After a while of noticing that the coolant temp gauge in my 97 C280 never goes above 95C, i began to get suspicious. So, I removed the aux fan fuse and let it sit for a while. Now, the mechanical cooling fan can do a good job cooling the engine, but when it is sitting at idle for an extended period of time on a hot day without the help of the aux fan, it can't meet all the airflow needs of the engine. I knowing this decided to experiment and monitored the actual engine temp with the live data feature of the climate control unit and compared it to coolant temp gauge in the instrument cluster.

Here is what I found: Temp gauge functions normally up until the needle reaches 95C. After that, the needle seems to be programmed to stay at 95C regardless of what the engine temp really is. I watched as the actual engine temp very slowly climbed up above 100C and the needle still stayed on 95C. In fact, the needle stayed on 95C until the actual engine temp reached 115C. Then the needle rapidly shot up in stages, first to 100C, then to 110C within a matter of seconds. At this point, I shut it off and put the aux fan fuse back in and let it cool off. So, whenever the actual engine temp is within a range of 95C-115C, the coolant temp gauge shows 95C no matter what. The coolant temp gauge functioned much as the coolant temp gauges in Japanese cars do. It stayed in the "normal" range regardless of actual engine temp then rapidly rose when it was almost too late.

Needless to say, I was disappointed. I guess Mercedes decided much like many other car companies that they didn't want people complaining that their car is overheating because it reaches 100C (which is normal), so they decided to program the gauge to function like an idiot light.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record