E39 540i, voltage problems... - BMW Forum - BimmerWerkz.com
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#1 (permalink) Old 05-31-2006, 04:46 PM
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E39 540i, voltage problems...

Hey all again, long time no see. Recently my 97 540i has been going through hell. When it starts up, the voltage rapidly fluctuates from 13.8 to 12.0 and back up again. Adding any kind of electronic consumer (lowering windows, headlights, changing from park to drive etc.) will some times kill the car completely as though the key were turned off and then back on. When its not killing the engine, the voltage shift causes my subwoofer to "pop." The alternator was replaced a little more than a year ago and I'm afraid its the problem again. Has anyone else had this issue before or have a possible solution?

I knew it was time to post this problem as I stalled it out in the middle of Michigan Ave. and was sworn at in about 8 different languages.
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#2 (permalink) Old 05-31-2006, 04:56 PM
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Dude, how many miles do you have on that thing now? Good hearing from you, altho not under the circumstances.

Have you checked your battery lately? I've heard that can cause sundry undesireable effects such as the one you describe. Let us know when/how you resolve the problem!

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#3 (permalink) Old 05-31-2006, 06:43 PM
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the battery is perfect. I thought at the beginning that the battery was to blame so I disconnected the battery with the engine going and volts freaking out....still the same deal. Its up to 183,xxx now and still doing pretty well other than this.
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#4 (permalink) Old 06-01-2006, 02:52 AM
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Did you replace the alternator last time with a factory part? Have it rebuilt?
If you monitor the voltage while running does it spike all over the place or does it stay constant until you put a load on it then it just cant keep up?
Maybe try checking the ground cable at the battery to the chassis. Take voltage readings at the battery and also at the jumper post under the hood. Make sure you have no voltage drop from 1 to the other.
Good luck,

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