BYD Han DM-p Cooling System Fault Diagnosis and Maintenance Guide

Fault Background

The Han DM-p cooling system is not just about "engine heat dissipation"; the manual clearly states that it is divided into four circuits: engine cooling circuit, motor controller cooling circuit, EHS motor assembly cooling circuit, and intake intercooler cooling circuit, which release heat from heat-generating components such as the engine, EHS assembly, motor controller, and on-board power supply to the outside, ensuring each component operates in its optimal temperature range. Components include radiator assembly, intercooler assembly, electronic fan assembly, electric water pump, auxiliary reservoir, cooling hoses, and intercooler pipes. The most practical on-site content includes: common fault and solution table (five categories: pump not drawing water / cannot start / not discharging / intermittent discharge / insufficient flow), dual-circuit coolant replacement procedure (engine circuit fills about 6.5L, motor controller circuit about 5.4L, both using "maintenance mode" for 5-minute drain), and removal/installation torque for each water pump/auxiliary reservoir/intercooler pipe (engine water pump 25±4N·m, clamp 4±0.5N·m, motor controller water pump and auxiliary reservoir 10±1N·m). This article is organized as an on-site guide in the order of "fault diagnosis → coolant procedure → removal/installation torque".

System Composition and Component Locations

Electrical control relationship: The engine circuit water pump's PWM speed control signal is driven by the VCU low-side; the motor controller circuit water pump PIN3 signal input connects to VCU B23 (PWM signal); the electronic fan connector includes power "+", ground GND, PWM input, and control signal, where both PWM and control signal connect to the VCU.

Common Faults and Solutions

The manual categorizes common cooling system faults into five major categories, providing ready-made countermeasures for each:

Auxiliary Reservoir Level and System Seal Check

1. After the engine, motor, and motor controller have fully cooled, carefully remove the auxiliary reservoir cap, add coolant so the level is between the MAX (maximum) and MIN (minimum) marks;
2. Install a pressure tester onto the auxiliary reservoir (use commercially available adapter T901122-09);
3. Apply 140~160 kPa pressure to the engine coolant auxiliary reservoir;
4. Check if pressure drops and if there are leaks at each coolant hose port;
5. Remove the tester and reinstall the auxiliary reservoir cap.

The auxiliary reservoir cap must only be removed after the vehicle has fully cooled to avoid burns.

Coolant Replacement (Separate Procedures for Dual Circuits)

This vehicle's cooling system is divided into engine cooling circulation circuit and motor controller cooling circulation circuit; the two circuits are drained and filled separately. Coolant must be BYD-specified or equivalent grade—mixing different grades of coolant or adding plain water can cause system corrosion and sediment.

Engine cooling circulation circuit:

1. After the vehicle has fully cooled, remove the engine coolant auxiliary reservoir cap;
2. Remove the rubber hose at the lower outlet of the engine radiator to drain the coolant;
3. After draining, reconnect the hose and ensure the clamp is securely tightened;
4. Remove, drain, and reinstall the auxiliary reservoir;
5. Add specified coolant to the MAX mark;
6. Refill capacity is approximately 6.5L;
7. Activate "maintenance mode" for 5 minutes to run the engine electric water pump (if level drops below MIN, top up to MAX). Maintenance mode must complete 4 steps within 60 seconds: Power switch ON → Select Park (P), fully depress accelerator pedal twice → Select Neutral (N), fully depress accelerator pedal twice → Select Park (P), fully depress accelerator pedal twice;
8. Tighten the auxiliary reservoir cap;
9. Start the vehicle at idle (do not turn on A/C), let the engine run and heat up for 5 minutes;
10. Turn off the engine, wait for the vehicle to cool, check the auxiliary reservoir level, top up to MAX if needed;
11. Repeat the previous two steps at least 2 times until no more topping up is needed;
12. Tighten the auxiliary reservoir cap, restart the vehicle, and check for leaks.

Motor controller cooling circulation circuit:

1. After the vehicle has fully cooled, remove the motor controller coolant auxiliary reservoir cap;
2. Remove the connection (A) between the electric water pump assembly I inlet fitting and rubber hose, drain coolant, reconnect and ensure clamp is tight;
3. Remove the connection (B) between the nylon pipe and rubber hose under the front compartment floor, drain coolant, reconnect and tighten;
4. Add specified coolant until the auxiliary reservoir level stops dropping; remove the rubber hose (C) at the motor controller water inlet, after observing coolant flowing from port C, reconnect and tighten, continue adding to MAX mark;
5. Activate "maintenance mode" for 5 minutes (if level drops below MIN, top up to MAX). Maintenance mode 4 steps within 60s: Power switch ON → Select Park (P), depress brake pedal twice (brake depth ≥80%, second depression requires depth ≥30%) → Select Neutral (N), depress brake pedal twice (same depth) → Select Park (P), depress brake pedal twice (same depth);
6. Until there is continuous water flow in the auxiliary reservoir inlet chamber (D), the circuit filling and bleeding is complete—during this process, the motor controller circuit water pump may start and stop multiple times, which is normal;
7. Exit maintenance mode, check auxiliary reservoir level, top up to MAX if needed;
8. Refill capacity is approximately 5.4L.

