Auxiliary fan W140 on all the time, high speed
 

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?????

Auxiliary fan high speed
 

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,

Go off if you pull the relay??

Auxiliary fan high speed
 

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..

Auxiliary fan high speed
 

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

You know what Arthur,
If you are in Ottawa, Canada, I would invite you to go out for few beer in a hot sunny weather. Just because your generosity.
And again, thanks a lot.

If I had a few beers , you would probably get a false diagnosis ..:)
..but thanks for the offer.

Hi All again,
I have replaced the ACC push button according to a mechanic and the fan is still on. I bring my car to other mechanic he told me that may be wiring problem. This guy is the guy who replaced my engine harness in Nov, 2006. He said there are 2 harnesses go into computer, he said may be the second harness having problem. I don't know what I'm going to do now. Any help?

Thanks,

Try this before changing any more parts

Pull the connector off the blue 2 wire sensor and connect a 1K ohm resistor [ just a little 1/2 watt one-Radio Shack/electronics store] across the connectors two female pins..does that stop the fan????

Do you mean that 1/2 Volts drop (not Watts) right?
I measured voltage drop when the connection is on and it's 0.445 V.
But I will go and buy resistor anyway.
Thanks,

Resistors come in wattage aside from resistance..so, I am just saying you only need a little 1/4 or 1/2 watt resistor , not a big 10 watt jobber...that is the sensor side of the cicuit and there is only miliamp draw, so it does not require a large wattage resistor.... jusy a little 1/2 watt/1K ohm resistor ... they are like .99 cents for a package of 5.
This is just for a test to take the sensor out of the circuit without tripping the default funtion of the CC unit.
The CC unit will see a cold sensor cuz you are fooling it.

I'm going to do so on the way home. I'm at the office now. I did try to short this connector... but it did not do any thing different. I guess that just the default short circuit kick in.
I will let you know as soon as I get this resistor.

Thanks,

[QUOTE=dinh_hai_tran;1536207]
. I did try to short this connector... but it did not do any thing different. I guess that just the default short circuit kick in.

Yes, but shorting the connector tells the CC that the cut-in resistance spec has been met and it then calls for fan.. High fan cuts in about 285 ohms.
The blue sensor is a Negative Coefficient Thermistor, simply meaning that as the coolant temp increases, the ohms decrease..so jumpering it just tells the CC you have past the cut-in spec...BUT, by using a 1K resistor, you are assured of complete circuit without reaching the 285 ohm cut-in fan trigger..get it??
It's best to just do the test and report the finding

Hi Arthur,
I did exactly as you said, resistor 1k 1/4 watts connect to the wire. The fan did not stop. Here is another hint. By accident, when I start the car for checking faulty code by pressing + holding REST key on ACC control panel, the fan stops and AC is ON. Of course, this is not a permanent solution but at least I can get AC on. By doing this, I have to close look at the coolant temp to be sure that the car is not over heated since AC ON fan OFF.
Does this suggest you something wrong?

Thanks,

Kinda strange. I am thinking that the harness they changed has got to be the problem.

I do not have the 140 schematic... I do have the 124 of same vintage..but the 140 uses a few different speeds and fan controls, so I don't like using the wrong one.
However, I did do some testing on 124 sensor/thermistor circuit on a live car w/working fan circuit . What I found that does not match up with your test is that the thermistor circuit is a 5V circuit, not 12v as your voltage drop test showed.
On my test, if I unplug the sensor , I get 5V at the feed side and trace on the return wire. This does not match up w/your findings , so this is something to consider...is the wire harnes they changed incorrect or does the 140 use a 12v feed to the thermistor ?? [ which I highly doubt]..I do not know the answer to that question, but I consider the answer to have something to do with the diagnosis to that point in the fan circuit.
I also mentioned to you earlier that if you could get to the fuse that feeds the coil side of the fan relay and pull it.....if that stops the fan, you know the sensor side of the circuit is at fault, rather than the fan load circuit..[ which would be a different /high amp fuse]..in other words , is the primary side of the relay being constant fed HOT or is the contact [ load side ] of the relay somehow picking up 12v somewhere ? [ you mentioned fan stops when relay is pulled , but pulling the relay disconnects both the relay coil circuit and contact circuit]. I would expect the fan to stop when you pull that fuse.. Check your fuse map for aux fan relay and try it...this would be what your guy is talking about when he mentions relay is alway HOT, but it would determine which side of relay is hot [ he probably also already knows that]
Tricky diagnosis and b/c of your complaint coming up when they did the wiring change, I think that needs some further looking at. Could 12v be at the thermistor due to wrong harness?? very well may be..
The 1K resistor test was just to satisfy the trigger default circuit of the CC, but if you show 12v there and it should be a 5v circuit, who knows how the CC sees that as..I just don't know the answer to either of those questions .

Thanks Arthur,
I do have a friend who has the same car. I will test his AC temp. sensor for the voltage difference, this is very easy test. For the fuse side, not the Aux. relay, I did unplug this fuse and it make no difference, I mean the fan is still ON. I will post here as what I find out the voltage difference of my friend's car.

Thanks,

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.