BMW electrical systems. Positive Ground?

[Pdaddy]

My daughter recently purchased a 1988 325 and it has a number of issues, The most daunting of which concerns the electrical system. The battery will go dead in a matter of hours if left with the battery hooked up when parked anywhere. The reason I'm asking about positive ground is because I found that the car will start even with the negative cable detached from the battery. Also a number of things went wrong when her boyfriend tried installing a car stereo. He said as soon as he turned the stereo on, there was a spark and several things like the speedo and dashboard lights stopped working. Does BMW use the old positive ground system like VW used to? Paul

[Manolito]

First answer is No, there were no positive ground 1988 BMWs.

With regard to the car starting with the negative battery cable disconnected, did you observe this personally? Because this is physically impossible unless the battery cable flopped back over so it was touching the terminal when someone turned the key.

I am not familiar with this system, but the connection between the stereo and the dash will be one of two things: A trigger that tells the stereo to dim when the headlights are turned on, or a line that illuminates the stereo display together with the dash so all the lights brighten and dim together manually or as the headlights are turned off and on. Whatever else is wrong, it sounds like that connection was improperly done, and/or was connected to an incompatible head unit.

I think that the only things that should be hot on an '88 when it is turned off are the clock and the ignition computer memory. You may be able to find a drain that large with just a voltmeter. Put a voltmeter between the battery cable clamp and the battery. It will read full battery voltage which should be 12.6 volts if the battery is fully charged. Then start pulling fuses and relays one at a time until the voltage drops a volt or two. Then you are on the right track. I would probably disconnect the line between the head unit and the dash cluster before I even started on fuses.

If the voltmeter does not drop, you may have to use an ammeter, but a drain that drains the battery in just a few hours will be way too much current for a typical in-line ammeter. Those normally only measure a few hundred milliamps and you are leaking amps.