W221 S320 CDI 2008 Yellow Engine Fan Faulty Symbol (picture)

mbenz1

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Hi All,

Hope you are well.

Need some help on a strange thing recently with my dad's 2008 S320 CDI.

After turning the engine on (from cold start), a yellow coolant symbol displays with no text (picture below) and the radiator fan turns on at full whack for about 1 or 2 minutes, then goes off, then comes on for like 10 seconds then goes off then comes on again randomly again. This will continue happening even after the engine is warmed up. Engine warms up to 90 degrees C as normal and does not overheat, however the temperature goes down slightly when fan turns on then goes to 90 degrees C when fan goes off... it never over heats or runs cold.

Took the car to our local MB specialist and his STAR showed no error codes!

I have been reading that this could be caused by a couple of things:

1) Coolant expansion tank AND/OR Washer fluid tank sensor: apparently these are wired in series (can anyone confirm this for this model?) and if one goes faulty, it could trigger the engine fan and put on that yellow coolant light.

2) MB specialist said it could be the fan clutch/module going faulty - is this built-in with the fan unit? If so, then it would mean a new fan unit?

3) Rear consumer battery? Its showing 11.8V with car off and 14V with car on. I asked the specialist and he said 11.8 is not very low? I thought 11.8V is like 10/20% charge and 12.8V is a fully charged battery??? Ca a low consumer battery trigger this yellow symbol and cause the fan to turn on or is this a unlikely?

Any ideas on this strange problem? Fan never comes on when the car is off. 20210125_141057.jpg
 

W1ghty

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Can’t help with the fan but 11.8 on a battery is dead , it just doesn’t know it yet
 

markjay

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I don't know if the two issues are connected, but before doing any work on electrical items (in this case the fan or temperature sensor etc) I would advise to ensure that both batteries are fully charged (or replaced if weak). Low voltage can cause all sorts of weird electrical faults, and if the batteries are not fully charged you might end up with a wild good chase on your hands. So I suggest to sort-out the low battery voltage first, then deal with the fan issue.
 
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mbenz1

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Can’t help with the fan but 11.8 on a battery is dead , it just doesn’t know it yet
Exactly, that was my thought when I read the voltage with the car off - 11.8V is very low! Why my MB specialist said that is not that bad I don't know. Even though its a consumer battery (in the boot) as the W221 has a separate starter battery in the engine, it doesn't mean 11.8V is not bad...
I don't know if the two issues are connected, but before doing any work on electrical items (in this case the fan or temperature sensor etc) I would advise to ensure that both batteries are fully charged (or replaced if weak). Low voltage can cause all sorts of weird electrical faults, and if the batteries are not fully charged you might end up with a wild good chase on your hands. So I suggest to sort-out the low battery voltage first, then deal with the fan issue.
I think you have a valid point here. Its better to spend some money on a new battery and then chase the problem if it still persists. The only thing that I am surprised at is that despite the consumer battery being down at 11.8V, there are not any other issues besides the the engine fan issue...one would think a low battery would trigger more than one error message or symptoms.

Will do another voltage check tomorrow with the car off and if its still low, I will look to replace the battery.
 

geoffus

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With such a small consumer battery that you likely have, 11.8 volts is not low, unless i'm mistaken it looks like a basic lead/acid battery fitted in domestic alarm systems as back up, if you have 14v when you say the car is "on", which I assume is ignition turned on but engine running not running, then its charging and won't cause the issues.
 

zipdip

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Yes I would get a new battery that is behind the rear boot back,it is very easy to change,make sure you get the right one,as you know that battery runs everything on the car except starting.
 
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mbenz1

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With such a small consumer battery that you likely have, 11.8 volts is not low, unless i'm mistaken it looks like a basic lead/acid battery fitted in domestic alarm systems as back up, if you have 14v when you say the car is "on", which I assume is ignition turned on but engine running not running, then its charging and won't cause the issues.

General rule of thumb says 11.8 is low, like 20/30% charge and 12.8/9 is 100% charge... Thats what I've read and heard over the years...
 

geoffus

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General rule of thumb says 11.8 is low, like 20/30% charge and 12.8/9 is 100% charge... Thats what I've read and heard over the years...
Maybe for a large starter battery, not so much if its a small consumer battery.

Did you fix your problem ?
 
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mbenz1

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Maybe for a large starter battery, not so much if its a small consumer battery.

Did you fix your problem ?
It is a large battery - its not a back-up battery, its a standard size 12V (60Ah). I think the facelift W221 had a back-up battery in the boot but not in the pre-facelift.
 
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mbenz1

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Update (for the record):

Got a used engine fan from eBay and connected it to the car and still had the same problem - engine fan light on and engine fan turning on. So this meant it wasn't the fan. The next thing we did was get a used expansion/header tank off eBay as there is a sensor inside this tank that can go wrong. We connected the tank and this did not solve the problem either. Luckily we were able to return these 2 items back so not much lost besides the fees to ship them back.

The FIX was the following... It turned out to be a break in the 0.5mm2 Red-Black wire running from pin 58 in the N3/9 ECU (located at top LHS of engine bay, when looking at the engine) to pin 4 of the engine fan harness/connector. This was causing the ECU to turn on the fan as a precautionary measure because it was not aware of what the problem was and it did not want the car to overheat hence turning on the fan all the time...

We managed to identify this problem by doing some simple continuity tests from the ECU connector to the engine fan connector and discovered no continuity in this wire!

To fix the problem, we ran a new wire between both connectors using the Red Splice connectors and then the error code disappeared and engine fan stopped turning on all the time. A big thanks to DieselBenz for his great support on this forum.

Hope this helps others. If in your case it does not turn out to be a broken wire, then I would recommend getting a 'engine fan speed control test' done by anyone with STAR to establish if the ECU is communicating with the fan. If the fan speed test does not function, that could most-likely mean the ECU is faulty and need replacing (or repairing, if possible).

We also changed the engine coolant temperature (£7 for a OEM Febi Bilsten part) as a precautionary measure...

P.S. The wind screen washer sensor and coolant tank sensor are not wired in series...they are separate circuits in the W221.
 

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