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The BMW Z3, released in 1995, is the first modern mass-market roadster. It is also known as a the BMW E36/7 or E36/8 for the coupé body style.

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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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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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How did you make that reroute wire set-up? What size fuse is in it and it looks like you have pulled fuse 31 and the other fuse slot?

par

HI, I have the same problem and looking for a quick fix and have seen this solution mentioned before. In the picture it looks like you are jumping fuse 16 and 20 - how does this effect fuse 31 circuits?

par

Hi Philip, the picture above is not of my BMW but another person's vehicle. I used the switched accessory fuse, and did a similar bridge to the fuse 31 in my BMW. I will provide my actual setup pic shortly

par

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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?

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@jurgenkoppen

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.

par

Did you guys ever figure this out?

I have a 2000 bmw z3 2.3 and it has a 200 milliamp draw with fuse 31 connected and a 10 milliamp draw when I pull fuse 31.

I'm not fond of tearing into the dash unless someone was successful.

It could be clock or antitheft or instrument cluster or onboard computer. Not sure what to do here and super frustrated.

par

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Hi folks, here is a more accurate picture of the actual BMW Z3 fuse box (instead of the generic one for other BMW models I used earlier to illustrate the idea of fuse bridging) and the jumper cable I installed. I used fuse 44 which is switched on only when the key is on, it is for the foglights and inserted a jumper to power fuse 31 (computer and dash functions) to make its components also switched by the key. This now enables the fuse 31 functions only when your ignition is on, and eliminates the power drain which occurs through fuse 31 all the time, draining the battery.

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Jurgen sera éternellement reconnaissant.
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