As an automotive technician specializing in electric vehicles, I recently encountered a challenging case involving a BYD EV model that exhibited a high-voltage power-up failure. This BYD car, a 2019 pure electric model with a battery capacity of 130Ah and approximately 50,000 km on the odometer, presented with an issue where the driver could not initiate the high-voltage system despite following the standard startup procedure. The owner reported that upon pressing the brake pedal and activating the one-touch start switch, the instrument panel illuminated normally but did not display the State of Charge (SOC) value, and the vehicle failed to power up the high-voltage system. This prevented the BYD EV from being driven, as it could not shift into drive or reverse gears. In this detailed account, I will walk through the entire diagnostic process, utilizing various tools and analytical methods to identify and resolve the fault, while emphasizing the importance of communication networks in modern BYD car systems.
The initial step in addressing this BYD EV issue involved gathering the necessary diagnostic equipment. I prepared a range of tools, including the BYD-specific fault diagnostic instrument VDS2100, insulated gloves and insulated tools to ensure safety when handling high-voltage components, a multimeter for electrical measurements, an oscilloscope for waveform analysis, a battery tester to assess the high-voltage battery, and a set of检修线组 (inspection wiring harnesses) for circuit checks. Having the right tools is crucial for efficiently diagnosing complex systems in a BYD car, as it allows for precise measurements and reduces the risk of misdiagnosis.
Upon verifying the fault phenomenon, I confirmed the owner’s description: when the one-touch start switch was activated with the brake pedal depressed, the instrument panel lit up but did not show the SOC value or the “OK” indicator light. Additionally, no relay engagement sounds were audible, and multiple warning lights illuminated on the dashboard, including those for the charging system, power battery fault, and powertrain system. Messages such as “check vehicle network” were displayed, further indicating a communication issue within the BYD EV. This combination of symptoms suggested a potential problem with the vehicle’s network systems, such as the start CAN, comfort CAN, or power CAN buses, which are integral to the operation of this BYD car.
To delve deeper, I connected the VDS2100 diagnostic tool to the BYD EV and retrieved three fault codes: “B12ECO0 – Power Network Communication Fault,” “UO15987 – Loss of Communication with VTOG,” and “U011087 – Communication Fault with MCU.” These codes pointed toward disruptions in the communication pathways essential for the BYD car’s high-voltage power-up sequence. Analyzing these codes in conjunction with the observed phenomena, I hypothesized that the root cause lay in the vehicle’s network architecture, particularly the power CAN system, which facilitates data exchange between critical modules like the Battery Management System (BMS), motor controller, and gateway controller. The inability to display SOC on the instrument panel reinforced this, as the BMS and instrument cluster communicate via the power CAN bus. A summary of these fault codes and their implications is provided in the table below.
| Fault Code | Description | Potential Impact on BYD EV |
|---|---|---|
| B12ECO0 | Power Network Communication Fault | Disruption in data exchange between powertrain modules, preventing high-voltage power-up in the BYD car. |
| UO15987 | Loss of Communication with VTOG | Inability to communicate with the vehicle traction motor controller, affecting torque and power delivery in the BYD EV. |
| U011087 | Communication Fault with MCU | Issues with motor control unit data, leading to failures in engine start or high-voltage system activation in the BYD car. |
Understanding the high-voltage power-up control principle is essential for diagnosing such faults in a BYD EV. The process begins when the driver initiates a start request by pressing the brake pedal and the one-touch start switch. This action triggers a series of communications over various CAN buses, including the start CAN, comfort CAN, and power CAN. The intelligent key system sends an unlock signal via the start CAN to the Body Control Module (BCM), which then relays this information through the comfort CAN to the gateway controller. From there, the signal is transmitted over the power CAN to the Vehicle Control Unit (VCU) and the instrument cluster. The VCU, upon receiving the unlock signal, coordinates with the BMS, motor controller, charge-discharge distribution box, and gear controller to verify conditions such as high-voltage interlock status, insulation resistance, SOC, gear position, brake switch status, and overall system health. If all parameters are normal, the VCU sends a power-up permission to the BMS, which sequentially controls the main negative relay, pre-charge relay, and main positive relay to energize the high-voltage system. The entire sequence can be modeled using a control flow diagram, and the communication integrity can be assessed using formulas related to network performance. For instance, the voltage levels on the CAN bus lines must adhere to specific thresholds for reliable data transmission. In a BYD car, the dominant and recessive voltage levels for CAN-H and CAN-L are critical; typically, the dominant voltage for CAN-H is around 3.5V, and for CAN-L, it is 1.5V, while recessive levels are approximately 2.5V for both. Any deviation, such as an increase in CAN-H dominant voltage or a decrease in CAN-L dominant voltage, can indicate faults like open circuits or shorts. The relationship can be expressed using Ohm’s law and network equations: $$V_{CAN-H} = V_{dominant} + \Delta V$$ and $$V_{CAN-L} = V_{recessive} – \Delta V$$, where $\Delta V$ represents the voltage shift due to faults. Additionally, the resistance in the CAN bus lines should be low to minimize signal attenuation; for example, the characteristic impedance is typically 120Ω, and any significant increase can lead to communication failures. The following table outlines the key parameters for the power CAN bus in a normal BYD EV scenario.
