BYD Song PLUS DM-i Low-Voltage Power Supply System Fault During Charging: High-Voltage Feed Path Inspection

Fault Background

The low-voltage power supply of the Song PLUS DM-i is handled by the bidirectional on-board power supply (DC-DC converter): during charging or driving, high-voltage from the traction battery is distributed via the dual motor controller to the bidirectional on-board power supply and the PTC heater; the bidirectional on-board power supply steps down the voltage to power the vehicle's 12V low-voltage system. If any point in this "high-voltage → bidirectional on-board power supply → low-voltage" feed path opens, the instrument cluster will alert "low-voltage power supply system fault." This article dissects a real case, showing how to trace the high-voltage path all the way to the root cause.

Fault Symptom

DTC Analysis

Interpretation: "Voltage too low" on the high-voltage side is the key phrase—the bidirectional on-board power supply cannot receive high-voltage input, so it naturally has no low-voltage output, and the VCU subsequently reports a series of downstream DTCs.

Diagnostic Procedure

1. Check wiring diagram, measure basic circuits: The bidirectional on-board power supply ground is normal; CAN bus voltages are 2.28V / 2.71V, both normal.
2. Measure output: Multimeter shows no low-voltage output from the DC converter; suspect internal fault in the bidirectional on-board power supply.
3. Swap test overturns hypothesis: Replace with a spare bidirectional on-board power supply; fault persists—the unit itself is fine, the problem is "upstream."
4. Check live data: Found pre-charge status is fault, indicating high-voltage is not being delivered normally to the bidirectional on-board power supply.
5. Measure along the high-voltage path: At the dual motor controller end, power to the bidirectional on-board power supply and PTC is normal; measuring at the bidirectional on-board power supply end shows no power—problem is isolated to the connecting wires between the two ends.
6. Find root cause: Measuring the negative lead of the bidirectional on-board power supply controller shows no continuity; after peeling back the harness sheath, found high-voltage terminal crimp barrel loose, not crimped properly. After replacing the bidirectional on-board power supply high-voltage wiring harness, the fault was resolved.

Repair Procedure

1. Disconnect the high-voltage service disconnect switch, follow OEM procedure for vehicle power-down and voltage verification.
2. Remove the bidirectional on-board power supply high-voltage wiring harness, confirm the loose terminal crimp barrel and document with photos.
3. Replace the bidirectional on-board power supply high-voltage wiring harness assembly, plug in securely and confirm locking.
4. Restore high-voltage power, read live data to confirm pre-charge status is normal and DC low-voltage output is restored.
5. Retest under both charging and driving conditions to confirm the fault does not reappear.

Key Parameter Quick Reference

Safety Warnings

• High-voltage harness work must be performed by qualified personnel; strictly follow the "power off → verify voltage → lock out" procedure, and wear insulated protective equipment.
• High-voltage connectors have anti-misinsertion and interlock features; always confirm locking is complete during installation, and never force insertion or removal.
• During swap tests, re-verify the interlock circuit is complete each time before restoring high-voltage power.

Repair Insights

The biggest lesson from this case is "preconception": after measuring no DC output, the habitual thinking directly targeted the bidirectional on-board power supply itself, and only the swap test overturned the hypothesis. The high-voltage architecture of new energy vehicles is chain-like—traction battery → dual motor controller → bidirectional on-board power supply → 12V system—an open circuit at any point will manifest as a fault at the "most downstream" point. When repairing, remember three steps: first check live data for direction, then measure step-by-step along the circuit path, and finally disassemble and inspect connectors to confirm contact quality—"invisible breaks" like a loose terminal crimp barrel only reveal themselves when the sheath is opened.

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【This article is for reference only】