Cooling Fan Running Full Speed After Turning Engine On: Case Study in Mercedes-Benz CLA 45s with M139 Engine
The issue of a Cooling Fan Running Full Speed After Turning Engine On can be alarming for any vehicle owner. This case study explores a specific occurrence in a Mercedes-Benz CLA 45s W118 equipped with the M139 engine, detailing diagnostics and repair steps to resolve the issue effectively.
Customer Complaint
The customer brought the vehicle to the workshop with a peculiar complaint: the Cooling Fan Running Full Speed After Turning Engine On. While the extra fan turned off when the engine was switched off, the persistent high-speed operation during engine runtime required immediate attention.
Workshop Findings
1. Fault Code Diagnosis
A quick test was performed using diagnostic tools, and the following fault code was identified:
- P2B837A: The RPM of the component ‘Coolant Pump’ is too high, indicating potential leakage or a defective seal.
2. Cooling Circuit Inspection
The cooling circuit was thoroughly examined:
- Visual Inspection: No leaks were detected in the hose area or at the connections within the cooling system.
- Bleeding the Coolant Circuit: Bleeding was successfully performed to remove any trapped air.
- Actual Value Analysis: The medium temperature coolant circuit values were compared to a normal vehicle. A significant deviation in actual values was found.
3. Sensor Swap
To rule out sensor malfunction, the medium temperature sensor was swapped. However, the issue persisted, confirming the sensor was not the root cause.
Measurements and Observations
Key electrical and thermal parameters were measured to isolate the issue:
Supply and Communication Voltages:
- Supply Voltage (M44 – Coolant Pump): 12.7 V
- LIN Bus (M44): 8.6-9.4 V
- Ground (M44): No corrosion or short circuits found.
- Supply Voltage (M4/7 – Extra Fan): 12.9 V
- LIN Bus (M4/7): 8.6-9.4 V
- Ground (M4/7): No corrosion or short circuits found.
Thermostat Check:
The thermostat had an 83°C stamp, indicating it was designed to open at this temperature. However, the actual value of the medium temperature coolant circuit reached a maximum of 89.88°C, causing the extra fan (M4/7) to operate at 100% speed.
Can a Bad Relay Cause the Fan to Stay On?
Yes, a bad relay can cause the cooling fan to stay on. The relay acts as a switch that controls the power supply to the fan. If the relay is stuck in the “closed” position, it may continuously supply power to the fan, causing it to run at full speed even when it’s not required. Therefore, it’s crucial to check the relay during the diagnostic process to rule out this potential cause of fan overactivity.
Resolution Steps
Thermostat Replacement
The existing thermostat (stamped at 83°C) was replaced with a new thermostat stamped at 73°C. The new thermostat ensured the medium temperature coolant circuit maintained a maximum temperature of 76.25°C, significantly reducing the workload of the extra fan.
Post-Replacement Results:
- Medium Temperature Coolant Circuit Maximum Temperature: 76.25°C
- Extra Fan (M4/7) Operation: Reduced to 37% actual value at the observed temperature (normal functioning).
- Cooling Fan Running Full Speed After Turning Engine On: Resolved.
Key Insights
- Diagnostic Accuracy:
- Conduct a thorough diagnostic test to pinpoint the fault code. In this case, the fault code P2B837A guided the troubleshooting process.
- Sensor Verification:
- Swapping sensors can eliminate the possibility of sensor-related errors, helping isolate the root cause.
- Thermostat Role:
- The thermostat’s opening temperature is critical in regulating coolant flow and fan operation. Selecting the appropriate thermostat for the system’s requirements resolved the issue effectively.
- Voltage and Communication Checks:
- Consistent voltage supply and communication signals are essential for proper functioning of components like the coolant pump and extra fan.
- Relay Inspection:
- Don’t overlook the fan relay, as a faulty relay can keep the fan running unnecessarily.
Conclusion
The case of the Cooling Fan Running Full Speed After Turning Engine On in the Mercedes-Benz CLA 45s W118 with the M139 engine was resolved by diagnosing and replacing the thermostat with an appropriate temperature rating. This not only corrected the coolant circuit’s medium temperature values but also restored the extra fan’s operation to normal parameters.
