Maintenance case of BMW air conditioner sometimes not cooling and cooling fan rotating too fast.

Fault phenomenon: sometimes the air conditioner does not cool (it is easy to appear when it is hot), and sometimes it is normal. When not cooling, the condenser cooling fan is noisy and has high speed. In addition, the air volume of the air outlet is not affected, but the temperature of the air outlet is problematic. Fault vehicle information: model code: E66 engine model: N52 mileage: 9 1700KM Maintenance process: ISTA diagnosis, no related fault. Connect BMW 2000 refrigerant filling machine with low pressure of about 1.6Bar and high pressure of about 2 1Bar. Judging from experience, the low pressure is too low and the high pressure is too high, which may be the blockage of the air conditioning system. Check the appearance of condenser and oil tank, and there are no foreign objects. Check whether the electronic fan and mechanical fan are normal. Recovered refrigerant 1 120g, and 25ml of refrigeration oil, without exception. After re-venting and adding refrigerant, the fault still exists. Check the voltage at pins 1 and 2 of the compressor regulating valve plug X 163, which is about 9.5- 10.5V, and there is no abnormality. Remove the high-pressure pipeline of the air compressor and the high-pressure pipeline of the expansion valve. After ventilation, the airflow is normal. Take out the dry bottle and check whether it is clean. No abnormality is found. Measure the front throttle (part number 645 18390990), and the power supply of the plug X984 1 pin and pin 2 of Y 13 is 13.5V, and the throttle resistance is about 22 ohms, which is normal. According to the change of plug pressure when the air conditioner is running, it cannot be completely explained that the throttle valve is always normal. After that, remove the throttle valve and the front expansion valve to check that there are no foreign objects and the ventilation is normal. Then put the expansion valve into the refrigerator for freezing. There is no doubt that it will be checked immediately after it is taken out. Although the front and rear throttle valves with the same part number were replaced and a new front expansion valve was borrowed for trial operation, the fault continued. After many trial runs and tests, it was finally found that the pipeline between the front expansion valve and the front evaporator was frozen. There is no problem with the expansion valve and speed regulating valve in front. It is incredible why this position will freeze. The master who adjusted the front and rear throttle later said that the front throttle was not easy to install. In contrast, the pipeline where the throttle valve is installed is really bent. The difficulty of fault diagnosis and installation shows that there is a large displacement between the air conditioning pipeline and the throttle valve, resulting in a large tension and stress. The valve holes of expansion valve and throttle valve are also elastically deformed relative to the needle valve under the action of force, and finally the needle valve cannot move freely as designed. Troubleshooting after replacing air conditioning pipeline.