Volkswagen Volkswagen Volkswagen 1. 6L limousine with a mileage of about 40 thousand km. The owner reflected that the engine idling speed was unstable during the night driving, and the speed increased and then fell back, but idling speed was normal during the day. Fault diagnosis: first, the cold car starting engine, the idle speed is normal, the speed is 680 ~ 720r/min, the warm machine to the normal working temperature of the engine, the use of fault diagnosis instrument to inquire the electronic control unit, all no fault code. Check the data flow of engine electronic control system in normal range. A preliminary examination of the relevant sensors on the engine has not revealed any unusual conditions. Considering the failure of the car in the night, the front light is opened in the night mode. The engine idling is still normal, but the engine speed increases slightly when the temperature of the engine coolant reaches 95, and the cooling fan starts to run at low speed. As the temperature of the engine coolant increased to 99 degrees C, when the cooling fan worked at full speed, the engine idle speed increased obviously (9600 ~ 1050r/min), but the engine idle speed returned to normal working condition when the cooling fan stopped working. Thus, as long as the cooling fan works at high speed, the idle speed of the engine will obviously increase, and the idle speed of the engine will not be normal until the fan stops working. It takes into account that the electric control engine is not enough voltage in the power supply system, especially at night, the power supply of the headlamp is increased. The electric control system corrects the fuel supply of the engine, and improves the idle speed to compensate the voltage shortage of the power supply system. Subsequently, when inspecting the power supply system, it was found that the wiring of the battery negative pole was seriously oxidized and the terminals were not in good contact. Clean the battery junction column and replace the negative iron wire to test the engine. When the cooling fan is running at high speed, the engine idle speed is slightly elevated to the normal speed, and the failure is eliminated. Fault analysis: in order to ensure that the battery has enough electricity to make the engine start and run smoothly, the electric load (electric energy) management system, that is, the power control unit of the vehicle, is used to evaluate the battery voltage according to the battery voltage, engine speed, and the phase of the DFM signal of the generator. When the battery voltage is lower than a certain value, the vehicle power control unit will improve the engine idle speed to compensate the voltage of the power supply system (the system is also used in the first Volkswagen model of the FAW), and the electric equipment for the comfort function is properly closed. The vehicle power control unit controls the power consumption system according to the following 3 management modes. (1) mode one line 15 is connected, the generator is in working state. If the battery voltage is less than 12. 7V, the on-board power control unit requires the idle speed increase of the engine. If the battery voltage is less than 12. 2V, the seat heating, the rear window heating, the rearview mirror heating, the steering wheel heating, the pit lighting and the door handle lighting are closed, the full automatic air conditioning energy consumption reduces or the air conditioning closes, and the information entertainment system is closed. (2) mode two line 15 is connected, and the generator is in a state of shutdown. If the voltage of the battery is less than 12. 2V, the air conditioning energy is reduced or the air conditioning is closed, the crater lighting, the door handle lighting, the upper and lower lights and the left home function are closed, and the information entertainment system is closed. (3) mode three, line 15 is disconnected, and the generator is in a state of shutdown. If the battery voltage is less than 11. 8V, the inner lamp, the foot hole lighting, the door handle lighting, the up and down lights, the off house function are closed and the information entertainment system is closed.