NIO ES8 Cooling System Fluid Replacement and Diagnostic Guide

Fault Background

The NIO ES8 cooling system is divided into two major sections: front/rear drive cooling system (expansion tank, radiator, low-pressure heat exchanger, radiator three-way valve, four-way valve, high-voltage battery pack three-way valve, front/rear drive motor electronic water pumps) and battery cooling system (high-voltage battery heater, expansion tank, battery cooler, four-way valve, high-voltage battery pack electronic water pump, high-voltage battery pack three-way valve, coolant hose quick-connect fittings). The control chain is three-tier: environmental control unit sends cooling request → vehicle control unit (VCU) → front/middle/rear electronic water pumps, with closed-loop control via multiple temperature sensors (inlet/outlet/entry/exit/ambient). The fluid replacement process has three on-site details: drain using compressed air blown from the expansion tank filler neck (blow until no coolant flows from the drain hose), filling with 2 vent valves loosened (fill to the MAX mark on the expansion tank, observe the vent valves—tighten when no bubbles and coolant flows out), and battery heating active heating mode (when lithium battery temperature is below 0°C, VCU enters active heating, involving diagnosis of the 40A fuse and UR09 relay). This article organizes the layout, control, fluid replacement, and diagnosis into a reference guide.

System Layout and Control

Control chain: Environmental control unit (cooling request) → Vehicle control unit (VCU) → Electronic water pump-front / Electronic water pump-middle / Electronic water pump-rear; Temperature inputs: Inlet coolant temperature sensor, Outlet coolant temperature sensor, Entry temperature sensor, Exit temperature sensor, Ambient temperature sensor.

Coolant Drain and Fill

Drain:

1. Remove the front underbody shield assembly, middle underbody shield, and front compartment cover middle assembly;
2. Loosen the clamp, disconnect the battery pump inlet hose connection—use compressed air blown into the expansion tank filler neck, stop when no coolant flows from the bottom drain hose;
3. Loosen the clamp, disconnect the rear motor inlet hose connection—similarly use compressed air from the expansion tank filler neck to blow until no coolant flows out.

Fill: Loosen the 2 vent valves → Add coolant to the MAX mark on the expansion tank, while observing the vent valves—tighten when no bubbles and coolant flows out.

Battery Heating Active Heating Mode and Diagnosis

• Active heating mode: When lithium battery temperature is below 0°C and conditions are met (within six minutes...), VCU triggers active heating, using the high-voltage battery heater to warm the battery circuit; this function falls under VCU integrated control along with torque management, energy management, thermal management, high-voltage control, and fault diagnosis;
• Diagnostic sequence: Read DTC → Inspect/replace VCU (vehicle controller) → Inspect/replace high-voltage power distribution box (including removing/installing the cover) → Replace 40A fuse (high-voltage power distribution box) → Check high-voltage battery heater installation position (correct position, securely fixed) → Remove and take out UR09 relay for visual inspection.

Disassembly/Assembly Points (Pipes and Heater)

• For pipe-type components (radiator outlet hose, battery pump to battery three-way valve hose, battery three-way valve to HVH hose, etc.), follow the sequence: "Remove expansion tank → Remove sound insulation panel → Loosen clamps and disconnect hoses → Replace → Reinstall → Refill";
• High-voltage battery heater: Disconnect low-voltage harness, disconnect high-voltage harness, remove ground harness nut, remove heater with bracket and hose assembly from the vehicle; install in reverse order.

Safety Warnings

• This article is for technical reference; cooling system maintenance must be performed by certified technicians per OEM specifications. Readers unfamiliar with the procedures are strictly prohibited from performing operations themselves.
• Before working on the high-voltage battery heater, disconnect the low-voltage harness, disconnect the high-voltage harness, and remove the ground harness; follow high-voltage safety procedures.
• Coolant drain/fill should be performed after the vehicle has cooled; control compressed air pressure during blow-out to prevent coolant splashing onto high-voltage connectors.
• Filling must use the dual vent valve method, with "no bubbles and fluid flow" as the completion criterion; maintain fluid level at the MAX mark.
• Active heating mode diagnosis involves the high-voltage power distribution box and fuses; de-energize the system before working.

Maintenance Insights

The most valuable takeaway from the NIO ES8 cooling manual is the "blow-out + dual vent valve" fluid replacement process. Draining is not gravity-based; it uses compressed air blown into the expansion tank filler neck, blowing until no fluid comes from the drain hose—performed once for each of the two circuits (battery pump inlet hose, rear motor inlet hose). Filling is not just "fill to MAX and done"; instead, loosen the 2 vent valves, fill to the MAX mark, wait until no bubbles and coolant flows out, then tighten—this process eliminates "air lock," the enemy of electric water pumps, from the procedure. On the control side, remember the three-tier chain: environmental control unit request → VCU scheduling → front/middle/rear three pumps execute, with closed-loop control via multiple temperature sensors; so if a pump doesn't run, first check the request chain rather than replacing the pump. On the diagnostic side, one main line: active heating failure → VCU → high-voltage power distribution box → 40A fuse → heater installation position → UR09 relay, follow in order; the relay and fuse are the cheapest root causes, don't skip them and go straight to replacing the heater.

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【This article is for reference only】