A 2020 Mercedes-Benz EQC350 pure electric vehicle with a driving range of about 20,000 km. Customer feedback: The car battery light alarm, the vehicle can not start.
Fault diagnosis: The vehicle enters the store through the trailer, tries to start the vehicle, the result is that the vehicle cannot start, the instrument appears red battery fault alarm and trailer prompt is not allowed.
Ask the customer to know that the vehicle is used for the customer's home, driving on the urban road, charging on the fixed charging pile, and there is no special way to use it. I bought a car for nearly 1 year, did a maintenance, and had no other maintenance records.
A quick test of the vehicle was carried out with a Mercedes-Benz special diagnostic instrument (XENTRY), and the results showed that the N82/3, N83/11 and N115/63 control units had fault codes, where the fault code P2C8500 was currently present, as shown in Figure 2.
From the fault code and fault phenomenon, it is obvious that there is an insulation fault in the vehicle high voltage system, and the inspection direction should start from the insulation fault code, so that the guidance test of the fault code (as shown in Figure 3) is performed, and the result is guided to the halfway can not continue to boot, which may be related to the diagnostic instrument software version.
Reading the actual value of the insulation resistance of the high voltage system (as shown in Figure 4), the result is that the insulation resistance value is 56kΩ, which is beyond the standard and is not normal.
Enter the Detection menu item, and then perform the positioning test of the insulation fault part, the result is that XENTRY tests the insulation resistance of the N129/1, N129/2, F34/6 and N116/5 (as shown in Figure 5~Figure 12), which is not normal.
According to the previous step of the inspection, it is necessary to further confirm which high voltage component has an abnormal insulation resistance. Thus, the circuit diagram of finding the battery pack in the WIS is shown in FIG. 13 to FIG. 15, where N116/5 charges the battery pack A100, while the A100 supplies high voltage power to N129/1 and N129/2, and supplies power to other high voltage components through F34/6.
According to the circuit diagram, the insulation resistance is measured by disconnecting the high voltage plug one by one, and it is found that the HV+ terminal and HV-terminal to ground resistance of N129/2 are 0.02MΩ and 0.03MΩ respectively (as shown in Figure 16 and Figure 17), and the standard is infinity, which is not normal, that is, N129/2 is faulty.
According to the results of the previous step, replace the pre-power electronic control unit (integrated with the front motor), then remove the fault code, the vehicle starts smoothly, and the fault is completely eliminated.