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Battery drain through fuse 21

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    Battery drain through fuse 21

    1991 318is. Battery is 10 months old but wouldn’t start the car after car sat for a few days. Battery was recharged at auto parts store and passed all the tests there.

    I am measuring a 0.12A current draw on the battery. Drops to 0.01A when fuse 21 is removed. Stays at 0.12A with fuse 21 installed and all remaining fuses are pulled separately. With fuse 21 installed, I still get between 0.10 and 0.12A draw after removing lights / unplugging power to the following:

    - LH and RH dome lights
    - trunk light
    - glove box light (glove box has no flashlight)
    - chime module
    - analog clock (replaced OBC)
    - aftermarket radio and amp (I have no antenna)
    - instrument cluster (SI board batteries read 3.07 volts)
    - main relay and fuel pump relay

    After unplugging everything listed above, I can’t figure out what is draining the battery through fuse 21. Any suggestions on what else to unplug or test or where to start looking for corroded wires?

    #2
    I would bet its your SI board. The SI board has a common problem in which the 20+ transistors fail, causing the LED or incandescent bulbs to be stuck on, with the key off. They can sometimes be so dim, you cannot actually see the light. There can be other processes that can be stuck on, still causing your drain.

    New SI board batteries should be 3.25V so yours is definitely dead.

    If your SI board is not been refurbished before, I would definitely assume that is the case. Remove your SI board from the cluster, plug it back it and you will see.
    Owner - Bavarian Restoration
    BMW and European Electronics Repair and Restoration
    www.BavRest.com
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