Fan stays on

[CroftyOriginal]

I came back to my car after work to find my battery was dead, I thought it must just be old so I went and got a new one, after I went shopping I came out to find my radiator fan was running, Im guessing this is what killed the battery. The thing is its not on when I lock the car so its turning on randomly some time after, the car isnt hot in any way, also when the fan is running, as soon as I open any of the doors it turns off. I am having to pull the fuse every time I leave my car now just in case the fans decide they want to turn on. What could it be?

 

[AlexMac]

Any sign of water in the battery area? Not unusual for a drowning ECU to throw up symptoms like this (hopefully not though!)

 

[CroftyOriginal]

Not that I can see no.

 

[AlexMac]

The only time I've personally encountered this it was down to the ECU issue, but from memory there's a variety of possible causes. Given that it cuts off when you open a door I'm guessing a temp sensor issue is unlikely (but I honestly don't know for sure). If it was me, the first port of call would probably be the Fan Control Module. Check the condition of the wiring looking for cracks/splits or brittleness. It's a combination of relays and resistors in there, so could be stuck or failing. I believe part number is 8E0959501 followed by letters which vary by engine/application. It's should be attached directly to the cooling fans and looks something like this:

 

[CroftyOriginal]

Awesome mate! Thanks for that! I will check it out and have a look, see what I can see.

 

[AlexMac]

Also worth doing a scan of the ECU if poss. I believe the cooling fans default to a full speed state if there are ANY faults recorded relating to engine temp (as a safety measure). I really need to have a look at the wiring diagrams, but I think there may be a fuse you can pull to confirm whether the FCM is being TOLD to stay on by the ECU or not... Not near a computer with ELSAwin at the mo, can anyone else post one up?

 

[AlexMac]

Right, assuming you don't have any fault codes stored or clearing them hasn't solved the problem, I've had a chance to look at the flow charts and another possibility is the 614 relays. There's normally a couple located under the ECU in the battery compartment, and one above the comfort control module under the steering wheel (have to remove the lower dash trim on drivers side to access it). Any of these going bad can apparently cause the ECU to trigger a default 'fan on' setting.

Which fuse did you pull to shut the fan down mate? If it was fuse 4 in the main box, I believe that just controls the supply from the ECU to the FCM. So if pulling fuse 4 worked and killed the fans, then it suggests the FCM is functioning correctly and not to blame! There's normally a much bigger thermal fuse (again located by the Comfort Control Module) which supplies the actual power to the FCM

I think the part number for the 614's should be 8E0951253. Maybe worth checking them visually for any signs of damage/heating, but you often have to butcher them open to see the problem. Alternatively, source a new (or used having seen the prices!) 614 and try swapping it out in each of the locations. Hopefully one of them solves the issue.

Still possible it's a temperature sensor issue, but wouldn't be top of my list of suspects...

Let us know how you get on, 'cos it's always nice to get more info on what ends up being to blame.