HELP please! Electric fan runs on full with ignition off / empties battery

Andy Skips

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Would really appreciate some expert advice.

Generally the car is running fine, no fault lights etc. However every now and again, the electric fan turns itself on FULL (making quite a noise!) after I have removed the key from the ignition and locked the car.

Car is a Mercedes-Benz R Class (W251, V251) R 300 CDI (190 HP) - year 2011

Typically:
  • park the car and get out (at which point electric fan is not running)
  • lock the car
  • then around 10-20 seconds later the fan starts on full
  • when this happens, the fan never seems to stop. it just keeps running until the battery is drained.
A few clues:
  • battery is new and holds charge fine generally.
  • this does seem to happen on hot days.
  • fan runs on full for 20 mins or so before battery is drained.
  • when it does happen I can resolve by unlocking the car and then turning the ignition on and off again. This seems to reset the fan and having done so it seems never to restart.
I would massively appreciate anyone's help on this as the garage can't seem to diagnose. It happens reasonable frequently e.g. every few weeks. And three times we haven't noticed before leaving, leaving us in need to a jump start.
  • Could it be anything to do with the AC Rest setting for instance?
  • Why would this happen?
  • When it does (e.g. if the engine is hot and needing cooling when stationary) why does it not stop when sufficiently cooled?
I'm hoping this may just be a sensor or something that needs checking. Thanks everyone!

Andy
 
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Andy Skips

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Thanks for replying. As yet we haven't been able to diagnose a fault so we have not changed anything.
 

LostKiwi

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If you don't use Aircon does it still do it? There is a pressure sensor in the A/C which will tell the fans to turn on. If the sensor is bad it may cause it. Similarly there is a temperature sensor on the engine which if the engine reaches a predetermined temp (95 degrees?) will cause the fans to kick in.
Finally there is a speed controller integral to the fan unit. This can also fail.
Also check the wiring between each of these component for cracked/bare insulation or loose connections.
 
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Andy Skips

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Thanks so much for this...

When driving the car seems to reach 95 degrees and holds that temperature perfectly - so that sounds like the engine temperature sensor is ok?

The AC unit seems to work fine in general, but I will ask the garage to look at the AC pressure sensor / speed controller and / or connections perhaps that is it.

If anyone else has experienced this issue then please post... it'd be really interesting to hear from you!
 

LostKiwi

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When driving the car seems to reach 95 degrees and holds that temperature perfectly - so that sounds like the engine temperature sensor is ok?
There is more than one temperature sensor.
The one for A/C fan control is a separate one to the engine temperature gauge.
 
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Andy Skips

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OK thanks for clarifying
 

alexanderfoti

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I think a fan controller failure can cause this, but best to check. It looks like it coincides with the can-bus going to sleep.
 

DREAMER NO2

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Andy you have a sensor in the car that monitors the air temperature inside the cabin, and it will turn on the heater fan blower for cold air to circulate till its cooler inside .This is not the one on the radiator .But if your sure its the fan in the radiator cowl it something else wrong
 
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Andy Skips

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Thanks everyone very much... it is definitely the large fan in the radiator cowl. From your comments, I think likely faults may be:

Most likely:

- fan controller (doing something strange when can bus goes to sleep)
- cracked wiring / bad connections somewhere

Less likely:

- pressure sensor in the AC (less likely as AC seems to work fine when running the car?)
- internal temperature sensor (as AC / heating works fine in normal operation)
- engine temperature sensor (as engine runs at 95c fine)

Is that a fair summary? Really appreciate your advice on this.....

Anyone know whether the fan controller sends anything to the diagnostics log? Or is it just a question of swapping it out and hoping that does it? I will ask the garage of course who seem to be very good in general but uncertain about this one as it is one of this intermittent faults and I cannot just make it happen on their test rig.

Best wishes - Andy
 

John Laidlaw

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Not 100% on this but I’d have thought that a Star diagnostic would have picked this up- is your garage an MB specialist?Where are you based?
 

LostKiwi

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- engine temperature sensor (as engine runs at 95c fine)
As I said earlier there are two sensors. One is purely for monitoring engine temp for the ECU/gauge and the other is a switch which tells the fans to turn on.
If the switch is faulty and turning on when it shouldn't that could be your issue. When driving you'd be unlikely to know if the fan was turning on.
That said it does seem coincidental that timings are around CANbus sleep time so the fan controller may have an issue. The controller isn't cheap as I believe its integral to the fan.
As John said it would be interesting to see what STAR can do with this issue.
 
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Andy Skips

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Thanks both. I think that's all clear. My local garage is an MB indie (Merc Services in Mitcham) and I think they are very good. Certainly have the specialist STAR equipment. I'll post an update here when I know more. Later in the year when the car is in for servicing. Cheers - Andy
 

alexanderfoti

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As I said earlier there are two sensors. One is purely for monitoring engine temp for the ECU/gauge and the other is a switch which tells the fans to turn on.
If the switch is faulty and turning on when it shouldn't that could be your issue. When driving you'd be unlikely to know if the fan was turning on.
That said it does seem coincidental that timings are around CANbus sleep time so the fan controller may have an issue. The controller isn't cheap as I believe its integral to the fan.
As John said it would be interesting to see what STAR can do with this issue.

I am not sure I have heard of the AC pressure/Engine temp causing the fan to run when ignition is off though?
 

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