Hi, My car is 1993 300D, 145K miles...
The aux Fan came on yesterday evening for a bout half minute or so at FULL speed when I stopped at home...the car temp gauge was indicating 85 - 90 degrees!!! I checked the diagrams of aux fan for 1993 300D it shows that the ACC (specifically the PBU) is receiving signal from a separate temp. sensor and decides when to turn on the aux. fan in FULL...The ACC was OFF at that moment!!! but obviously it performed its job (it might be designed to do so)....Unlike the old systems where there is a temp sensor with known temp. to turn on the fan at (usually 105 degrees)...this new system is used by many cars ( 124 Gas & deisel) according to the diagrams...

>>>>I need clarifications on these points:

1. What is the temp. (in the new system) at which the fan FULL speed will be turned on (where it is listed or mentioned in the CD!). Is it a function of the ACC with known temp settings!!??Is it related only to the highest temp, or another factors within the ACC??

2. What if the ACC is defective, will it still perform this crucial function?

3. If in my case the dash temp. gauge was giving the wrong temp. reading (it has a separate sensor), how to verify this..which sensor could be at fault..or the ACC???

4. The car running fine, with good cooling pressure and new radiator...there is no sign of overheating...it is almost always at 80 - 90...should the car run hotter than this!!thermostat is 2 years old..

Please help me clarify this mystery.
Thank you for the great help....