Becker Electronics

E30 SI Board Replacement, Also removing instrument cluster

If you can not reset your Service light then you have to change the SI BOARD batteries.
These batteries are intended to save the service indicator memory in case you disconnect the car's main battery.  When the car is running it recharges the SI board batteries.  But over time they lose their ability to hold a charge and that is when the SI system starts to go haywire.  The LED's will come on and cannot be reset.  Eventually your gauges can start to act funny (tach, temp etc.)  At his point it is time to replace the batteries.  BMW did not make this particularly easy, the whole instrument cluster has to be pulled out to get at the batteries.  Some people solder in a remote connection so that the batteries can be stored farther down above the pedals.  This makes them much easier to access the next time they need replacing.  However, the modern class of batteries is far superior to what our cars originally came with.  The new (NiCad) batteries should last for quite some time.  Below you will find an outline of the procedure that I followed to replace the SI Board (and the offending batteries).
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