Fault phenomenon: a 2004 production of the Passat B5 car, equipped with BFF 2.0L engine, the use of BOSCH ME 7.5 electronic control fuel injection system, equipped with manual air conditioning. The car opens the air conditioning switch when the cold car has just started, the air conditioning works well, the air vent can blow out the cool wind, after a period of time, the air conditioning slowly is not normal, the wind blows out of the wind elbows are cool, but sometimes it can work normally, when it is bad, and there is no law to follow. Fault diagnosis: the main causes of intermittent uncooling of the air conditioning system of Passat car are: (1) evaporator frosting; (2) the system has water, dirty plugging or condenser heat dissipation; (3) compressor variable adjustment failure; (4) system control circuit barrier. Through inspection, it is found that when the fault occurs, the compressor does not work, the opening test lamp measure the air conditioning compressor clutch coil without "electricity", pull out the electromagnetic clutch plug, use the cross wiring to send the compressor clutch coil separately, the clutch suck in operation, at this time, the air conditioning outlet can blow out the cool wind, it seems that the cause of the failure is that the cause of the failure is that the cause of the failure is that the reason for the failure is that the reason for the failure is that the reason for the failure is that the reason for the failure is that the reason for the failure is that the reason for the failure is that the reason for the failure is that the reason for the failure is that the reason for the failure is that the reason for the failure appears to be the reason why the fault is caused by the failure. There is a problem with the control line of the compressor. Then check the control circuit of the compressor. The control circuit of the vehicle's air conditioning system is shown in the diagram. Open the blower switch E9, press the air conditioner switch E35, the 12V voltage from the 503 connection point of the unloading line 75X, the fuse S225, the blower switch E9, the air conditioning switch E35 and the outside temperature switch F38; then, it is divided into 3 ways: one way through diode G28 enters the fortieth foot of the engine electronic control unit J220, which is used as the air conditioning request signal; The low pressure contact of one way through double pressure switch F129 enters the 9 foot of the air conditioning cut relay J314. If all the working conditions of J314 are satisfied, the 12V voltage of the electromagnetic clutch of the compressor is supplied by the No. 8 foot, the compressor is working normally; the other way enters the fan relay J26, and the 126 coil of the cooling fan relay is done work, After the contact is closed, the coil working circuit of the fan low speed relay J79 is switched on, and the 279 work is provided to the cooling fan at low speed for power supply, and the fan operates at low speed. Of course, J279 is also controlled by the low speed contacts of the thermistor switch F18 (closed at 95 degrees centigrade). Similarly, the high speed relay J513 of the cooling fan is controlled by double pressure switch F129 high pressure contact (1500kPa closed) and the high speed contact of the thermosensitive switch F18 (102 centigrade). The fan operates at high speed when J513 works. The main components that affect the normal work of the compressor are: 1. when the pressure in the air conditioning system is lower than 220kPa or higher than 3200kPa, the double pressure protection switch F129 is disconnected and the working compressor is stopped. Prevent the system from working too low or too high and continue working to damage the compressor. 2. when the external temperature is below 5 C, the external temperature switch F38 will cut off the compressor that is working, and under this condition, the air conditioning system will not start. 3. air conditioner cut off relay J314. The relay first feet are connected to the forty-first legs of the engine control unit J220 and are controlled by J220. When the fortieth foot of the electronic control unit J220 receives the request signal of the air conditioning switch, the following conditions are detected: (1) when the engine is in full load state; (2) when the engine is in the emergency operation state, the engine control unit J220 will control the air conditioning cutting controller J314 to cut the 12V voltage of the electromagnetic clutch of the compressor, the compressor will stop Stop work. When the engine control unit (J220) fails to control the cutting control of the compressor, the function of the data block can be read and measured by the reading of the fault diagnosis instrument, and the detection and diagnosis of the air conditioning cutting control can be carried out. The specific method is: adjust the 050 display groups and observe the display area: when the air conditioning is closed, the display area 3 should be displayed as "A / C - Low", and the display area 4 should be displayed as "KOMPR, AUS", connect air conditioning, compressor operation, and display area 3 should be displayed as "A / C one High", display area 4 should be shown as "KOMPR, EIN". If the contents are not correct, check the air conditioning cut-off relay 1314, the engine control unit J220 and the relevant lines. In addition, the fourth feet of the relay are connected to the T32b / 12 end of the instrument control unit J285. If the coolant temperature is too high, J285 will cut off the fourth feet of the relay T314 through the T.32b / 12 terminal control air conditioning, and the compressor will stop working. In the inspection, it was found that when the compressor stopped working, the air conditioning switch was closed and then opened, and sometimes it could be absorbed normally, but it would break and no longer absorb. According to E analysis, connect fault diagnosis instrument into engine control system, check no fault code, use read data block function, transfer 050 display groups, open air conditioning switch observation and find that the third display area is "A / C - High'" when the fault occurs, and the fourth display area is "KOMPR EIN", indicating that the compressor is not working. The electronic control unit J220 has not cut off the compressor. Then check the air conditioner to cut off the relay J314 fourth feet to the cooling liquid superheat signal of the T32b / 12 terminal of the instrument control unit J285. When the signal line is broken, the air conditioning refrigeration is restored to normal, and the circuit is measured without short circuit or circuit breakage. The fault diagnosis instrument is used to read the engine temperature data by the instrument control unit. It is shown to be 97. %: it is suspected that the fault of the instrument control unit resulted in the abnormal cut-off of the air conditioner compressor. Troubleshooting: replacing the instrument control unit and matching the anti-theft system, the air-conditioning refrigeration is restored to normal and troubleshooting. Fault summary: the instrument control unit J285 has cut off the compressor when the engine temperature is too high. In fact, it is realized by the J285 T32b / 12 terminal controlling the fourth feet of the relay to cut off the relay of the relay. The concrete is as follows: when the water temperature is higher than 119, the J285 control J314's fourth feet are grounded, and the compressor is cut off. When the water temperature is less than 114 degrees, J285 stops the J314 fourth month grounding control. At this time, the line voltage is 12V, and the compressor works. In this case, if the cost of the replacement instrument is higher, the line is disconnected directly without replacing the instrument, and the air conditioning can be restored to normal refrigeration. Only when the engine temperature is too high, it loses its protective effect and violates the original intention of designing the function.