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, 369 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.

I suspect the cable ..
The CC unit has a default feature that triggers the high fan... if there is an open circuit in the sensor thermistor or wires going to it..that is a safety feature in case of open/faulty sensor so car wil not overheat and cause damage. Check that you have a good connection at that blue ,2 wire sensor and that the sensor reads spec from temp/ohm chart..

Hi again,
I have used the Voltage meter to measure the two poles of the connector. When the switch is ON position, I measured 11.85 Volts. So there is NO short circuit on the cable. Gee, what's next.
Thank you for your suggestion.

Connect the cable back to sensor and take V from each side of sensor to ground w/key on

I forget to mention that, even the cable was disconnect from the sensor, the fan is still on, permanently.

-Re-read my other post..I just told you CC Panel defaults to full fan if there is an open circuit, so ..if you unplug the sensor,[creating an open circuit ],the fan comes on Full speed..that is how one would test for high fan circuit..you pull the connector off and you should then see full speed fan if the circuit is working.

I'm sorry Arthur,
It's raining in Ottawa, Canada.
I just did what you say.
One pole to ground is 10.79V
and the other one to ground is 11.9V.

Do you think AC pressure switch?

Thanks,

Doubt it b/c you have high fan..but , disconnect the red on on the drier and see what happens just for kicks

Hi All,
I have brought my car to the shop and the mechanics found out that my ACC push button control panel having problem. It sends out hot signal all the time to the relay. This panel would cost me for $2500.00 + tax + labor.
Expensive eh? There is any way to repair this panel?

Thanks,

Well. it s possible that the CC driver for fan is shorted closed, but...before spnding that kind of $$ , I would try 2 test to make sure.
Look on fuse chart and find aux relay fuse ..might be #7, [ not sure on 140 ].. This is NOT the fuse for aux fan motor [ load side]..it is the relay primary [ coil] side. Find that and take the fuse out ..does fan stop??

Next test is to get the ohms value of the blue sensor at cold start temp.
I am not disagreeing with your guy, just checking...
I am confident he probably knows that the CC defaults to
an always hot sig to relay if the sensor circuit is open between the CC and sensor, including the sensor..but one never knows