Hi,
I have a 2009 BMW 520D E60 and last week on a cold morning I noticed that the wipers were running slow. After a few miles I got a transmission error followed by an ABS light, then the I drive display went off. The engine then cut out when I stopped at traffic lights. Dash lights were very dim and car wouldn't crank.
I got the car was towed to a mechanic, he jump started it and it ran perfectly. He tested the battery and alternator charging and everything was ok.
The car ran fine all week, I bought a cheap plugin voltage reader to keep an eye on the cars charging and it was fine all week.
Yesterday was another cold morning ( Light frost on car ) and when I started the engine I noticed the voltage was 11.5v and not increasing as I drove.
I drove home and turned off the engine and restarted, this time the voltage rose to 15.1v and varied between 12.5 and 15 as I drove the car.
It has been fine since. I ran a Protool diagnostic after the first issue and it reported lots of under voltage errors but nothing much else. I have tested the battery with a BT20 tester and it reports it as good.
This has only happened twice so far, both frosty mornings, Any ideas what might be the cause. I have checked for loose connections but found none.

Thanks
Diarmuid

 

There are two possibilities.

One (and I believe by far the most likely) is that your alternator is on its last legs, and is exhibiting thermal intermittency. You'll probably be able to fix it with "just" the voltage regulator (which includes new brushes) but you could also consider a rebuilt or new alternator (just get a quality brand from a trusted vendor, if you go that way).

The other is that you could have an intermittent connection somewhere in the alternator wiring.

But if the alternator has a whole lotta miles on it, I'd suggest swapping the regulator or the whole alternator... of course, I can't be sure just what kind of alternator fits in a 1996 E60 (it would be the first one I ever saw). ;-)

 

Not sure where 1996 came from. It's 2009.
thanks

 

Your wrote it was a 96 but I corrected it. Have you done a test on the alternator yet. If not I would perform one.

 

The alternator test was good.

 

Understand the meaning of the word "intermittent" and you'll have your solution. ;-)

 

I might be the only guy in the world that likes installing new voltage regulators proactively in alternators (for just the exact intermittent problems stated above).

Its only $40, but worth it.

 

I do a lot of "unnecessary" maintenance myownself, because I do a lot of long (6-7,000 mile) road trips. I figure it's better to spend an extra few hundred bucks to replace parts that are working just fine if there's any chance of them failing in the middle of one of those trips. The thought of trying to find a competent, affordable mechanic in NYC makes me shudder (and by "affordable" I mean "the quote doesn't produce an immediate heart attack).