Failure phenomenon
A 2009 Brilliance BMW 523Li, equipped with N52B25BE models, 2.5L engine and 8-speed AMT automatic transmission, has a mileage of 14,971km. During the driving, the engine is unable to accelerate and the engine is trouble light lit up.
Fault diagnosis and remove dimension
After the maintenance personnel pick up the car, first confirm the fault phenomenon. It is found that the engine needs to be started continuously for 2 to 3 times before it can be started. After the car, the central display screen prompts the words "The driving device is faulty, it can continue driving, the acceleration ability is reduced, and BMW after-sales service will check as soon as possible", as shown in Figure 1.
Figure 1 central display prompt
On the road test of the faulty vehicle, it is found that the maximum speed of the vehicle on the horizontal road can only reach 30 km/h, and the vehicle on the road with a slight slope will not pass.
Use BMW special diagnosis equipment to test the engine and report multiple fault codes, as shown in Table 1.
According to the prompt of the fault code, the maintenance personnel suspected that the engine crankshaft position sensor was working abnormally. Therefore, a new spare parts store was borrowed from crankshaft position sensor for loading. After the test, the failure was still the same, indicating that there should be no problem with the original car crankshaft position sensor.
According to the diagnosis equipment "search function", maintenance personnel found that the crankshaft position sensor 12V power supply was controlled by F03(20A) fuse in the engine compartment fuse box. Turn off the engine and pull off the F03 fuse for inspection. The fuse is intact, but it is found that one end of the terminal at the lower end of the F03 fuse is corroded and oxidized, as shown in figure 2.
Figure 2 F03 fuse installation position
Carefully handle the oxidation terminal, plug the fuse again, and start the vehicle, wire and related lines again. The engine starts the car smoothly, its idle speed runs smoothly, the acceleration is smooth and powerful, and the engine trouble light goes out. After repeated road tests, the fault did not occur again. So far, the fault is eliminated.
Maintenance summary
Referring to the circuit diagram (figure 3), it can be seen that the fault of the car is mainly due to the corrosion and oxidation of the lower terminal of F03 fuse in the engine compartment fuse box, resulting in unstable power supply of relevant sensors of the engine electronic control system and DME, cause the engine to start difficult, speed up weak, trouble light alarm. Maintenance personnel took some detours during fault diagnosis. Due to the lack of working experience of maintenance technicians, after finding the fault codes of relevant sensors, the first step only thought of replacing the sensors. However, the faults still remained after replacing the sensors, to find the relevant lines of the sensor. It is understood that it is very difficult to measure the power supply line of crankshaft position sensor crankshaft position sensor on the car because the installation position of connector is relatively secret. Under the author's prompt, the maintenance personnel later measured the resistance value of crankshaft position sensor of the car, and the resistance between sensor Terminal 1 and Terminal 2 was about 3.5 MΩ.
Figure 3 circuit diagram
After receiving this fault case, find a 2010 BMW 523Li and pull off F03 fuse, connect diagnosis equipment, and read the fault code, as shown in figure 4.
Figure 4 read the fault code
From the point of view of fault codes, although it is slightly different from the author, maybe the software version or the annual model of the vehicle is different, there are indeed crankshaft position sensor fault codes.
In the actual maintenance diagnosis, the first step is to clarify the diagnosis idea. The first step is to strictly follow the inspection plan and fault guidance provided by the factory, so as to avoid many detours during diagnosis, improve work efficiency, and the detection plan of this fault is directly pointing to F03 fuse and related lines.
