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Old 12-13-2012, 02:53 PM
bimmerfan52 bimmerfan52 is offline
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Location: Arizona
Join Date: Feb 2012
Posts: 1,255
Mein Auto: 545i
Not sure I follow your post.

If I read you correctly, voltage is a low 11.5V before starting but alternator is outputting 14.5 after starting?

If this is correct then battery is not holding a charge (especially if there is not a significant time between turning the car off and restarting it).

These cars are designed so that almost all systems are tied together electrically to computers and depend upon a solid 12V supply to operate accurately.

I think the first step is to go to a national auto parts chain (O'reilly, Autozone, etc.) and have your alternator, starter and battery computer analyzed for free.

If the diagnosis is that the battery is bad then replace it. Your big decision will be, do a dealer battery or buy aftermarket, and then should you pay to have the computer reset to acknowledge the new battery. My car happened to be in the shop for a replacement iDrive screen, and the battery needed to be disconnected and the computer reprogrammed anyway so the dealer quote was $289 for a new battery installed and reprogrammed. However, I have heard of $600 and $700 quotes from dealers for a new battery and programming so I would proceed carefully. A replacement of the correct size and amperage can be as little as $160 from an auto store, and the programming can always be done later. Lots of posts on this forum regarding battery replacement and correct battery model numbers.

If it is a weak battery, once the battery is replaced you can again assess the transmission and see if it cleared up.

If the battery is not the cause the next thing you can try is a trans fluid flush. Sometimes sticky solenoids can be corrected by two drain and fills. If you haven't had this done to date this is excellent preventive maintenance anyway.

Re post after assessing your battery.
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