W204 Radiator Fan Woes (Cannot Fix It)

[gibsonk]

I am hoping someone can help me here, i have a issue with the Aux fan on my 2010 W204 350cdi.

The fan comes on randomly when the ignition is off and drains the battery overnight. If i leave my keys in and the ignition on its fine it seems to occur when the can bus goes to sleep.

So far i have done the following.

• Fit new AC pressure sensor and updated HVAC software
• Replaced the Fan with the updated pert as per the MB Bulletin
• Checked the F-Sam for moisture ingress
• Checked the wiring for shorts
• Replaced the water temp sensor
• Removed all after-market audio kit
• Checked Fan plug for corrosion
• Checked ME ECU for corrosion or moisture on the plugs

I have a new oil temp sensor on the way



None of the above has sorted it, as soon as you plug in my scanner the fan stops as it requires the ignition to be on, i have no fault codes, star is not reading any abnormal requests for the fan to turn on.

The odd part is if i disconnect the fan and drive the car it smokes like a steam train, plug the fan back in and its golden, i get no power issues or problems when driving it, its fine.

I am now at a point of replacing the F-Sam and Motor ECU as nothing else requests the fan to operate, when i check the reference voltages to the fan when the car is off it looks fine the majority of the time, however, i did watch the request signal fluctuate at one point which seems to be when the fan starts and stops randomly, hence now looking at the Motor ECU. .

If anyone has any ideas i would really appreciate it, i am currently haemorrhaging money into it , but with no fault codes, shorts or corrosion i can see other than throw parts at it i am at a loss.

Thanks to all in advance

 

[Tonygw]

I don't know for sure but what condition is the main battery in? What voltage is it reading with the engine off. I do know these mercs do funny things if the battery voltage is low or the battery isn't in excellent condition.

Does seem like an odd fault that.

 

[gibsonk]

I don't know for sure but what condition is the main battery in? What voltage is it reading with the engine off. I do know these mercs do funny things if the battery voltage is low or the battery isn't in excellent condition.

Does seem like an odd fault that.

Thanks for the reply, my battery is in good condition its only a year old.

I have seen some cases where the ME ECU is bad and needs to be replaced, i have a FSAM now on the way , I will try that next and if not that i am left with the ME ECU, crazy that something like that can go bad

 

[Uky]

Problems like these normally occurrs when the Fan controller has lost the connection to the Engine management unit.
In order to protect the engine, the fan runs at maximum speed. If this happens only when the ignition is on or
if it can happen when the vehicle is in one of the sleep-modes, I do not know. What I would do is to monitor
the battery (drain) current from the moment the ignition is turned off and see how long it takes for the
vechicle to come to sleep. You will have to invest in a clamp-on current meter capable of reading down to mA
however.

I would also make a complete investigation of all connections in the engine compartment, looking for possible
bends over sharp metal.

I have seen examples of connectors where the pins seems perfectly aligned inside the connector house
but with crimps inside that does not make contact. Mercedes has previously used cable harness suppliers in very-low-wage
countries.

Know then also that in order to save money, supply voltage and GND junctions can be found INSIDE the harness
insulation. It is unfortunately perfectly possible the a bad joint inside a harness is causing your problem.

 

[gibsonk]

Thank you for your reply, i will start to pull and check each connection

 

[J19YTP]

Sorry for the thread revival. Happy new year all

I have a similar problem on my CLS (w219) diesel.

The radiator fan is on full blast even when the car is cold. Sounds pretty loud even inside.

From reading it's likely to be the ( in order of least to most horrific)
Coolant sensor/switch - seems nice and simple
SAM ( can't figure out what that stands for?)
ECU - is this the main engine ECU - let's all pray it's not that

The air con being on or off doesn't seem to impact. And the temp gauge on the dash seems to be operating normally.

Everywhere's closed at the moment and need to make a motorway journey... I think the car will be ok. Ambient temp is relatively low, and air flow should be good on the motorway.

I'll check my coolant and oil level. Any other suggestions?