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....

Full [ high] fan is at 105 and is triggered by the thermistor sensors resistance value at thermo housing.. this triggers a
switching transistor circuit in the control unit which grounds the aux fan relay coil . [ thus, giving you high fan]. The control unit does not have to be on for this to work, the circuit is simply in the control circuitry...
To find out which is out of calibration, you have to test both the gauge sensor [ single wire sensor] and the aux fan sensor [ 2 wire] with an ohm meter and compare it to a temp/resistance chart.
I believe that chart is in the CD
If not, I have listed both somewhere in the archieves..
I would start with the gauge one first, just so you know where you are at , temp wise...

Gauge temp sensor is single wire at thermo housing..
Resistance sustitution [ RS close tol -1% ] will verify gauge calibration..OR...
With ohm meter , one can also verify sensor accuracy after gauge test..

60C-110 ohm
80C -67
100C - 38

Thanks "Arthur Dalton" for clarifying this issue...
I have easier access to the temp. sensor for the ACC (2 poles on top of the head)...could you please help me on how to verify its accuracy...should I measure the resistance between the two poles and compare to your readings...but should I wait untill the fan comes (high) and see the matching resistance for 105 degrees...is there other way..
thank you very much,

The readings on the two pole are different than the single for the gauge..
The reason to verify the gauge first is that you can then use the gauge to compare the aux sensor temp/ohm chart by using the gauge [ that you now know is accurate] as the base temp..
Then you will know if the acc unit is tripping at proper time
[ when resistance of 2 wire is at correct ohms in reference to the gauge...]

That chart is also in the CD