Battery Light Issue

[Matthew A.]

So having some electrical issues this morning (car was dead, jumped it, she ran, turned off and tried turning on and she wouldn't start) I replaced the cable ends for the battery (the old ones where corroded and wouldn't seat tightly.

Now I've got everything wired up, jumped it and it worked. Cut it off after about 5 minutes and she didn't restart. Checked the fuses and they're still good so we charged the battery for a bit and then ran her longer and she cut off and then came back to life. But when off the battery light was still on.

Fearing that the radio (that a previous owner had wired directly to the battery) had shorted I've disconnected it from the negative terminal and ran her some more. Cut her off again, pulled the key and the battery light was still on, and doesn't turn off. Disconnected the wire to the positive end as well (just in case) and have her running at the time of this post to see if that helps any.

Any ideas? I'm working on a 1980 528i who follows up solving one gremlin with giving you a new one (classic really). I can get her running and she stays running, but I don't know how long she'll stay that way if the battery light continues to stay on when she's off.

EDIT: Forgot to mention something else I noticed. When the key is turned to the point where the electrical is on, the battery light turns off.

Double EDIT: Went ahead and shut her off and disconnected the battery so I don't drain it while I try and work this out.

[GripGreg]

I'm not even average electrically, so, I pass!
Someone will chime in we hope!
Greg

[Mike W.]

Any ideas? I'm working on a 1980 528i who follows up solving one gremlin with giving you a new one (classic really). I can get her running and she stays running, but I don't know how long she'll stay that way if the battery light continues to stay on when she's off.

EDIT: Forgot to mention something else I noticed. When the key is turned to the point where the electrical is on, the battery light turns off.

Unless someone really, really butchered the wiring, classic symptoms of a bad alternator diode. And yes, that will kill the battery.

BMW alternators rarely fail, I've not had to replace one in way over half a million miles, but nothing lasts forever. Especially the light coming on with the key off says alternator, not regulator, which do fail.

[Matthew A.]

Any ideas? I'm working on a 1980 528i who follows up solving one gremlin with giving you a new one (classic really). I can get her running and she stays running, but I don't know how long she'll stay that way if the battery light continues to stay on when she's off.

EDIT: Forgot to mention something else I noticed. When the key is turned to the point where the electrical is on, the battery light turns off.

Unless someone really, really butchered the wiring, classic symptoms of a bad alternator diode. And yes, that will kill the battery.

BMW alternators rarely fail, I've not had to replace one in way over half a million miles, but nothing lasts forever. Especially the light coming on with the key off says alternator, not regulator, which do fail.

Any good way to check this before I jump up and start trying to order parts?

EDIT: Found a nice little video that explained how to do it:



That way anyone else who has this problem can narrow it down and solve it.

EDIT mark II: So digging about some more I found a video on testing for a parasitic battery drain as well. I felt that since this is a kind of parasitic battery drain that it might be worth linking as well:



I'm going to pick up a multi-meter tomorrow, I'll keep you guys informed when I find the answer.

I appreciate the help Mike! It's good to have a direction to go instead of just running around like a chicken with my head chopped off.

[Mike W.]

Many auto parts stores will test them for free if you bring the alternator in. Of course then you almost need to buy a new one there which may or may not be the best price, but it's an option.

[Matthew A.]

Many auto parts stores will test them for free if you bring the alternator in. Of course then you almost need to buy a new one there which may or may not be the best price, but it's an option.

Ideally I'd like to narrow it down to the smallest possible part rather just hucking the alternator out, but I'll keep that in mind.

[Mike W.]

Diodes aren't really a normally replaceable part. I might have an alternator if you get stuck, or even a diode bridge if you really want to go there, but I don't think you do. Really, they very rarely fail. Regulators yes, alternators, no.

[Matthew A.]

Diodes aren't really a normally replaceable part. I might have an alternator if you get stuck, or even a diode bridge if you really want to go there, but I don't think you do. Really, they very rarely fail. Regulators yes, alternators, no.

Fair enough. What my plan has come down to is pulling the alternator, getting it tested and picking up a multimeter so I can track anything else down as I need too. Last thing I need to is to fix this problem only to have another sneak up on me. I'd rather be prepared for it early than be surprised later.

[Matthew A.]

Got it out and tested this morning. Diode was bad, as predicted. I really appreciate the help Mike. Having a starting point gave me a much better direction to go than being completely lost like I was.

New Alternator is on order, it'll be here by the end of next week so Samantha should be up and running about Saturday at the latest (pending any other issues).

[Blaise]

Got it out and tested this morning. Diode was bad, as predicted. I really appreciate the help Mike. Having a starting point gave me a much better direction to go than being completely lost like I was.

New Alternator is on order, it'll be here by the end of next week so Samantha should be up and running about Saturday at the latest (pending any other issues).

Good job

[GripGreg]

Matthew; you may as well change the belts, then you'll know they're new.
Greg

[Matthew A.]

Matthew; you may as well change the belts, then you'll know they're new.
Greg

You must work for my local auto parts store.

Of course I'm only joking. Belts are actually in good condition. No cracking or anything, but swapping them now would be easier than doing it later.

[GripGreg]

How do you know I don't!
If they look good, then OK.