Red Battery Light - 98 528i - Need Help!

[goatboy]

Hello all,

I am new here, and a first time BMW owner. I appreciate any help you can suggest about the following post. Sorry for the length, I wanted to be as thorough as possible.

I have a 1998 BMW 528i with a 2.8L 6-cylinder with about 230k miles.


The Red battery light came on roughly 2 weeks ago. I did the following to try to correct the issue.

• Tested the battery (Testing decent at 12v)
• The white compartment, located just above the battery with all of the fuzes/connectors was taken apart and all of those tested positive in working order. All of the fuzes in the panel just to the right tested good as well, along with the large 40 Amp fuze (not sure what this is even for) test good.
• I then removed the alternator, found ALOT of debris in the air intake cooler for the alt and cleaned all of that out. The alternator (Bosch) bench tested good.
• After cleaning all terminals and putting everything back together, the car started right up and the battery light was no longer on.

It ran fine for the first few hours, but then the battery light lit back up. I did notice that when I accelerated, it turned off, and when i was driving at normal speed, it came back on. But only for a while. It was then on constantly.


The car sat for roughly 1.5 day. I fired it up, ran about 4-5 miles and the battery light started doing the same thing. On until I accelerated, then off. After the car was warmed up, it stayed on.


I directly went to the tensioner thinking that it may be loose and as the belt warmed it quit turning the alternator fully, generating the light, but to no avail…the tensioner is fine and the belt is in good shape and plenty tight.


Any thoughts about this issue? I have done everything in my power to try and run this down.

[al_r]

How old is the battery? - E39s get wonky with an old / borderline battery.

[Beitie]

The battery light should just be letting you know if the alternator is charging. I'd be curious how well it actually tested, and I'd venture to guess that's where your issue will be.

[d21spike]

Not sure if you'll be in the same boat as me but I had my alternator fail. Replaced it with one from a local parts store. Within a couple days the light came back on. Threw it on the tester and it tested good, replaced it anyways and no issues since. My point is the tolerance of the tester may not be that of your car. As well as failing brushes and/or regulator on the alternator may be intermittent to begin with. Just something to look out for. I'd say when the light comes on, keep it running and hook up your volt meter and see what it is putting out (would help adding load also like headlights, a/c, radio, etc).

[goatboy]

Resolved: I replaced the battery and alternator and everything is working correctly.

I also replaced the plugs and boots, cleaned and brushed all the grounds I could find and made sure all terminal connections are tight and clean.

Thanks to all for your help!