Symptoms: A Shanghai Buick sedan experienced a gradual reduction in airflow from the air conditioning vents during driving, eventually disappearing completely. However, after approximately 6 to 7 minutes, the airflow would return to normal.
Failure Analysis: The Buick vehicle is equipped with a variable displacement compressor, where the cooling capacity depends on the operation of the air conditioning pump. The system automatically adjusts the cooling based on pressure changes, but it does not monitor the temperature at the evaporator outlet. Throughout the cooling process, the air conditioning pump remains continuously running. The adjustment of cooling intensity is entirely controlled by the pressure regulator valve located inside the compressor. When the low-pressure side drops, the pressure regulator reduces the effective stroke of the five small pistons, thereby decreasing the cooling output. Conversely, when the high-pressure side rises, the valve shortens the piston stroke, reducing cooling intensity. If the high-pressure side drops and the low-pressure side increases, the valve extends the piston stroke to enhance cooling. The key factor in this mechanism is the spring force within the pressure regulator. If the spring becomes too stiff, it causes the pressure cut-off point to shift upward, leading to excessive cooling. This can result in frost forming on the outer surface of the evaporator, causing no airflow from the vents. Once the frost melts, the system returns to normal.
Failure Cause: The Buick’s air conditioning system was originally designed for R134a refrigerant. However, if the system was contaminated with R12, it would significantly increase the cooling capacity. Over time, this excessive cooling could cause the spring coefficient of the pressure regulator to increase, leading to system failure.
Remedy: The refrigerant should be completely evacuated from the system, the pressure regulator replaced, and the system recharged with pure R134a refrigerant. This should resolve the issue and restore normal operation of the air conditioning system.
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