BYD Song DM instrument prompts “Please check the ESP system” and “Please check the HDC system”

Q: A BYD hybrid DM Song car purchased in August 2017 has a mileage of 80,000km, and its power and fuel consumption are very stable. Recently, during a long-distance driving, the car suddenly appeared on the dashboard with the prompts “Please check the ESP system” and “Please check the HDC system”. The owner was very nervous at the time and immediately stopped the car for inspection. After restarting the car, the fault prompt disappeared. The owner thought it was an occasional fault and did not care. But recently, this prompt has appeared frequently, so the car had to be sent for repair and inspection. The maintenance technician inspected the main components involved in the instrument circuit and driving, and did not find any abnormalities, but did find in multiple test drives that the fault prompts described by the owner occasionally appeared, without any regularity, and had no direct correlation with the speed, curve or straight driving, road conditions, uphill or downhill. What is even more strange is that when the fault prompt appears, it has basically no effect on the driving of the vehicle. The car owner is worried about the potential safety hazard, and the maintenance technicians are also confused and difficult. Teacher, how should we start to repair this fault?

Answer: For this kind of difficult fault, it is recommended to detect the fault code and dynamic data flow first, find out the rules, and then analyze the possible fault location. The remote guidance of the vehicle maintenance process is as follows: Use the VDS2000 diagnostic instrument dedicated to this model for detection. In the long-term tracking detection, it was indeed found several times that in the ESP, EPB and TCU modules, there were faults related to ECM loss of communication (Figure 1). ECM is the engine electronic control module, which has the function of continuously monitoring and controlling the normal operation of the engine. It is a very important core module. From the maintenance experience, it can be known that the communication between ECM and ESP, EPB, TCU and other modules belongs to the subnet of the vehicle’s power system. Once there is a fault of interruption of communication with the core module, the fault code will be generated first. From the above-mentioned occasional fault situation, it can be seen that the fault of this vehicle should be generated early, because if the communication is often seriously interrupted, it will inevitably affect normal driving.

Why do multiple modules such as ESP, EPB and TCU have communication failures? According to logical reasoning, it should not be that multiple modules have CAN bus disconnection or other faults at the same time. The resistance between the high and low CAN buses is 62Ω, the static voltage on the bus is 2.5V, the grounding terminal is connected normally, and the circuit connectors are also good. It seems that the power subnet is in normal condition. What factors may cause this fault? Recalling that during the test, when the prompts “Please check the ESP system” and “Please check the HDC system” appeared alternately on the instrument, the prompt “Please check the charging system” occasionally appeared, accompanied by the flashing of the charging indicator light. At that time, this was also analyzed, and the measured battery charging voltage was 13.92V, which met the requirements, so it was considered that the charging system was in normal condition.
This phenomenon of flashing charging indicator lights should be alerted, and the fault may be related to the generator or battery. There will be rectified ripple voltage during charging, and the power grid fluctuation may also produce a higher surge peak voltage. When the voltage peak is severe, the electronic control system will also interrupt the communication system for protection measures. Although the charging voltage is normal when checked with a multimeter, the battery is actually a large load, which has a good voltage stabilization effect and can buffer the peak voltage of the power system. However, if the condition of the battery becomes worse, the buffering degree of the peak voltage will be greatly reduced. Considering that the vehicle is 4 years old and the battery has not been replaced, the service life of a general low-voltage battery is about 3 years. It is estimated that its performance has deteriorated, and a bad battery may be a common factor that causes communication failures in multiple modules. After replacing the battery, the fault was no longer found after the test drive. The vehicle was tracked for two months and the vehicle fault did not reappear.