Maintenance mode operation notes: After entering maintenance mode, the water pump starts working and its sound can be heard; do not shift gears, turn off power, or shut down during this period; the system will automatically exit maintenance mode if it detects driving/reversing gear engagement, power off/shutdown, or the electric water pump running continuously for 12 minutes.

Removal/Installation and Torque

Radiator and fan replacement: Follow the previous section to drain coolant → remove oil cooler inlet/outlet oil pipes, high-temperature radiator inlet/outlet hoses, A/C pipe connected to condenser, and recover refrigerant → remove fan harness connector and vehicle harness secured to electronic fan → remove front bumper → remove intercooler inlet/outlet air pipes, motor controller radiator inlet/outlet hoses connected to radiator, and motor controller radiator outlet hose tie straps → remove front-end module, radiator upper mount bracket and mount bushings → remove radiator and fan assembly → remove electronic fan assembly and other components. Install in reverse order, confirm upper and lower damping bushings are seated and secure, then fill coolant, bleed air, and charge refrigerant per the motor controller circuit method.

Engine electric water pump (mounting bolts Q1860835T1F61, three bolts A~C): Removal sequence: vehicle power-down → remove harness connector → remove surrounding pipes and harness → remove inlet/outlet pipe assembly → remove mounting bolts and pump. During installation, first mount to bracket, tighten to 25±4N·m; inlet/outlet hose assembly A-type worm clamps (D, E) torque 4±0.5N·m; connector fully seated and cannot be pulled back out.

Motor controller circuit electric water pump: Power down → remove connector → remove inlet/outlet pipe assembly (A, B) → remove mounting bolts (C, D) and pump. During installation, mount to EHS powertrain, standard torque 10±1N·m.

Auxiliary reservoir assembly / motor controller circuit auxiliary reservoir (battery thermal management auxiliary reservoir): Power down → remove air, fill (or inlet/outlet) pipes → remove mounting bolts and bottom locating rubber sleeve to remove reservoir. During installation, ensure mounting points are seated, tighten mounting bolts to 10±1N·m, then connect pipes.

Intercooler pipe assembly I: Power down → remove front bumper → remove clamps (A, B) connecting intercooler pipe and intercooler hose → remove CRV valve, coolant pipe tie straps, fuel evaporation pipe quick connector → remove intercooler pipe mounting bolts (C, D). Install in reverse order.

General precautions: Wear gloves to prevent injury; do not damage pump damping rubber pads when tightening bolts; ensure pump exterior is clean and free of scratches or dents before installation; prevent foreign matter from entering pump during assembly; do not drop or impact pump; before inserting connector, ensure no coolant residue on pump side and harness side.

Safety Warnings

• This article is for technical reference; cooling system inspection and repair must be performed by skilled technicians per OEM specifications. Readers unfamiliar with these procedures are strictly prohibited from performing operations themselves.
• The first prerequisite for cooling system work is complete cooling: removing the auxiliary reservoir cap and draining coolant must be done only after the vehicle has fully cooled; otherwise, there is a risk of high-temperature burns.
• Coolant is corrosive; use BYD-specified or equivalent grade, and never mix different grades or add plain water.
• Draining and filling coolant requires working under the vehicle; appropriate knowledge, tools, and equipment are required, and must be performed by skilled technicians.
• During maintenance mode operation, do not shift gears, turn off power, or shut down to prevent interruption of the draining procedure.

Maintenance Insights

The most valuable takeaways from this cooling system manual are two sets of "dual-circuit thinking." First, fault diagnosis follows the "five pump phenomena" table: not drawing water—check blockage, sealing, and air bleeding; cannot start—check power, fuse, and wiring; not discharging—first bleed air, then check rotation direction, then use oscilloscope to check current signal; intermittent discharge—usually air ingress or blockage on suction side; insufficient flow—check fluid level, pressure, and leaks—following the table in order is much faster than disassembling the pump immediately. Second, coolant replacement must be done per circuit, and the maintenance mode procedures differ between the two circuits: the engine circuit requires depressing the accelerator pedal within 60 seconds (P-N-P, two times each), while the motor controller circuit requires depressing the brake pedal (initial depth ≥80%, subsequent ≥30%); the completion indicators also differ—the engine circuit relies on "idle for 5 minutes to heat up + top up and repeat at least 2 times," while the motor controller circuit relies on "continuous water flow in the inlet chamber." Finally, remember three numbers: engine water pump 25±4N·m, clamp 4±0.5N·m, motor controller water pump and auxiliary reservoir 10±1N·m.

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【This article is for reference only】