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Stan, Mercedes Technician
Category: Mercedes
Satisfied Customers: 3526
Experience:  Technical support engineer
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2000 Mercedes Benz: Radiator cooling fan not turning on

Resolved Question:

My radiator cooling fan is not turning on.. I checked the power to the fan and none is present even at ~82 C on the car temperature gauge... I had the oil changed reticently and after that the car ran hot. Last night the temp went up to 95 C (for the last 10 years it has run at 80 C). When I arrived, I opened the hood and the fan was not turning. When moved manually it turned with ease... Is there a fuse box diagram (relay diagram) I can look at to help troubleshoot the problem ???
Optional Information:
Year: 2000
Make: Mercedes-Benz
Make (other):
Model: S500
Engine: 5 liter
Already Tried:
- no power to radiator fan - fan turns freely - water level is fine - car heats up to 95C - normal is 80 C
- #48 is fine visually and test OK with a volt meter
Submitted: 6 years ago.
Category: Mercedes
Expert:  Stan replied 6 years ago.

Just to make sure i get correct wiring diagram for you, could you please tell me the VIN of your car?

Did you find 4-wire connector at the fan assembly?



Customer: replied 6 years ago.
Hi Stan, The fan has 2 wires on it, vin: WDBNG75J8YA084049 . . I guess what I really need to know is the sensor, relay, fuses, etc.. that controll the fan so I can check / measure their function... Thanks...Paul
Expert:  Stan replied 6 years ago.

Hi Paul

Click here for the wiring diagram.

I suggest to remove the plug on fan control module and check all the positive (30 ,15)

and negative (31) supply.

On your model, this module is separate from the fan assembly. Usually they malfunction. ECM sends control signal to this module depending on the engine temperature. Theoretically temperature sensor can be bad, but in this case engine performance would not be satisfactory as well.

Let me know if you need further clarifications,



Customer: replied 6 years ago.
OK here is the scoop...... I have power (14.2V) on #30,15 and 31 is Ground (0V). The Y/B wire from the main ECU has a constant 1.3V even at 90C (while idling the car). Put power on the fan terminals #1 & 4 , and the fan started spinning (the fan motor is OK)....

So is this mean the "Fan Control Module" is toast ??? OR

There is no signal from the main ECU to the Fan Control Module ??
Customer: replied 6 years ago.
BTW, with this VIN, what transmission do I have, GM or sometiong German.... It has been making whinning noises in first & reverse for some time (145K on the car).... AND how expensive is that ! ! ! !
Expert:  Stan replied 6 years ago.


It is hard to judge by the signal voltage what ECU is telling fan module to do. But you can check how much the voltage changes during igniton off/on, engine running/AC on to maximum cooling. If you see the change- this means ECU is giving out comands and wiring is ok. So that leaves Fan module as the weak link.

Mercedes uses their own transmissions, which are repairable. The price for the new MB box would be 9000$ at the dealer. But that would be a last resort, as OEM parts are avaliable, just have to know what and how to replace.



Customer: replied 6 years ago.
OK , here is the result from all the collective wisdom, .... The fan starts WHEN I turn on the AC and stops again if AC turned OFF.. While the AC and the fan were running, at one point the fan turned off and shortly there after it turned ON again... Looks like the Fan Control Module, relay, fuses, wiring are all OK. This leaves out the engine temp sensor .. $25 at the local dealer but they are out till tomorrow.... OR am I missing something else ??

BTW - dealer German rebuilt transmission is $2,700 and the NEW torque converter is $1,240 (do I really need a new torque converter ?) GOOD - BAD ?????
Expert:  Stan replied 6 years ago.

In this case, i would suggest to check the engine management to compare the Instrument cluster indicated temperature to engine temperature. Same sensor is used, but at 95C, the fan must be at least slowly spinning.

If Star Diagnosis is used, it is possible to check how much fan actuation is requested from ECM and how much from AC. Aso the fan can be actuated on demand. This will give the complete overview. It seems very strange that ECM does not actuate the fan at 95 in the cluster, since one sensor is used for both.

If you get a year warranty on the rebuilt transmission, that is a good deal, since the repair will cost as much, but will immobilize the car for a while. Since the tranny makes noise in specific gear, do not see any justification for convertor replacement from this side of the screen)).



Customer: replied 6 years ago.
Thanks again for you advice in this adventure....

The temperature sensor measures 225 ohm at ~90C (Ohm measurement slowly climed as I was measuring, assuming the engine was cooling off)... While I was rinning the engine and the AC was ON, I disconnected the sensor wireing clip from the sensor and the fan accelerated to a VERY FAST speed ! ! ! !

I guess the ECU is getting good input, but still doesn't answer the question why is the fan not turning on during normal operation (without AC ON).....

Thanks fr the trany advice......
Customer: replied 6 years ago.
Hi Stan,
Ultimatelly, Thanks for al your help, it elliminated the following;
1) Fan Motor
2) Fan Control Module
3) Fuse # *****
4) Relay K40
5) Engine temperature sensor - 225 ohms at 90C
6) Wiring between all units - everything works

So what do we know;
- Fan runs with AC ON (on and off as temperature varies arount 80C)
- Fan runs very fast with engine temp sensor disconnected
- Fan does NOT run at all when AC is OFF (temp went up to 95C)dfz

Sorry but I don't know what elese plays a role here other than the ECU or some missing logic..
A mystery of the universe.... or Mercedes

Expert:  Stan replied 6 years ago.

Let me clarify something- with AC off and engine temp sensor disconnected- fan runs at full speed? Is that correct? If all is in order it should work like this.

On some older Benzes, it is ok for temperature to rise with AC off, to reasonable extent (102C) of course.

At this point, fan actuation must be checked with Star Diagnosis. I think the Fan module does not actuate the fan at lower end of output request. When temperature is below 100C- fan output request could be under 10% of full power. Due to internal fan module failure- it can only actuate the fan with 20% power. This is just a theory, which must be confirmed with Star Diagnosis actuation. Some independent Mercedes shops have it, not only dealers.



Customer: replied 6 years ago.
Hi Stan,
Sorry as well but I just got to my computer again to sign off... I will check tomorrow if the fan runs at full speed with AC off and temp sensor disconnected..

To your other point though, I have owned the car since it was new, and it has never run over 80C... With AC on or off.... That is why this is so wierd... And why would this be going on right after I had the oil changed at Jiffy Lube.. Could they have done something in the oil change process to cause this ??

Thanks again for all your help in this saga...

Expert:  Stan replied 6 years ago.


I agree with you, that the temperature should be rock-stable at 80 degrees. It is a topic with earlier models

Maybe coolant level is too low after service for some reason, but if the engine heats up too much due to oil- that must be some really bad one. I do not see a connection to thermostat because the sensor detects the temperature and the fan must be on So i do not think that is the case.

To put a point to the story, Star Diagnosis must be connected to find which of the parts is not working properly and get the whole picture.



Stan, Mercedes Technician
Category: Mercedes
Satisfied Customers: 3526
Experience: Technical support engineer
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