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Changed battery, now have electrical issues...

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Hi all - good to see the forum is still going strong!

The battery in MY 2002 LMB E39 M5 was dying / not holding charge so I replaced it with a new one today. However, on swapping out the battery, I suddenly:

1. Could no longer lock / unlock the car, whether from the keyfob or even by turning the key in the driver's door;
2. Have no interior lights, windscreen washers, or windscreen wipers, and the trunk has to be unlocked with the key.

I've checked the fuses in the glove box and in the boot (trunk), and they all seem fine. From my searching, it seems likely the General Module is responsible and may have failed. Hard to understand why changing the battery would cause that but the car's 20 years old now so I suppose this may be par for the course.

New GM3s are fairly cheap on eBay but I'd have to recode it somehow - I managed that when my light control module failed a while back and I have the cables etc. but not sure if I'll have the necessary software anywhere and god knows if it will run on a modern laptop...! I know there is a chap called Scott on the west coast of the USA who can potentially fix the GM3 but sending the unit to him for a fix is around £150 plus my shipping costs to him so if there's another cheaper / quicker solution then I'd be keen to hear it.

Any suggestions very gratefully received.
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Discussion Starter · #3 ·
Thanks! I will check those fuses though none of the fuses in the glove box or trunk were out. Are there others elsewhere? I found reference to Fuse 53 on a number of posts but that one was fine…

how do I check for 12V either side of the fuse - with a multimeter? I have one but not used it much.

help much appreciated!
 

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All of those fuses are in the glovebox. You can check the fuses with a multimeter or a test light. Touch the probe to the small opening on the each corner of the fuse. You must get 12V or light at BOTH corners of the fuse to prove it is good.
Your key needs to be switched to the Run position.
 

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Maybe try leaving trunk open, wait 20 minutes for the car to go to sleep, then disconnect the new battery for 15 minutes, then reconnect it. Maybe it will reset the GM.
 
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