Battery light on only when cars shut off?

[Marshmellow]

Hello fellow e30 enthusiasts! Recently acquired a 91 318i and am having trouble diagnosing my battery light woes. When the car is on the light goes away but as soon as i shut it off boom light illuminated. I checked my charging system and put a brand new interstate battery in and its reading at 12.7 volts when at idle and hooked up to volt meter. With the throttle depressed to what I am assuming is 2k (tachometer non operational) it goes up to around 13.7. The values seemed okay so i started pulling fuses for radio, OBC, radio amplifier, trunk light etc. but still the lights on. Any help would be appreciated!

[e30davie]

Thats how the battery light works. The light comes on when the alternator isnt charging and its not charging when the engine isn't running :)

Only worry if the light comes on when the engine is running.

[Marshmellow]

Yes, I understand how the battery light functions but am trying to figure out why it remains on when the key is out of the ignition :) if i leave it the way it is in the morning when i try to start the car my battery is dead.

[Jaxx_]

Inspect ignition switch. You may have a circuit back feeding as well. After market alarm or stereo, for example?

[Marshmellow]

Inspect ignition switch. You may have a circuit back feeding as well. After market alarm or stereo, for example?

yes the car does an aftermarket stereo, but after further inspection i'm thinking it has something to do with either my alternator or voltage regulator. Read on another thread that there are two wires that come from the back of the alternator one of them being a blue wire. When i disconnected the blue wire running from the back of my alternator the light went away. What would be the next step in figuring it out? Should I go down to the local autozone and have them test my alternator?

[Jaxx_]

That’s the exciter circuit. I would suspect perhaps that an internal component of the alternator failed, but I am not sure on the interior components of the alternator. I am sure that you could have it tested if you are planning to take it out anyway.

The blue wire does complete the circuit for the light to work, so that alone cannot verify the alternator is faulty.

[Marshmellow]

That’s the exciter circuit. I would suspect perhaps that an internal component of the alternator failed, but I am not sure on the interior components of the alternator. I am sure that you could have it tested if you are planning to take it out anyway.

The blue wire does complete the circuit for the light to work, so that alone cannot verify the alternator is faulty.

Back again... after throwing a new voltage regulator on the problem still persists. How would I go about examining the ignition switch? I tried searching through the forums but am unsure if there is a difference between the m42 and m20 variants.

[Jaxx_]

Wiring diagrams are available if you search the web. You can find your specific model and year—look for the ignition switch circuit and use a multimeter or 12v probe/test light to confirm the switch is functioning the same as the diagram

[TobyB]

Hmm, I would have expected the regulator to fix it. That light dead- ends in the alternator.

http://www.wedophones.com/BMWManualsLead.htm

Here's the stinker- as far as I remember, there's no specific manual for the
M42E30 engine. So look at the E30 manual AND the E36M42 manuals, and you'll
get close enough.

I still suspect your alternator.
Quick check? Unplug the connector from it. If the light stays on, that ain't it, and
it's time to get the voltmeter warmed up.

hth

t

[Marshmellow]

Quick update to provide reference for other owners who may experience this issue in the future, it turned out to be my alternator. I suspect the diodes went bad in my previous one which is why the light remained on. No issues with the new unit installed and no light! Cheers!