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Just to make sure i get correct wiring diagram for you, could you please tell me the VIN of your car?
Did you find 4-wire connector at the fan assembly?
Click here for the wiring diagram.
I suggest to remove the plug on fan control module and check all the positive (30 ,15)
and negative (31) supply.
On your model, this module is separate from the fan assembly. Usually they malfunction. ECM sends control signal to this module depending on the engine temperature. Theoretically temperature sensor can be bad, but in this case engine performance would not be satisfactory as well.
Let me know if you need further clarifications,
It is hard to judge by the signal voltage what ECU is telling fan module to do. But you can check how much the voltage changes during igniton off/on, engine running/AC on to maximum cooling. If you see the change- this means ECU is giving out comands and wiring is ok. So that leaves Fan module as the weak link.
Mercedes uses their own transmissions, which are repairable. The price for the new MB box would be 9000$ at the dealer. But that would be a last resort, as OEM parts are avaliable, just have to know what and how to replace.
In this case, i would suggest to check the engine management to compare the Instrument cluster indicated temperature to engine temperature. Same sensor is used, but at 95C, the fan must be at least slowly spinning.
If Star Diagnosis is used, it is possible to check how much fan actuation is requested from ECM and how much from AC. Aso the fan can be actuated on demand. This will give the complete overview. It seems very strange that ECM does not actuate the fan at 95 in the cluster, since one sensor is used for both.
If you get a year warranty on the rebuilt transmission, that is a good deal, since the repair will cost as much, but will immobilize the car for a while. Since the tranny makes noise in specific gear, do not see any justification for convertor replacement from this side of the screen)).
Let me clarify something- with AC off and engine temp sensor disconnected- fan runs at full speed? Is that correct? If all is in order it should work like this.
On some older Benzes, it is ok for temperature to rise with AC off, to reasonable extent (102C) of course.
At this point, fan actuation must be checked with Star Diagnosis. I think the Fan module does not actuate the fan at lower end of output request. When temperature is below 100C- fan output request could be under 10% of full power. Due to internal fan module failure- it can only actuate the fan with 20% power. This is just a theory, which must be confirmed with Star Diagnosis actuation. Some independent Mercedes shops have it, not only dealers.
I agree with you, that the temperature should be rock-stable at 80 degrees. It is a topic with earlier models
Maybe coolant level is too low after service for some reason, but if the engine heats up too much due to oil- that must be some really bad one. I do not see a connection to thermostat because the sensor detects the temperature and the fan must be on So i do not think that is the case.
To put a point to the story, Star Diagnosis must be connected to find which of the parts is not working properly and get the whole picture.