As an automotive technician specializing in electric vehicles, I recently encountered a challenging case involving a 2016 BYD Qin EV300, a prominent model in the BYD EV lineup. This BYD car had accumulated approximately 52,000 kilometers and was used as a training vehicle for hands-on exercises. After students completed a practical session involving the disassembly and measurement of the IGBT module within the motor controller, the vehicle exhibited a series of critical issues: it failed to establish high-voltage power, the “OK” indicator on the instrument cluster did not illuminate, and both cooling fans on the radiator operated at high speed, indicating that the front junction box module was in emergency mode. This scenario highlighted the complexities of modern BYD EV systems and underscored the importance of meticulous diagnostic procedures.
Upon initial inspection, I confirmed the fault symptoms and proceeded to connect a diagnostic scanner to the vehicle. The scan revealed that all modules on the powertrain network were unresponsive, while modules in other CAN networks logged fault codes related to the powertrain. Key fault codes included U0111, indicating loss of communication with the battery management controller, and U0122, signaling a communication fault with the low-voltage battery management system (BMS). This pointed to a potential failure in the powertrain network, which is crucial for the operation of any BYD car. The powertrain network in a BYD EV facilitates communication between critical components like the motor controller, battery manager, and other control units, and any disruption can lead to operational failures.
To systematically address the issue, I referred to technical documentation for the BYD EV model, which outlines the powertrain network circuitry. Based on the fault codes and symptoms, I hypothesized that possible causes included malfunctions in powertrain-related modules or faults in the associated wiring harnesses. Adhering to a methodical approach—starting from external checks and moving inward, and prioritizing simpler tests over complex ones—I began by measuring the CAN signal voltages at the low-voltage BMS (LBMS). Using a multimeter, I measured the voltage between terminal BK50/3 (CAN High) and ground, obtaining a reading of 2.2 V, which is abnormal as the typical range for CAN High in a BYD car should be around 2.5 V to 3.5 V under normal conditions. Similarly, the voltage between terminal BK50/1 (CAN Low) and ground was 0.2 V, far from the expected 1.5 V to 2.5 V range. After disconnecting the low-voltage battery negative terminal, I measured the resistance between terminal BK50/1 and ground, which was 9.23 Ω, indicating a short circuit to ground on the CAN Low line. This deviation can be expressed using Ohm’s law: $$V = IR$$, where the abnormal resistance and voltage suggest a fault path.
Given that the fault emerged after the motor controller IGBT module exercise, I suspected issues within the high-voltage control unit. This unit in a BYD EV integrates multiple modules, including the motor controller, DC-DC converter, onboard charger, DC leakage sensor, and active discharge module. To isolate the problem, I disconnected connector B28(A) on the high-voltage control unit, which severs the motor controller’s link to the powertrain network. Upon remeasuring the CAN signals at the LBMS, the voltages returned to normal: CAN High measured 2.7 V, and CAN Low measured 2.3 V, confirming that the high-voltage control unit was the source of the fault. This restoration of normal values can be summarized in a table for clarity:
| Measurement Point | Condition | CAN High Voltage (V) | CAN Low Voltage (V) | Resistance (Ω) | Status |
|---|---|---|---|---|---|
| LBMS Terminal BK50/3 | Before Disconnection | 2.2 | N/A | N/A | Abnormal |
| LBMS Terminal BK50/1 | Before Disconnection | N/A | 0.2 | 9.23 | Abnormal |
| LBMS Terminal BK50/3 | After Disconnection | 2.7 | N/A | N/A | Normal |
| LBMS Terminal BK50/1 | After Disconnection | N/A | 2.3 | N/A | Normal |
Next, I focused on the high-voltage control unit, which houses five modules. Employing a step-by-step disconnection method, I removed each module’s connector while monitoring the powertrain network. When I disconnected the active discharge module, the network communication resumed normally, indicating that this module was faulty. The active discharge module in a BYD EV is responsible for safely dissipating high voltage from supercapacitors, and its malfunction can disrupt the entire system. To verify, I replaced it with a known-good module from another training BYD car, but the fault persisted. However, I observed a critical detail: the powertrain network communicated normally when the active discharge module was not grounded (i.e., “floating”), but failed once it was mounted to the chassis, which provides ground through the aluminum housing. This suggested a grounding-related issue.
In the floating state, I measured the voltage between terminal 1 (a black wire) of the active discharge module and ground, which was 2.3 V—identical to the CAN Low signal. This was highly abnormal, as the black wire should not carry CAN signals. Upon tracing the wiring, I discovered that the harness connector for the active discharge module was identical to that of the onboard charger, and it had been mistakenly swapped during reassembly. This misconnection caused the CAN Low line to be linked to the module’s ground terminal, creating a short circuit. The relationship between voltage and current in this fault can be described by $$I = \frac{V}{R}$$, where the low resistance path resulted in excessive current flow, disrupting communication. This error stemmed from repeated training sessions where harnesses were not properly secured, leading to student mistakes during reconnection.
To resolve the fault, I correctly reconnected the active discharge module’s connector, ensuring proper alignment. After reassembly, I tested the BYD EV, and all systems returned to normal: the high-voltage power engaged, the “OK” light illuminated, and the cooling fans operated at standard speeds. This case emphasizes the importance of careful handling during maintenance of a BYD car, especially in educational settings where multiple interventions occur. Additionally, when working with high-voltage components like the active discharge module in a BYD EV, it is crucial to implement safety measures, such as using insulated tools and following isolation protocols, to prevent electric shock hazards.
In summary, this incident underscores how simple human errors, such as connector misalignment, can lead to complex faults in advanced BYD EV systems. The diagnostic process involved a logical sequence of voltage and resistance measurements, combined with network isolation techniques, to pinpoint the issue. For future reference, technicians should double-check connector identities and secure harnesses to avoid similar problems. The integration of multiple modules in a BYD car demands precision, and this experience reinforces the value of systematic troubleshooting in maintaining the reliability and safety of electric vehicles.
To further illustrate the principles involved, consider the CAN bus voltage characteristics in a BYD EV. The differential voltage between CAN High and CAN Low determines signal integrity: $$V_{diff} = V_{CAN\_H} – V_{CAN\_L}$$. Under normal conditions, $V_{diff}$ should be around 2 V for dominant bits and close to 0 V for recessive bits. In this fault, the short circuit caused $V_{diff}$ to approach zero, disrupting communication. This can be modeled using network theory equations, such as those for transmission line impedance, but in practice, simple measurements suffice for diagnosis. The table below summarizes key parameters for CAN bus operation in a typical BYD car:
| Parameter | Normal Range | Fault Condition | Impact |
|---|---|---|---|
| CAN High Voltage | 2.5 V – 3.5 V | 2.2 V | Reduced signal amplitude |
| CAN Low Voltage | 1.5 V – 2.5 V | 0.2 V | Short to ground |
| Differential Voltage | ≈2 V (dominant) | ≈0 V | Communication failure |
| Termination Resistance | 120 Ω | 9.23 Ω | Impedance mismatch |
Ultimately, this case with the BYD EV demonstrates that even in controlled environments like training workshops, vigilance is essential. The powertrain network’s sensitivity to wiring errors highlights the interconnected nature of modern automotive systems. By applying fundamental electrical principles and methodical diagnostics, I successfully restored the BYD car to full functionality, reinforcing the critical role of detailed inspections in electric vehicle maintenance.