W204 C300 fan runs full speed at all times - PeachParts Mercedes-Benz Forum

daveshea · #1 09-05-2012, 08:48 PM

Hello!

I am a new owner of a 2010 C300 4Matic that I bought from an insurance auction - the car was a salvage car due to a front impact.

I have repaired all of the damage, but there is one issue that I cannot figure out.

Immediately upon starting the car, the radiator fan begins to run full speed, even though the car is stone cold. The AC is OFF. The radiator and condenser are BRAND NEW. This problem existed before and after the radiator was replaced.

The fan continues to run at its maxiumum speed, regardless of the engine temperature. I allowed the car to run today from cold to just above 80 deg. C before shutting it off today.

When one feels the upper radiator hose(driver side), it is HOT, and the air blowing near that side of the radiator appears to be hot.

The lower radiator (passenger side) is COLD, and the air blowing near that side of the radiator appears cool.

The heat is also turned on full blast, and blows lukewarm/cool air.

What is causing the fan to run full blast, and why is the coolant apparently not circulating?

My only guess is that the thermostat is stuck shut? Does that even make sense? Is there a sensor that is malfunctioning making the fan run full blast??

Please advise if able!
-David

mbdoc · #2 09-06-2012, 08:20 AM

The engine fan is controlled with a PWM signal from the fuel system computer.
Any interruption in that signal wire causes the fan to run at full speed.

And yes, the T/stat can cause issues but not the fan to run wide open.

porkface · #3 09-06-2012, 08:32 AM

had a car 2 years ago that needed an ecm for the same reason. means a trip to the dealer because it's a trp part. good luck, chuck.

daveshea · #4 09-06-2012, 08:47 PM

Quote:

Originally Posted by mbdoc

The engine fan is controlled with a PWM signal from the fuel system computer.
Any interruption in that signal wire causes the fan to run at full speed.

And yes, the T/stat can cause issues but not the fan to run wide open.

Using a standard OBD-II code reader, I was ableto read code P0116 - Engine Coolant Temperature Circuit Range/Performance Problem.

That would seem to confirm the idea of either a bad sensor or a broken wire/connector.

Does any sort of diagnostic flow chart exist to help me fix this? What sensor am I looking for on a W204 sedan? Where is it located, etc?

Thanks for any help you can provide!
-David

amerahmed16 · #5 09-18-2016, 09:39 AM

hi did u end up finding the issue with ur car engine fan