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Battery drains in two days through fuse 31

I have a little mystery to figure out. I have my battery drained by a component connecting via fuse 31, namely: clock anti theft, immobilizer, instrument cluster power, buzzer, board computer, they are all on there. The draw is 0.25 amp for 60 seconds, then drops to 0.05 amp , which would be nice if it would remain there, but alas, it comes back to 0.5 amps after 60 seconds precisely. One local tech told me about the BMW board computer having a dedicated small watch battery which died and caused a similar effect in another BMW. It sound very plausible to me, given the regular nature of the intervals, one could nearly rule out a stuck relay or short.....or am I mistaken? So did anyone ever dig out the computer and find such a little battery which can be replaced? Or did anyone ever have a similar pattern and found the solution? Another weird thing is, that when I pulled the fuse, the car started up as if nothing was missing, except for the cluster no longer indicating its stuff. What gives here?

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Solution retenue

After reading just about anything I could find on the draining issue, I think I found a very slick solution: I will just reroute the power supply for that circuit from the current "always hot" fuse, and instead supply the power from one of the accessory fuses which only become hot when the key is on. The I only need connect the clock to an always hot wire,and I will lose my trip meter settings which I do not care about, and voila....

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Hi @jurgenkoppen ,

Don’t know the answer but I was wondering if it were possible to unplug each component one at a time to see where the problem is emanating from.

maybe start with the Board computer -low A35y as you suspect, which is located in the centre console.

just a thought.

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Thank you jayeff, that link to the manual helped and now I can at least know where to find the components... Do you know what they mean with computer -low?



I have no idea.

The best that I can surmise is that it is an interface between the components connected to it and the central body electronics module

I think that the central body electronics module is the BMW name for a BCM unit found in other vehicles (body control module). This one handles most of the vehicle's electrical "body" functions i.e. door locks and windows, lights, horn, wipers, security, air con etc and works in conjunction with the other control units in the vehicle i.e. powertrain control module (PCM), engine control module (ECM)etc.


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Jurgen sera éternellement reconnaissant.
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