| Parameter | Normal Value | Description |
|---|---|---|
| CAN-H Dominant Voltage | 3.5 V | Voltage level when transmitting a dominant bit on the CAN-H line in a BYD car. |
| CAN-L Dominant Voltage | 1.5 V | Voltage level when transmitting a dominant bit on the CAN-L line in the BYD EV. |
| CAN-H/L Recessive Voltage | 2.5 V | Voltage level when no data is transmitted, indicating idle state in the BYD car network. |
| Bus Resistance | 60 Ω (between CAN-H and CAN-L) | Termination resistance to prevent signal reflections in the BYD EV CAN bus. |
| Data Rate | 500 kbps | Communication speed for power CAN in most BYD car models. |
Proceeding with the fault diagnosis, I focused on the power CAN system, as the fault codes and symptoms indicated a communication breakdown. Using an oscilloscope, I measured the signal waveforms at the BMS connector terminals BK45B/16 (CAN-H) and BK45B/17 (CAN-L) relative to ground. In a properly functioning BYD EV, the waveforms should show clean transitions between dominant and recessive states, with CAN-H rising to around 3.5V and CAN-L dropping to 1.5V during dominant bits, and both settling at 2.5V during recessive bits. However, the实测波形 (measured waveforms) revealed anomalies: the CAN-H dominant voltage was elevated beyond the normal range, while the CAN-L dominant voltage was reduced, suggesting a potential open circuit in the CAN-H line. This deviation can be quantified using the formula for voltage imbalance: $$\Delta V_{imbalance} = |V_{CAN-H} – V_{normal}| + |V_{CAN-L} – V_{normal}|$$, where $V_{normal}$ represents the expected values. In this case, the calculated $\Delta V_{imbalance}$ was significant, confirming an abnormality. To further investigate, I used a multimeter to measure the resistance between the BMS’s BK45B/16 terminal and the gateway controller’s G19-9 terminal. The resistance reading was infinite, indicating an open circuit—a clear fault in the wiring harness of the BYD car. This break in the CAN-H line disrupted communication between the BMS and other modules, preventing the high-voltage power-up sequence from completing. The following table compares the measured values against the normal benchmarks for this BYD EV.
| Measurement Point | Normal Value | Measured Value | Deviation |
|---|---|---|---|
| BK45B/16 (CAN-H) to Ground | 3.5 V (dominant) | 4.2 V (dominant) | +0.7 V, indicating possible open circuit |
| BK45B/17 (CAN-L) to Ground | 1.5 V (dominant) | 1.2 V (dominant) | -0.3 V, consistent with CAN-H fault |
| Resistance: BK45B/16 to G19-9 | ~0 Ω (continuous) | Infinite Ω | Open circuit confirmed |
After identifying the open circuit in the CAN-H line, I proceeded to repair the fault by locating the break in the wiring harness of the BYD EV. This involved tracing the path from the BMS to the gateway controller, inspecting for any physical damage, corrosion, or loose connections. In this BYD car, the break was found near a connector junction, likely due to vibration or wear over time. I repaired the wire by splicing and soldering, ensuring a secure connection with proper insulation. Following the repair, I rechecked the resistance between BK45B/16 and G19-9, which now read approximately 0 Ω, indicating continuity. I then repeated the oscilloscope measurements and observed that the waveforms had returned to normal, with CAN-H and CAN-L levels matching the expected values. To verify the fix, I performed multiple start attempts: upon pressing the brake pedal and activating the one-touch start switch, the instrument panel displayed the SOC value and the “OK” light, relay engagement sounds were audible, and all warning lights extinguished. The BYD EV successfully powered up the high-voltage system and allowed gear shifts into drive and reverse, confirming that the fault was彻底排除 (completely resolved). This repair highlights the critical role of the power CAN bus in the overall functionality of a BYD car, and the importance of thorough diagnostic procedures.
In summary, this case of a high-voltage power-up failure in a BYD EV underscores the complexity of modern electric vehicle systems and the interdependence of communication networks. The fault stemmed from an open circuit in the CAN-H line of the power CAN bus, which disrupted data exchange between the BMS, instrument cluster, and other control modules. This prevented the BYD car from completing the high-voltage power-up sequence, leading to the observed symptoms. The diagnostic process involved a methodical approach, using tools like the oscilloscope and multimeter to measure waveforms and resistances, and applying principles of CAN bus communication. Key formulas, such as those for voltage levels and resistance checks, were instrumental in pinpointing the issue. For instance, the general equation for CAN bus voltage validation is $$V_{bus} = \frac{V_{CAN-H} + V_{CAN-L}}{2}$$, which should remain around 2.5V in recessive state; any significant deviation can indicate faults. Additionally, the power-up process can be modeled as a state machine, where each module must report readiness via the CAN bus. In this BYD EV, the failure occurred because the BMS could not communicate its status, halting the sequence. This experience reinforces that technicians working on BYD car models must have a deep understanding of network systems and be equipped with advanced diagnostic tools. Regular maintenance and inspections of wiring harnesses can prevent such issues, ensuring reliable operation of the BYD EV. The successful resolution of this fault not only restored the vehicle’s functionality but also provided valuable insights into the robustness of BYD car designs and the importance of proactive fault diagnosis in electric vehicles.