Coolant Leak in EV Cars: Diagnosing and Fixing the Issue in a Mercedes-Benz EQB 250
Electric vehicles (EVs) like the Mercedes-Benz EQB 250 rely on a liquid-cooled electric motor and battery system to maintain optimal temperatures and performance. Unlike traditional combustion engines, coolant leaks in EVs pose unique challenges due to the high-voltage components involved. This article provides a step-by-step guide to diagnosing and resolving a coolant leak in EV cars, using a case study on the Mercedes-Benz EQB 250+.
Do EV Vehicles Use Coolant?
Yes, EVs use coolant to regulate the temperature of electric motors, inverters, onboard chargers, and battery packs. Maintaining the correct coolant level is essential to preventing overheating and ensuring efficient energy transfer. EV cooling systems are often more complex than those in internal combustion engine (ICE) vehicles, requiring specialized bleeding and diagnostic procedures.
Can EV Cars Overheat?
Yes, EVs can overheat, especially if the cooling system is compromised. Overheating can occur due to low coolant levels, blocked radiators, faulty cooling pumps, or software-related thermal management failures. Unlike ICE vehicles, which primarily generate heat from combustion, EVs generate heat from battery discharge and electric motor operation. Effective cooling is crucial to prevent damage to the battery pack, inverter, and electric motor.
What is the Cooling Liquid for EV?
The cooling liquid used in EVs is typically a specialized, non-conductive coolant designed for high-voltage components. Unlike traditional engine coolants, which are primarily water-based with additives, EV coolants must be:
- Dielectric (non-conductive) to prevent electrical faults.
- Thermally efficient to dissipate heat quickly from batteries and electric motors.
- Corrosion-resistant to protect sensitive components like the inverter and battery pack. Many automakers, including Mercedes-Benz, use ethylene glycol-based or silicate-free organic coolants specifically formulated for EV cooling systems. Using the correct coolant type is essential to prevent damage to the cooling system and high-voltage electronics.
Case Study: Coolant Leak in a Mercedes-Benz EQB 250
Customer Complaint:
A customer brought their Mercedes-Benz EQB 250+ to the workshop, reporting a coolant level warning on the dashboard. While coolant issues are common in internal combustion engine (ICE) vehicles, diagnosing a coolant leak in EV cars requires specialized knowledge and tools.
Step-by-Step Diagnosis
1. Performing an Initial Diagnostic Scan with Xentry
Using Mercedes-Benz Xentry diagnostic software, I conducted an electronic scan to retrieve fault codes. The scan showed:
- Fault Code P25567B – Coolant Level Low
- No other active faults related to the battery or power electronics
This indicated a physical coolant loss rather than a sensor malfunction.
2. Physical Inspection and Identifying the Source of the Leak
To pinpoint the coolant leak, I performed a thorough visual inspection:
- Removed underbody guards: Found coolant pooled in the splash shield.
- Checked electric motor housing: Discovered pink residue (antifreeze coolant) around the lower section of the motor.
- Cleaned and bled the cooling system for further examination.
3. Coolant System Bleeding in a Mercedes-Benz EQ EV
Bleeding the coolant system in electric vehicles is different from ICE vehicles. The correct procedure involves:
- Unlocking coolant valves using Xentry to allow flow through the system.
- Vacuuming the coolant system before refilling to prevent air pockets.
- Refilling the coolant and performing automated bleeding using Xentry to ensure proper circulation.
After the bleeding process, I monitored for any additional leaks.
4. Endoscopic Inspection for Micro-Leaks
Using an endoscope, I examined the motor casing and coolant lines under different load conditions. The inspection revealed:
- A small coolant leak from the electric motor casing, which worsened under driving conditions.
- Significant coolant loss over time, causing the coolant level warning.
The Solution: Electric Motor Replacement
Unfortunately, the electric motor unit in the EQB 250 is sealed, meaning internal leaks cannot be repaired. The only solution was complete replacement of the electric motor.
Motor Replacement Procedure:
To safely replace the electric motor, I followed Mercedes-Benz high-voltage safety protocols:
1. Deactivated the high-voltage system and ensured the vehicle was electrically isolated.
2. Removed the damaged electric motor, ensuring all connections were carefully disconnected.
3. Installed the new OEM Mercedes-Benz electric motor, verifying coolant line integrity.
4. Reassembled and reconnected the high-voltage system, ensuring proper torque settings on all fasteners.
5. Refilled and bled the coolant system again to confirm proper function.
6. Test drove the vehicle to verify that the issue was completely resolved.
After replacement, the coolant level remained stable, and no further leaks were detected. The EQB 250+ was back on the road, performing as expected.
Key Takeaways: Fixing a Coolant Leak in EV Cars
- Coolant leaks in EV cars differ from those in ICE vehicles due to high-voltage safety concerns and sealed components.
- Always use Xentry or manufacturer-approved diagnostic tools to perform a complete system check.
- Proper bleeding procedures using Xentry software are crucial to preventing air pockets in the cooling system.
- Some EV components, like the electric motor, are sealed and non-repairable, meaning full replacement is necessary for internal coolant leaks.
- Always follow manufacturer safety guidelines when working with high-voltage components.
- Overheating in EVs can cause severe damage, making proper coolant maintenance essential.
- Using the correct non-conductive coolant is crucial for protecting EV high-voltage components.
By following these steps, mechanics and EV owners can properly diagnose and resolve coolant leaks in EV cars, ensuring safe and efficient operation of their Mercedes-Benz EQB 250 or other electric vehicles.
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