Time must pass in the system to equalize the pressures (balancing), otherwise the readings will be distorted by the residual pressure in the condenser and evaporator.
Another mistake is ignoring the ambient temperature. Trying to diagnose a refrigerator that has just been brought in from the cold without first warming it up will give incorrect results. The refrigerant may be in the liquid phase in the compressor crankcase, which will lead to water hammer at startup or incorrect pressure gauge readings.
Also, do not rely on only one parameter. Pressure is only part of the puzzle. Always correlate it with the current consumption of the compressor, the temperature of the discharge and suction tubes, as well as the temperature in the refrigerator chambers. An integrated approach allows you to avoid replacing a serviceable, but incorrectly diagnosed compressor.
What pressure should be in the switched off refrigerator?
In the switched off and warmed up to room temperature refrigerator, the pressure in the system should correspond to the saturated vapor pressure of the refrigerant at a given temperature. For R134a at +25°C this is approximately 6-7 bar, for R600a - about 3.5-4 bar. If the pressure is significantly lower, there is a leak in the system. If it is equal to atmospheric (0 bar according to the pressure gauge), the system is empty.
Is it possible to check the compressor without pressure gauges?
Qualitatively - no. You can only indirectly evaluate the work by vibration, sound and temperature of the tubes. However, a “finger on the tube” will not give accurate compression data. For professional diagnostics, a pressure gauge station is required.
Why does the compressor create a vacuum, but the refrigerator does not freeze?
This indicates a lack of refrigerant circulation. Most likely, the capillary tube or filter drier is completely clogged. The compressor pumps gas out of the evaporator, creating a vacuum, but due to the plug, new freon does not enter the evaporator, and the cooling capacity is zero.
Is it dangerous to work with R600a?
Yes, isobutane (R600a) is explosive. At concentrations as low as 30 mg per liter in air, a spark can cause an explosion. Therefore, when working with such refrigerators, it is prohibited to use open fire, solder without prior ventilation and burning with a gas burner, and also store gas cylinders in a room with operating equipment.
What is “overheating” of a compressor by pressure?
If the temperature of the discharge tube exceeds 80-90°C, and the discharge pressure is abnormal high, this may indicate overheating. Reasons: the condenser is dirty, the fan is faulty, there is air in the system or an excess of freon. Long-term operation in this mode will lead to jamming of the compressor.