What causes low low pressure and high high pressure in car air conditioning?
3 Answers
The reason for low low pressure and high high pressure in the air conditioning system is poor circulation in the system. Focus on checking the following aspects: clogged capillary tube (or expansion valve), such as dirt blockage or ice blockage. Clogged dryer filter. Kinked connection pipes causing flow obstruction, shut-off valve not fully open, four-way valve not properly aligned, or dirty condenser tubes. Here are the relevant details: 1. Measure the high and low pressure of the air conditioning system while the engine is idling. 2. The normal high pressure for car air conditioning should be 1.3-1.7MPa (13-17kg/cm2 or Lbf/in2); the normal low pressure should be between 0.1-0.25MPa (1-2.5kg/cm2 or 15-35Lbf/in2). If the low pressure is higher or lower than this range, it indicates improper adjustment of the expansion valve. Compressor issues are not included in this category. When the engine throttle is increased, the measured low pressure should decrease with engine speed—the faster the speed, the greater (faster) the pressure drop. This indicates good compressor performance; otherwise, it suggests a compressor fault. 3. Note that car air conditioning pressure is highly dependent on ambient temperature (the working environment of the condenser).
I've repaired many car air conditioning systems, and the condition of low pressure on the low side and high pressure on the high side is quite common. The main cause is usually a clogged expansion valve. The expansion valve controls the flow rate of the refrigerant. Once it gets blocked, the pressure on the high side shoots up, while the pressure on the low side drops significantly, resulting in the air from the AC not being cold at all. It could also be due to excessive dirt accumulation in the dryer or the entire system getting air or moisture inside, disrupting the normal cycle. If you notice the AC performance deteriorating, you should promptly check the expansion valve and dryer, and replace them if necessary. Don’t delay too long, as pressure imbalance can damage major components like the compressor and condenser. For regular maintenance, it's best to clean the AC filter once a year to prevent contaminants from entering.
Last time I drove, I encountered an issue with the AC not cooling. At the repair shop, they found low low-pressure and high high-pressure readings. The mechanic checked and said the expansion valve was clogged, probably due to my lack of regular maintenance and too much dust buildup. After replacing the expansion valve, the AC immediately returned to normal. The whole process was quite simple and didn’t require much professional equipment. Now, I make sure to check the AC pressure before summer, buying a pressure gauge to measure the high and low-pressure values to see if they’re within the normal range. A reminder to fellow drivers: don’t neglect regular maintenance, otherwise refrigerant leaks or system contamination can easily cause such troubles.
