The mileage is over 200 thousand km, with 3. 3L electronic control engine and automatic transmission of the Dodge dragon car. The car starts the engine and opens the air conditioning system. After a while, the coolant temperature appears to be on the high side, and the air-conditioning refrigeration function fails. Troubleshooting: basic inspection of the car condition, it is found that the cold fan is running in the condition of cold car, but the cooling fan does not turn in the hot car, and the engine warning light of the dashboard is always in a constant state. Check the cooling fan relay. It is found that the housing of the plug is missing and the line is connected. Using the multimeter to detect the 4 wires of the relay, it is confirmed that the black line is a ground wire, the gray line is a smart power supply line, the dark green line is connected with the two cooling fans, and the light green / blue line is connected to the power system control module PCM. The diagnostic system is used for self diagnosis of the power system, and the result is a malfunction code for the cooling fan circuit. The fault code can not be cleared by clearing the fault code. In order to confirm the cause of the line cause or the power system control module, the 12V power supply was introduced into the light green / blue line with the test lamp, and the fault code was removed and the engine warning light was normally extinguished. The above test indicates that the terminal connection of the cooling fan relay is wrong. Reconnect the circuit according to the circuit diagram, test the car, cool the fan relay, and cool the fan. About 10min, touch the cooling fan relay manually, feel the shell is very hot. Quickly extinguish, cooling a while, find the original installation of the cooling fan relay position, after cleaning, the cooling fan relay is fixed, test car, the failure completely eliminated. The car's cooling fan relay is very special. It is not an ordinary quadruped relay, but an electronic contactless relay integrated with circuit board and power pipe. The terminal arrangement of this kind of relay is very special. It is difficult to distinguish terminal use through terminal obstruction measurement. If the wiring harness plug is broken and no circuit diagram is used as reference, it is easy to get the wrong line. In addition, the relay has a larger load current and more heat, so it is not placed in the central distribution box with the relay in the design, but is installed on the left front frame alone to better heat the heat. In addition, it is necessary to explain that the cooling fan does not operate after the air conditioning, but the power system control module automatically controls the cooling speed of the cooling fan according to the air conditioning load and the cooling liquid temperature signal, so the cooling fan relay is also called the cooling fan control module.