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Engine Cooling Fan Running at Constant High Speed

Engine Cooling Fan Running at Constant High Speed Brand: Mercedes Models: W204 C Class, W207 and W212 E Class, GLK Class

The customer complaint is that the cooling fan runs constantly and that the air-conditioning is not as cold as usual. 


This problem is often caused by a faulty pressure sensor located on the air-conditioning condenser. If accessing fault codes, there are no relevant DTCs stored in the engine control module (ECM) or signal acquisition module (SAM). 


Use the AssistPlus:
Run a Quick Test to check for any relevant fault codes. 
If there are none, check actual values in the bair-conditioning control module: AIR-CONDITIONING >> ACTUAL VALUES >> CLIMATE CONTROL
  • Look at refrigerant pressure, specially noting if it too high for the ambient temperatures. For example, at a coolant temperature of 51.0°C (123.8°F) the refrigerant pressure should be no more than 12 bar. If it is higher, the air-conditioning control module presumes that the pressure is high and assigns the fan control module to run at 90% PWM actuation.
  • Also check the AAC pressure at stand still / cold. This should be approximately 5 - 6 bar.
Electrical checks
  • Confirm that there are no wiring issues to the sensor.
  • Check the power supply, signal and ground of the 3-pin connector at the pressure sensor:
    • Pin 1 - Ground (Circuit 31) 
    • Pin 2 - Signal. Reference values: 
    • 0 bar - 400mV 
    • 10 bar - 1.3V 
    • 30 bar - 4V 
    • Pin 3 - 5V supply feed
    • Confirm electrical line resistances to the SAM using the appropriate wiring diagram.


  • If electrical checks return results that are within specifications, replace the pressure sensor. Before replacing, first fully evacuate the air-conditioning system.
  • Caution: Evacuate the air-conditioning system of all R134a gas before the system is opened. There is no Schraeder valve on the sensor and removing the sensor allows the gas to escape into the atmosphere.
  • Replace the sensor and confirm good location of the wiring connector. Locate wiring in order to avoid chafing of harnesses.
  • Use the AssistPlus to check the reading in the climate control module. As the system is still currently empty, this should read 0 bar.
  • Fill the system with the specified amount of refrigerant and oil. Capacity information is found on the AAC refrigerant fill sticker, located under the engine hood (bonnet).
  • Confirm that there are no leaks at the sensor, that the air-conditioning is operating as expected and that the fan no longer runs at a constant high speed.
  • Finally use the AssistPlus to run a Quick Test and ensure there are no new fault codes.