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A BYD F3 car with a mileage of about 108 thousand km. The owner reflected that the coolant temperature of the vehicle engine was high. Fault diagnosis: first, verify the failure phenomenon after receiving the vehicle. It is found that the cooling fan does not rotate. After connecting the air conditioning switch, the condensing fan is rotated. It is preliminarily determined that the high temperature of the engine coolant is due to the cooling fan is not working. Find the control circuit of the cooling fan and the condensing fan of the car (Figure 1). Under normal circumstances, when the ignition switch is placed in the ON gear, the No. 1 ignition relay is absorbed. At this time, the air conditioning switch is pressed, the engine ECM controls the fan 1 relay and the fan 3 relay. The condensing fan relay starts working, the cooling fan is low speed spin. When the engine cooling temperature reaches 98 to 100, or when the air conditioning pressure switch terminal 2 and terminal 4 are connected (medium voltage protection), the engine ECM control fan 2 relay is absorbed and the cooling fan rotates at a high speed. At this time, the electronic fan adjustment resistance is no longer in series. Cooling fan circuit. Because the condensing fan is working normally, only the cooling fan is not working, the failure of the engine ECM control can be eliminated. The main faults may be due to the bad contact of the cooling fan wire harness, the fault of the line itself, the damage of the cooling fan itself, the fan 1 relay or the internal fault of the fan 2 relay. According to the principle of simple to complex fault diagnosis, first check the connection condition of the cooling fan wire connector, no anomaly is found; for the cooling fan alone, the cooling fan can rotate normally, thus the possibility of the cooling fan's fault can be eliminated; the fan 1 relay is replaced, the cooling fan is still not working; overhauled over and overhauled During the process, try to unplug the coolant temperature sensor wire connector and find the cooling fan rotating at high speed (if the coolant temperature sensor is broken, the engine ECM thinks that the engine is in a large load state, thus controlling the high speed rotation of the cooling fan, thus determining the fan 2 from the fan after the speed regulation resistance. " There is a problem in the low speed work control circuit. Pull down the fan 2 relay and check that the car is installed with a 4 terminal relay to check the circuit diagram. The fan 2 Relay installed by the car should be the 5 terminal relay. The car owner was asked to know that the car had previously maintained an air conditioning fault in other repair plants. It was considered that it was possible that the maintenance technician had not paid attention to the relay when it was replaced, and that the 4 terminal relay was replaced by a 4 terminal relay. The cooling fan is not working at low speed. Troubleshooting: replace the 5 terminal fan 2 relay test, cooling fan work normally, the engine is not in the cooling liquid high temperature failure, the fault is completely eliminated.