Hey there! I'm pretty new to this forum, but I've been having this weird issue with my 2006 750Li. The car has lately just gone at the most random moments into the Transmission Fault state and the CEL has come on too. Every time I check it now it shows me low voltage codes on several items such as most injectors and the MAF. A simple restart of the car eliminates the Transmission Fault and the codes clear themselves after a quick drive. I'm really frustrated and want to avoid going to the dealer if it could be something simple to fix. Anyone with similar issue or suggestions, would be great to get your input! Thanks. -Bill
Can't speak for 7's, but can't imagine it would be much different - modern BMW's that require battery "registration" are prone to all kinds of inexplicable wonkiness when the battery is at or near the end of it's life and not delivering full voltage/charge. If your batteries have only been replaced once, and that happened around 6-7yrs. in (which wouldn't be an untypical battery life for BMW's these days, it seems), then it could well be you're due for a new battery (or -ies; I could see a 7 having two, don't know if it does though). If your battery(ies) have been recently replaced, if they were registered, that wouldn't be the first obvious armchair-analysis guess, but that doesn't mean battery still isn't a possibility - I've heard mechanic friends comment that they've seen brand new dead/bad batteries more commonly than they ever used to and that battery quality often isn't all it used to be. Also just throwing batteries in without registering tends to kill the battery faster than normal. Beyond battery or power delivery, I don't have enough familiarity with the complex systems for a further guess, but our resident tech Charlson should have some thoughts when he sees your post here.
If you are getting low voltage faults your alternator could be dropping out. To test it I would take a multi meter and hook it up to the positive and negative terminals of the battery and set the meter to min/max function. By doing this it will capture the highest and lowest voltage reading. So if the voltage drops below 12v with the engine running then the alternator is dropping out. They should keep voltage from 12.v to 14.8v normally. If it is dropping out you will need to replace the alternator.
