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Old 02-21-2007, 07:50 AM   #1
Sergey
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Default Intermittant dim battery light puzzle

Hi all,

I have a '95 ti w/205K on it and it just gave me its first electrical puzzle. Everything electrical is original except for the battery that's 2 years old. Battery terminals are clean and battery cables are tight.

When I turn ignition on, not starting the engine yet, the battery light comes on dimly, at about a third of its usual brightness.
Once the engine is started, the light usually goes off. There was one time when the light stayed dimly on for a couple of miles, then went off.
Sometimes, if I leave ignition on, without starting the engine, the light would go from dim to bright on its own.
Except for this bizzare battery light, everything electrial works fine, the cars starts and runs.

Voltage checks that I made:
battery - 12.6
B+ alternator terminal - 12.6
D+ alternator terminal - 7.8 (ignition on, dim light)
D+ alternator terminal - 1.3 (ignition on, bright light)

This D+ voltage got me puzzled. Bentley says there's supposed to be "battery voltage" on D+ with ignition on, I assume I should've seen slightly less then 12.6 there. I pulled the instrument cluster and figured that the voltage drop happens across the cluster, that is it's close to battery voltage to the lock, across the lock with ignition on and into the cluster, but I'm seeing those 7.8 or 1.3 at any point after the cluster.

I also did the usual crude tests of the alternator when the light was off and got about 14.1 with no load and 13.9 with some load (headlights, defroster, blower) across battery terminals. No changes as I revved the engine. I once caught the dim light in the act and measured 11.5 across battery terminals.

I also did a static draw test and saw good solid 15 mA.

What do you guys think this is? I saw the 11.5 on the battery with the engine running and the battery light dim, so that suggests that the alternator is going, which wouldn't be surprising. But what about that D+ voltage? I really don't want to need a new instrument cluster

TIA,
Sergey.
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Old 05-17-2007, 07:26 AM   #2
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Default

I have a similar problem but a little worse. Earlier today my car died. This morning, on my way to work, I notice the battery idiot light on really dim. I thought I was imagining it but when I got to the parking garage it was clearly on but very faint. I figured the alternator may be failing but not totally cooked yet. BTW I have a 96 ti, original electrics sans battery. Anyway, there was no indication of anything wrong on my 45 min commute. On the way home, after about 30 minutes of driving, the car started to feel funny. It was sluggish. Several additional dash lights came on but dim. By now I realized that the car had been running off the battery and it was running out of juice. I turned off the radio and the ac fans and it got better but eventually it died about 1 mile from my home. (I did manage get the car home eventually)

I was looking up alternators on pelicanparts.com and saw this.
http://www.pelicanparts.com/bmw/tech...Alternator.htm
They mention that the regulator assembly attached to the alternator could be worn.

If you haven’t looked into this maybe this is something to consider. Otherwise, if you have solved the issue, please post.

Thanks
-jcc
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Old 05-17-2007, 04:52 PM   #3
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A rebuilt alternator solved it.

Please see http://tech.bentleypublishers.com/th...8838&tstart=60 and http://www.bmwcca.org/node/7147 for more details.

My impression about rebuilt alternators from reading online forums is that it's a crapshoot, so I went for one with 'lifetime' warranty from the dealer.

Regards,
Sergey.
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Old 05-21-2007, 02:41 PM   #4
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Yup. Same here. Thanks for the reply.
-jcc
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Old 05-21-2007, 04:59 PM   #5
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yup.. happened to me last week, alternator died..battery light went on at 1/3 or so of it's brightness.
I rebuilt the alternator, new rotor, voltage regulator and both bearings, everything works fine now..
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Old 05-21-2007, 05:38 PM   #6
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uaza,
Just curious, How many miles on your car? Mine went at 160K. As a side note, I bought my car new in 96 and only replaced the battery last year. I think 10 yeasr is pretty good for a battery.
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Old 05-21-2007, 06:36 PM   #7
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Quote:
Originally Posted by jccaruso View Post
I think 10 years is pretty good for a battery.
affirmative
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Old 05-21-2007, 07:41 PM   #8
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Quote:
Originally Posted by jccaruso View Post
uaza,
Just curious, How many miles on your car? Mine went at 160K. As a side note, I bought my car new in 96 and only replaced the battery last year. I think 10 yeasr is pretty good for a battery.
pretty good indeed..

my car have 85000km.. thats around 52.000miles only.. but nearly all the time driven in the city traffic..
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