BYD Song PLUS DM-i On-board Power Supply Assembly Fault Diagnosis and Removal/Installation Guide

Fault Background

The Song PLUS DM-i on-board power supply assembly is mounted on the left rear side of the trunk and is a "bidirectional" device: during charging, it rectifies 220V AC into high-voltage DC to feed the battery pack; during driving, it steps down high-voltage DC to 12V low voltage for vehicle loads; when parked, it can also convert battery pack power back to 220V AC to power external devices (vehicle-to-load discharge). It is also included by BYD dual-mode vehicles in "emission-affecting functional devices"—if it fails, the MIL light on the instrument cluster will illuminate. This article compiles system functions, low-voltage pins, diagnostic procedures, and removal/installation points into a field guide.

System Functions and Pins

Main pins of the low-voltage connector: CAN_H / CAN_L, discharge trigger signal, charge connection signal, three-phase temperature detection, constant power ground, OFF-12V-1 / OFF-12V-2, high-voltage interlock input / output, AC interlock, as well as AC side N line, L line, PE line, and body ground.

Diagnostic Procedure

1. Check the battery: Standard voltage 11~14V; if below 11V, charge or replace first.
2. Refer to the fault symptom table / DTC table: If the symptom is listed, handle per the entry; if not, proceed to comprehensive analysis and diagnosis.
3. After repair or replacement of the on-board power supply assembly, test to confirm.

DTC Diagnosis Key Points

• AC input verification: Connect the charging equipment to the vehicle's AC charging port, use VDS to read the on-board charger AC input side voltage and frequency, compare with the grid-side voltage and frequency measured by a multimeter; if inconsistent, replace the on-board power supply assembly.
• Low DC side voltage: Clear the DTC, reinsert the charging gun to charge and see if it recurs; if it recurs, measure whether the on-board charger fuse in the high-voltage distribution box is intact; if blown, replace the fuse.
• Low-voltage battery voltage too low/high: After clearing codes, reinsert/remove the charging gun, measure whether the battery voltage is within 11~14V (replace or charge if abnormal); then measure whether the voltage at the low-voltage connector of the on-board power supply assembly is normal; if abnormal, check the low-voltage wiring harness.
• Instrument cluster / multimedia communication timeout: After clearing codes, reinsert/remove the charging gun to see if it recurs; if it recurs, check the CAN communication module; if it does not recur, it is a history DTC and can be cleared. If the MIL light remains on after clearing codes: Drive the vehicle to a location where charging is possible → shift to OK (READY) and insert the charging gun to charge → remove the gun → power down the vehicle, repeat three times to extinguish the MIL light.
• Hardware protection: After clearing codes, reinsert/remove the charging gun to charge; if the DTC recurs, replace the on-board power supply assembly; also check whether coolant is sufficient, coolant lines are unobstructed, and the water pump is normal; if abnormal, first top up/clear/replace the water pump.
• Discharge system leakage: First check whether the discharge equipment is leaking (replace if leaking); then check whether the on-board power supply assembly itself is leaking; if leaking, replace it.
• Low AC side output voltage during discharge: After clearing codes, reinsert the gun to discharge and see if it recurs; if it recurs, measure the on-board charger fuse in the high-voltage distribution box; if blown, replace the fuse; then verify whether the high-voltage side voltage of the on-board power supply assembly matches the actual voltage of the distribution box; if not, replace the assembly.

Comprehensive Diagnosis (When Charging Fails)

1. Check whether the charging equipment supplies power normally;
2. In ON gear, use a multimeter to measure the constant power voltage at the low-voltage connector of the on-board power supply assembly (pins 10/11 of the assembly to body ground should be 12V);
3. Connect the charging equipment and observe whether the instrument charge indicator light illuminates; if not, measure the charge indicator light voltage at the low-voltage connector;
4. In OFF gear, measure the voltage at the low-voltage connector of the on-board power supply assembly (pin 12 of the assembly to body ground);
5. Check the distribution box on-board charger fuse: Disconnect the battery → open the distribution box → measure the on-board fuse for continuity;
6. Check the AC charging port: Pull out the connector, measure continuity between corresponding pins on both the charging port and the connector; connect the AC charging port to a charging station or household power, use a multimeter to measure the voltage at the low-voltage harness end of the on-board power supply assembly (to body ground 2.5~3.5V);
7. Check the vehicle loop: If the vehicle reports "EV power limited", resolve that fault first before attempting to charge; if no such fault, check modules such as the distribution box, battery management system, and front drive motor controller.

Removal and Installation

1. Power off: Ignition switch OFF → disconnect battery negative terminal → disconnect front motor controller busbar connector → wait at least 5 minutes;
2. Remove the left rear interior trim panel of the trunk;
3. Disconnect external connectors (high-voltage connector, low-voltage connector);
4. Loosen the ground wire M8 hex flange nut (torque 20N·m), and remove the hex flange serrated M8 nut securing the assembly (torque 20N·m);
5. Use water pump pliers to loosen the water hoses, plug the hose openings;
6. Carefully remove the on-board power supply assembly.

Installation points: Wear gloves, place the assembly on the trunk, align holes, tighten diagonally, then apply torque, finally secure the ground wire; connect high-voltage connector at the anti-error angle, and low-voltage connector with alignment and locking; ensure the assembly exterior is clean and free of scratches before installation. Operators should wear gloves to prevent injury; must wear insulating gloves when connecting/disconnecting high-voltage connectors.

Safety Warnings

• This article is for technical reference; high-voltage repairs must be performed by certified personnel per OEM specifications; readers unfamiliar with such work are strictly prohibited from performing it themselves.
• Strictly follow the power-off sequence: OFF gear → disconnect battery negative → disconnect front motor controller busbar connector → wait at least 5 minutes.
• Replacement of high-voltage circuit components must use components of the original design capacity; in case of fire, only dry powder extinguishers and dry sand are allowed; water-based extinguishers are strictly prohibited.
• High-voltage work must be supervised; the operator works with one hand and, in principle, does not work on live circuits.

Repair Insights

The most common misdiagnosis of this "bidirectional power supply assembly" lies in the fact that it combines charging, step-down, and discharge functions in one housing—so the same "cannot charge" symptom could be due to a faulty AC input side, a blown DC side fuse, or abnormal low-voltage output. The three most practical field tips: First, "compare data before replacing parts"—cross-check the AC input voltage/frequency from VDS readings with multimeter measurements; if they do not match, the assembly is faulty; Second, "the fuse is in the distribution box, not in the assembly"—for faults like low DC side voltage or low discharge voltage, first measure the on-board charger fuse in the high-voltage distribution box, do not rush to replace the assembly; Third, "there is a specific remedy for the MIL light"—if the MIL light remains on after clearing history DTCs, perform "OK gear, insert charging gun to charge → remove gun → power down" three times in a row to extinguish it, without returning to the factory.

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【This article is for reference only】