BYD Han DM-i Bidirectional On-Board Power Supply Assembly Fault Diagnosis and Removal/Installation Guide

Fault Background

The Han DM-i's "bidirectional on-board power supply assembly" (HCHQ project) is a truly bidirectional power hub: during charging, it rectifies 220V AC into high-voltage DC to feed the traction battery; during driving, it steps down high-voltage DC to low voltage for vehicle loads; when parked, it can also invert battery power to 220V AC for external devices (vehicle-to-load). BYD dual-mode vehicles include it in "emissions-affecting functional components"—a fault will illuminate the MIL lamp, and the MIL lamp is extinguished in a special way: by sending a 7DF diagnostic message to clear it. This article compiles system functions, DTC diagnosis, comprehensive diagnostics, and removal/installation points into a field guide.

System Functions and Interfaces

Assembly interfaces: high-voltage DC input/output connector (DC positive/negative), low-voltage signal connector (discharge trigger signal, charge connection signal, temperature sensors, etc.), AC connector, AC charge port, PE wire. Electrically, it connects via the high-voltage distribution box to the charge port and low-voltage battery, and interacts with the BMS.

Diagnostic Procedure

1. Check battery voltage, standard 11~14V; if below 11V, charge or replace first;
2. In ON mode, use VDS to scan the power supply assembly module information;
3. Refer to the DTC table: if the symptom is in the DTC table, proceed to the DTC diagnostic procedure and handle item by item.

DTC Diagnostic Key Points

Comprehensive Diagnosis (When Unable to Charge)

1. Check if charging equipment supplies power normally; if abnormal, replace charging equipment or check supply circuit;
2. Check instrument charge indicator: in ON mode, measure assembly low-voltage connector constant power voltage; measure charge connection signal (charge connection signal pin 5 to body ground, standard 2.5~3.5V); connect charging equipment and observe if indicator lights; if not, measure charge indicator voltage at low-voltage connector → if abnormal, replace assembly;
3. Check low-voltage power input: in OFF mode, measure assembly low-voltage connector voltage;
4. Check distribution box charge fuse: disconnect battery → open distribution box → measure power supply assembly fuse continuity; if blown, replace power supply assembly charge fuse;
5. Check AC charge port assembly: pull out connector, measure corresponding pins on both charge port and connector for continuity; connect AC charge port to charging station or household power, measure voltage at assembly low-voltage harness side.

Removal/Installation and Torque

Power supply assembly (located in left rear of trunk, behind trim panel):

1. Power down: ignition OFF → disconnect battery negative terminal → disconnect front motor controller busbar connector → wait at least 5 minutes → unplug high-voltage DC busbar connector at front compartment controller assembly end → use voltage measuring tool to confirm voltage between busbars less than 60V DC → insulate and seal connectors with insulating tape;
2. Remove left rear trunk trim panel;
3. Disconnect external connectors (high-voltage connector, low-voltage connector);
4. Use ratchet to loosen ground wire and DC positive wire M8 hex flange nut (torque 20N·m), and remove the hex flange serrated M8 nut (torque 20N·m) securing the assembly;
5. Use water pump pliers to loosen water hoses, plug hose openings;
6. Carefully lift out the power supply assembly.

Installation: wear gloves, place assembly on trunk, align holes, tighten diagonally, then apply torque, secure ground wire and DC positive wire; connect high-voltage connector at anti-error angle, connect and secure low-voltage connector. Must wear insulating gloves when plugging/unplugging high-voltage connectors; before installation, ensure assembly exterior is clean and scratch-free.

AC charge port assembly: OFF mode, disconnect negative terminal → remove rear trunk trim panel, hinge cover → disconnect AC output connector → loosen cable ties securing cables → loosen ground, latch, and other fasteners → remove M6×20 hex flange serrated bolts securing charge port base → gently push charge port outward. Installation: tighten four bolts to 8N·m, note cover opens to the right; secure cable ties to body sheet metal and hinge in sequence; connect connector to assembly; wear insulating gloves when plugging/unplugging high-voltage connectors, and protect male/female terminals with insulating tape.

Safety Warnings

• This article is for technical reference; high-voltage repairs must be performed by certified personnel per OEM specifications; readers unfamiliar with the procedures are strictly prohibited from performing them.
• Strictly follow power-down sequence: OFF mode → disconnect battery negative → disconnect front controller busbar connector → wait at least 5 minutes → confirm busbar <60V and insulate/seal.
• When replacing high-voltage circuit components, replace with original design capacity; after removing water hoses, plug hose openings.
• MIL lamp is cleared using 7DF message (01 04 00 00 00 00 00 00) or VDS OBD module; do not use methods like disconnecting battery to "mask" the fault.

Maintenance Insights

The Han DM-i power supply assembly service manual writes "bidirectional" into every diagnostic entry: for high-voltage side step-down anomalies, first compare DC bus voltage with BMS voltage; for low-voltage side anomalies, first check the 9~16V output window—check high and low voltage separately, and only then consider replacing the assembly. Three most valuable field details: first, "disconnect positive output wire before measuring"—for low-voltage side voltage too high/low, first disconnect DC-DC positive output wire and measure between output terminal and housing to eliminate load interference before judging the assembly; second, "MIL lamp 7DF message clearing method"—send 01 04 00 00 00 00 00 00 or use VDS OBD module to clear, a dedicated "lamp reset" procedure for BYD dual-mode vehicles, more thorough than battery disconnect; third, "fuse is in the distribution box"—for low DC side voltage, first measure the assembly fuse in the high-voltage distribution box, don't rush to loosen the 20N·m M8 nuts.

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【This article is for reference only】