Radiator fan running after car turned off - draining battery.
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Junior Member
Joined: May 2007
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Radiator fan running after car turned off - draining battery.
Ok guys having a bit of a strange issue and I'll do my best to explain what has led up to now.
My problem right now is that when I turn my 08 c350 off after a minute or so the radiator fan comes on full blast and will run until the battery is dead. It stops for a little of I unlock the cat, lock the car, open the door, basically anything that uses a bit of electricity will temporarily stop the fans while the carb is off.
I recently had the front engine fuse cover off (kind of a long story butt my esl went out and I had to get the cat home so I pulled apart the starter relay and removed the esl fuse to get the vehicle started and driving) and I think a little bit bit of condensation got into there but it seems dry now and everything on the car seems to function fine. Any ideas on what I can do? It's there a fuse or relay for the resistor fan or fan module that I can temporarily pull to prevent the battery from draining?
My problem right now is that when I turn my 08 c350 off after a minute or so the radiator fan comes on full blast and will run until the battery is dead. It stops for a little of I unlock the cat, lock the car, open the door, basically anything that uses a bit of electricity will temporarily stop the fans while the carb is off.
I recently had the front engine fuse cover off (kind of a long story butt my esl went out and I had to get the cat home so I pulled apart the starter relay and removed the esl fuse to get the vehicle started and driving) and I think a little bit bit of condensation got into there but it seems dry now and everything on the car seems to function fine. Any ideas on what I can do? It's there a fuse or relay for the resistor fan or fan module that I can temporarily pull to prevent the battery from draining?
Junior Member
Joined: Apr 2023
Posts: 20
Likes: 6
Merc R Class
I have the same thing. I have an R class W251. Unfortunately I haven;t got to the bottom of it but in my case:
- the fan tends to come on after a slow hot journey, up a hill. So perhaps correctly as it realises the engine block is hot.
- HOWEVER like you, the fan never stops. It will drain the battery,
If I then turn the ignition back on and off again, the fan always stops, and never restarts. Which is again strange... if it decided the first time to turn the fan on, why not again? Anyway that's my work around.
Further to all this... I have read that the cars may be designed to turn on the fan in 'limp mode' i.e. when the control systems detect some sort of fault which they don't understand... the fan is turned on as a defence mechanism, 'just in case'.
By the way, it's pretty easy to pull the power connector to the fan. That may help you. I have a feeling the fan power fuse is one of the ones that is hidden (in the front pre-fuse box) i.e. not really accessible to us. A 100A biggie. So easier to pull the plug. HOWEVER you may still see the electric coolant pump still running.... which will draw something. Though not as much as the fan.
- the fan tends to come on after a slow hot journey, up a hill. So perhaps correctly as it realises the engine block is hot.
- HOWEVER like you, the fan never stops. It will drain the battery,
If I then turn the ignition back on and off again, the fan always stops, and never restarts. Which is again strange... if it decided the first time to turn the fan on, why not again? Anyway that's my work around.
Further to all this... I have read that the cars may be designed to turn on the fan in 'limp mode' i.e. when the control systems detect some sort of fault which they don't understand... the fan is turned on as a defence mechanism, 'just in case'.
By the way, it's pretty easy to pull the power connector to the fan. That may help you. I have a feeling the fan power fuse is one of the ones that is hidden (in the front pre-fuse box) i.e. not really accessible to us. A 100A biggie. So easier to pull the plug. HOWEVER you may still see the electric coolant pump still running.... which will draw something. Though not as much as the fan.
Last edited by skippety; Feb 8, 2024 at 04:07 AM.
Junior Member
Joined: Apr 2023
Posts: 20
Likes: 6
Merc R Class
Hey sorry for the delay on replying and thanks agin for the information you sent through.
I'm sorry to say that I've run out of ideas on this....
- I tried unplugging various sensors associated with that fuse and the CDI unit and didn't manage to find anything that seemed to be the cause.
- I'm now wondering whether the CDI controller itself has a minor issue or whether it is just 'older electrics' and that that a 120mA drain is in spec.
I've followed up with a few electrical specialists for advice and I will post back here if there's any more developments.
Best wishes
I'm sorry to say that I've run out of ideas on this....
- I tried unplugging various sensors associated with that fuse and the CDI unit and didn't manage to find anything that seemed to be the cause.
- I'm now wondering whether the CDI controller itself has a minor issue or whether it is just 'older electrics' and that that a 120mA drain is in spec.
I've followed up with a few electrical specialists for advice and I will post back here if there's any more developments.
Best wishes
Out Of Control!!
Joined: Apr 2019
Posts: 11,921
Likes: 6,775
From: Silicon Valley
W212 MY'14 M276-3.5NA @75kMi
troubleshooting
I have the same thing. I have an R class W251. Unfortunately I haven;t got to the bottom of it but in my case:
- the fan tends to come on after a slow hot journey, up a hill. So perhaps correctly as it realises the engine block is hot.
- HOWEVER like you, the fan never stops. It will drain the battery,
If I then turn the ignition back on and off again, the fan always stops, and never restarts. Which is again strange... if it decided the first time to turn the fan on, why not again? Anyway that's my work around.
Further to all this... I have read that the cars may be designed to turn on the fan in 'limp mode' i.e. when the control systems detect some sort of fault which they don't understand... the fan is turned on as a defence mechanism, 'just in case'.
By the way, it's pretty easy to pull the power connector to the fan. That may help you. I have a feeling the fan power fuse is one of the ones that is hidden (in the front pre-fuse box) i.e. not really accessible to us. A 100A biggie. So easier to pull the plug. HOWEVER you may still see the electric coolant pump still running.... which will draw something. Though not as much as the fan.
- the fan tends to come on after a slow hot journey, up a hill. So perhaps correctly as it realises the engine block is hot.
- HOWEVER like you, the fan never stops. It will drain the battery,
If I then turn the ignition back on and off again, the fan always stops, and never restarts. Which is again strange... if it decided the first time to turn the fan on, why not again? Anyway that's my work around.
Further to all this... I have read that the cars may be designed to turn on the fan in 'limp mode' i.e. when the control systems detect some sort of fault which they don't understand... the fan is turned on as a defence mechanism, 'just in case'.
By the way, it's pretty easy to pull the power connector to the fan. That may help you. I have a feeling the fan power fuse is one of the ones that is hidden (in the front pre-fuse box) i.e. not really accessible to us. A 100A biggie. So easier to pull the plug. HOWEVER you may still see the electric coolant pump still running.... which will draw something. Though not as much as the fan.
Now you may want to troubleshoot further with a good OBD scanner.
You need to see faults and live data.
The big 100Amp fan can be temperamental with its remote control by ECU PWM signal.
Last edited by CaliBenzDriver; Feb 20, 2024 at 05:08 PM.
