Dear experts,
I have a 1994 S420, lately my auxiliary fan on all the time with a very high speed and of course make lots of noise. I have change two temperature sensors (not the one connect to the gauge). I have checked the auxiliary fan relay and it's OK. The cable harness is new.
What should I do next?
Any advise would be much appreciated.

does your car A/C is on too? I think that the aux fan on that base on A/C system or high engine temp... If your A/C feron low, the aux fan on to help feron cool down.... then, make the A/C cold..... If your engine temp is fine and A/C is on.

It should be lower feron on your car.... That means somewhere leaking on the A/C system....

Very expensive to fix A/C on W140.... Hope your car A/C will keep working fine this summer... good luck

<< I have change two temperature sensors (not the one connect to the gauge). I have checked the auxiliary fan relay and it's OK>>

So, which sensor did you change?????

Thank you for answering my question.
The two sensors that I replaced are:
A/C ACC Temp Sensor
and the Water temp sensor (see attachment)

The fan is still on when I turn off AC (EC mode).

Thank you,

Attached Files

temp sensors.pdf (57.9 KB, 433 views)

Go off if you pull the relay??

Yes, it went off if I unpluged the relay. I borrowed a good one from my friend's car and put on, the fan started on again. This is headache problem.
According to what I knew from internet for testing AC system from: http://pages.prodigy.net/jforgione/MB_AAC.html
to read sensor values. For Test Step 6, the value is HI would means that "Coolant temperature sensor B10/8) is short circuit. But mine B10/8 is brand new so it could be the cable harness (also new). I remember that when the mechanics put this new cable harness on my car, the problem started. Maybe it is the $1300.00 job has a BAD cable?
I am really nut now.

Following fuse descriptions might be of help:

Engine compartment fuse box

Fuse 2 - 30 amp main supply to auxilary fan relay contacts
Fuse 18 - 15 amp air conditioning aux coolant pump control relay module
Fuse 20 - 10 amp automatic climate control base module (might power relay coils)

Rear compartment (boot)

Fuse 1 - 15 amp rear climate control

hopeit helps

I would like to thank all of you, especially to Arthur with his knownledge and generosity. Without him, I could not find something wrong with the car (I believe that many mechanics don't know either).
The actual problem is the : HARNESS. I checked the cable from my friend's car, the actual voltage difference of the AC temp sensor cable is 5 volts as Arthur stated (as opposed to mine 12V). I looked at part number of the two harnesses and they are the same 000 545 4881. From my friend's car, I used Ohm meter to link the wire from the sensor to other end (computer), one goes to pin 6 and the other goes to pin 7 (wire color is brown and blue). And my wires go to pin number 31 and 5 (brown and red). I also measure voltage difference between between pin 6 and 7 of my ECU (computer) and It is 5 Volts. So the conclusion (hard to believe but it's true) is that the Harness wiring is wrong from MB or whatever supply part to MB.

Again, thank you very much,

Cool..

Thanks for the outcome reply..it was starting to bother me too. We do like to see a diagnostic conclusion/ remedy.