The mileage is about 110 thousand km, carrying the Dongfeng Citroen Elysee 8V car with manual transmission. When the car presses the air conditioning control switch, the air conditioning work indicator lights up and the compressor clutch has a sucking action, but it is immediately broken and the air conditioning system can not work normally. Fault diagnosis: according to the working principle of Alela 8V engine model air conditioning system, it is believed that the failure of the compressor clutch in the air conditioning system may be caused by the following aspects: the air conditioning switch is malfunction; the compressor clutch or the working line fault; the air conditioning system pressure is abnormal to protect the system automatically; the evaporator temperature Pass low protection; air conditioner regulator work bad; air conditioning relay work bad; engine control unit based on the speed to protect the working state of double seal relay; engine temperature protection and so on. The following sequence of diagnostic operations are performed: (1) replace a new air conditioning switch assembly, and the malfunction still exists. First, check the state of the clutch and the working line of the compressor, replace a new compressor clutch assembly, and the malfunction still exists. The working line is checked by multimeter and checked by resistance state, and no abnormal phenomena are found. 3. Remove the evaporator temperature sensor of the air conditioning system and replace it with a new assembly. The problem is not solved. The working line is checked and checked, and everything is normal. The pressure measurement of the high and low pressure pipe of the air conditioning system of the air conditioning system is measured by the air conditioning system refrigerant injection machine. The pressure value of the high and low pressure line of the air conditioning system is different from the standard value under the normal condition, and the possibility that the air conditioning compressor can not work normally is excluded because of the excessive or too little refrigerant in the air conditioning system. The state of the pressure switch is carefully checked with the special diagnosis wire harness of the air conditioning system, and the state of the pressure switch is carefully checked. After that, a new pressure switch assembly is replaced, and the fault still exists. The above check results show that the pressure switch is in good working condition. Check the working condition of the air conditioner and replace a new regulator assembly. Disconnect the connector on the jack and check the voltage value of each foot with multimeter. The measurement results fully meet the requirements of the working feet under normal conditions. Under the condition of opening the ignition switch, the voltage of the 15N9 foot of the double sealed relay is checked and the measured voltage value is the battery voltage. It shows that the double sealed relay is normal to control the working state of the line related to the work of the air conditioning compressor. The working state of the air conditioner relay 805 is checked, and it is found to supply the power supply line 12V of the compressor clutch (the power supply of 12V in the normal condition, the compressor clutch can form the working loop) line is disconnected, and the short connection of the 5N3 and 5N4 feet of the air conditioning relay 805 is found. When the compressor clutch is normally absorbed. The replacement of a new air conditioning relay assembly is still a problem. Accordingly, the fault is independent of the air conditioning relay and its circuit. Replace a new engine control unit assembly, the failure still exists, the engine control unit internal engine speed signal problem, cause the engine control unit internal control relay 822 (engine air conditioning compressor cut off relay) 5N1 and 5N2 electricity, this time 5N3 and 5N4 disconnect, air conditioning compressor clutch The interruption of the power supply foot basically eliminates the possibility that the function of the engine control unit controls the function of the relay. He continued to check the working feet of the water temperature control box. It was found that the 15N11 always attached the iron, while the voltage values of the other working feet met the normal requirements. According to this situation, the engine temperature information was read by the PROXIA diagnostic instrument, and the engine temperature parameters were found to be shown at 115 degrees C, while the water temperature meter on the dashboard showed only about 80 C at this time. Replace a new engine temperature sensor (water temperature control box), press the air conditioning control switch, air conditioning compressor recovery work, after a period of time road test failure phenomenon did not appear again. Failure summary: the water temperature sensor was dismantled and found that the internal failure occurred. According to the analysis of the failure phenomenon, it is believed that the internal failure of the sensor (Electrical number 910) sends engine temperature information to the water temperature control box, and the engine temperature information transmitted to the engine water temperature control box is higher than 112 C (the actual engine temperature is only about 80 c), according to the engine water temperature control box. The operating characteristics (air conditioning system is protected under the condition of engine temperature above 112 degrees C). Under this condition, the internal control system of the water temperature control box will automatically guide the 15N11 and 15N8 feet (the water temperature control box hits). The 5N3 and 5N4 of the air conditioning relay 805 disconnect, causing the air conditioning compressor clutch to disconnect and the compressor stop working at this time. The main purpose of entering this state of protection is to avoid the high temperature of the engine and cause more serious problems in the air conditioning system.