Symptoms: A Shanghai Buick sedan was experiencing an issue where the airflow from the air conditioning vents gradually decreased and even disappeared during driving, only to return to normal after about 6-7 minutes. This intermittent behavior made it difficult to diagnose at first glance.
Failure Analysis: The Buick vehicle is equipped with a variable displacement compressor, which adjusts its cooling capacity based on the pressure within the system. Unlike some systems that use temperature sensors at the evaporator outlet, this model does not monitor the outlet temperature directly. Instead, the compressor continuously runs, and the cooling intensity is controlled by an internal pressure regulating valve located inside the compressor.
When the low-pressure side of the system drops, the pressure regulator reduces the effective stroke of the five small pistons, decreasing the cooling effect. Conversely, when high-pressure levels rise, the valve shortens the piston stroke, reducing cooling. However, if the pressure fluctuates too much, the spring inside the pressure regulator can be affected. If the spring’s elastic force increases, it causes the pressure cut-off point to shift upward, leading to excessive cooling. This can result in the evaporator's outer wall freezing, blocking airflow and causing no wind from the vents.
Once the frost melts, the system returns to normal operation, but the cycle repeats. The root cause of the problem was traced back to refrigerant contamination. The system originally uses R134a, but if R12 was mistakenly added, it can significantly increase the cooling capacity. Over time, this leads to excessively low temperatures, which in turn alters the spring coefficient of the pressure regulator, causing the system to malfunction.
Remedy: To resolve the issue, the entire refrigerant system was evacuated and purged. The faulty pressure regulator was replaced, and the system was refilled with pure R134a refrigerant. After these steps, the air conditioning functioned properly, and the intermittent airflow problem was resolved. This case highlights the importance of using the correct refrigerant type and maintaining proper system pressure balance to avoid long-term damage.
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