Another possibility is the BST module on the positive battery terminal.

https://bimmerscan.com/bmw-battery-safety-terminal/

I don't know if that can cause a problem once running. It will prevent a start but it's specifically designed to not possibly reconnect once it opens.


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Given the symptoms, it is most likely that the alternator is overheating and hence, by design, the DME temporarily reduces power (as a precautionary measure) until the alternator has cooled. This explains why the vehicle behaves fine after a few minutes of being off. It shows up as BSD interface (communications line) between the alternator and the DME being interrupted. The relevant fault would be DME- 2E95 Generator - Communications loss. If you have that error code, then very likely that is the issue.

See newtis.info:

https://www.newtis.info/tisv2/a/en/e...-light/XcGCxRr

Controlled load reduction at high temperatures: The alternator is overloaded and the alternator voltage is reduced as a precautionary measure; it remains at a reduced level until the alternator has cooled. The charge indicator lamp does not light up.


I had this very issue and multiple voltage tests determined there was nothing wrong with the charging system, battery, connections etc. I did it myself, and later deferred to a professional when I could not find anything wrong. The pro found nothing, but I continued to have sporadic power failures while driving. The car would be fine for weeks on end and, out of nowhere, while driving, the vehicle would behave as if the battery were dead, warning lights come on, trans goes into fail safe mode, engine dies, etc. I would pull over, restart the car and it would fire right up! But I would always get the code DME- 2E95. The rest of the codes were the usual low power codes as expected. I eventually just replaced the alternator and all has been fine since. That was 8 years ago, I think.

True, if it’s working as designed. But if it’s old and has corrosion or wear then vibration might cause an intermittent disconnect.

Probably not a common occurrence, but it’s a possible point of failure.

Nope, power Ball odds impossible. There is just no way a bst could be intermittent. If it ever loosened enough to act up while driving the increased resistance would absolutely melt it the first attempt to start the car.

It's not a normal kind of "contact" where it could get loose: it takes an explosive charge like a bullet to open. Once opened, barbs hold it open permanently.

The overheating theory holds some water, however, it doesn't explain a low voltage start attempt, but that could be a secondary symptom.

Get a plug in volt meter that shows the voltage at the cigarette lighter socket.

Keep a constant eye on the car system voltage. Odds are, the Alternator is taking mini vacations periodically, and the car is running off stored energy in the battery. This will be quite apparent if you monitor the battery voltage regularly. If the alternator runs just 2/3 of the time, and you don't drive at night much, you might never know. 80AH battery could run a car for an hour or two with an alternator that is running 1/2 the time or even less

Do you have air cooled or water "cooled" alternator (funny that it's filled with 100C water to me). I'm pretty sure my air cooled Alternator spends 99.8% of it's life cooler than 100C!

A couple of times I did experience the low voltage start attempt after a failure. I would then just pause for a few minutes, after which the engine would fire right up on the next start attempt! I had this issue for almost a full year before deciding to replace the alternator. Incidentally it first happened not long after the CPO warranty expired. It's a difficult issue to diagnose as everything would check out great, and then it suddenly hits. Everything would be fine again for weeks or months and then suddenly recur.:yikes: Hopefully the OP updates us with info of error codes read off the vehicle. A DME- 2E95 would be a give-away.