Suginami, I should have been clearer. At speeds above 20 mph, no fan is needed for cooling. If the temperature is around 87-90 at high way speeds, that indicates that the basic cooling system is working properly.

Now if the car is stopped with the AC running and the temperature starts to go up, that indicates that the aux fans are not providing sufficient air flow. In my car, the temperature approaches 100 C while stopped with AC on but does not reach it.

The only time it went above 100 C was when the aux fans were bad. They were rotating very slowly in slow speed mode and blowing the fuse as soon as I removed the temp sensor connector for high speed mode. After I replaced the aux fans and pre-resistor, temperature does not reach 100 C, with outside temperature around 85 C.

If in the future, it starts to go over 100 C, I will install the cool harness. It is a whole lot cheaper to replace the aux fans than all the electronic components and wiring in the engine. Mechanical parts like gaskets and engine blocks do not like the high temperatures either.

87-90 C seems to be the optimal temperature for this engine as evidenced by the highway operation. For some reason, MB was not willing to operate the aux fans at high speed for extended periods of time, one reason maybe extending the life of the fans and possible noise reduction.

In any case, the location of the aux fans is such that they are very susceptible to contamination and moisture from the road, reducing their service life.

I would like to add that MB considers temperatures approaching 120 C under certain conditions normal, but remember that the warranty runs out at 48K miles! Long before thermal cycle of heating and cooling have had a chance to degrade the components.

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Zafar
94 E320 58000 Miles