Seen several posts but no definitive answer regarding this and even a video posted. Last night the temp gauge started jumping from 80-90 degrees every 15 seconds or so and doesn't stop. When it did the coolant light (not red, just white) would flash on or off related to the temp shown on the gauge. Then the A/C quit working. It also appears that the radiator fan does not come on either. I am in 90F + weather now in the south so the fan should be running.


I have checked the coolant level and topped it off. I let the car sit over night and started it up. Everything appeared to be working fine until the engine got up to temp. A/C worked fine. Then the light and gauge started fluctuating again and the a/c quits. Let the car run longer and the fan never comes on.


So, I know these are all related but trying to determine where to start. Has nothing to do with the thermostat or low coolant as some have suggested. I don't want to start just throwing parts at it either. Here is where the questions start:


The temp sensor is fairly easy to change but not sure that is the place to start. Saw posts about fan module but have no idea where it is located. Fan is expensive and look like a pain to remove. How can I test to see if it is related to a bad fan, fan module, temp sensor or hopefully nothing else.


**Updated**
So did a live stream of the coolant temperature. When I started the car back up the stream showed 80c. The coolant light was not on and the a/c was blowing cold. Let the car run a few minute and the temperature gauge climbed slightly. The stream showed 85c. At about this time the a/c stopped working and the gauge started to fluctuate again. The radiator fan never came on. Allow the stream to continue to go until it reached 90c. Fan never came on. The gauge would still fluctuate between 80-85c.


** Update 2**
Decided to change the temp sensor so went to Napa and got one. Installed and still doing the same thing with the gauge. A/C does not work and the fan does not seem to come on. At what temp should the fan normally come on? How can I test the fan to see if it is any good?



Thanks for you help.

 

Strange electrical symptoms can occur when system voltage is low. Have a battery load check & alternator output check.

 

Any luck Shawnee ?

 

No news yet. Did determine that you can apply 12v and ground to the two large wires. While doing so, connect 12v to the small 12v "ignition" wire and the fan will turn. Now I am wondering whether the fan is getting a signal from the ECM to turn on.


Other thought was that there are two separate coolant temp sensor. One is in the back of the cylinder head (which I have changed) and the other is part of the thermostat housing.


I know my issue is all inter-related to one another but not sure what is causing it.

 

How old is the battery? .

 

Battery was install 12/2014 so less than 3 years old.

 

Do you have a Bluetooth ODB-II adapter and something like Torque? If so you can probably log the sensors.

It's probably not the battery but my factory one needed replaced before it was 4 years old. See if your brake hold works consistently.

 

On my reader the coolant temp sensor just steadily climbs. No fluctuation noted. Took the temp all the way to 100c and the fan never came on. A/c never worked from the time I started the car.


Fan appears to be working. After I reinstalled it (took it out to test as mentioned earlier) in the car to do the test above as soon as I turned the key on the fan was running wide open. Turned the key back off and on and the fan never came back on.


For some reason I am not getting a signal to the fan to turn on. Think this is all controlled by the ecm. If the ecm doesn't think the fan will work I assume it shuts the a/c off in order not to overheat.


Sounds like I am headed to the shop as I am lost with what to do.

 

So the verdict came out to be the module in the fan. Replaced the fan and all good to go. So you can jump the fan to see if it works or not but that does not tell you if the module is bad. Sucks to have to buy a complete fan for the module.

 

I have the exact same problem! Thank god I am not alone!

I've changed temp sensor (back of the cylinder head), washer pump (not directly related, but I've read that they are related somehow and I did have washer pump leak issue).

https://www.fcpeuro.com/Mercedes~Ben...3&keywords=fan

A look at FCPEURO, there are 3 fans sizes (16, 12, 10), do they all come with fan module? And which one actually fits in a 2008 C300 Sport?

Also, notice that RockAuto has these fans at $300 a piece. A big price difference...what's going on?

 

Your module is probably bad so just replacing the fan with a universal is not going to work. I got an OEM fan for $400 from my service guy.

 

There are a couple of fan controllers listed from the link above. Would those work?

 

Just wanted to say thanks. Your issue set and diagnosis path was a mirror of my own and made investing on a new fan easier to do. Also on a 2008 C300.