Battery light now working back to front?

[Chuffy]

Since removing and replacing the alternator, the battery light is now on when the ignition is off, but goes off when you switch the ignition on, once the engine is started it stays off as normal. The only thing I'm not sure of is the wiring connections on the back. There is the red and blue which have obvious connections but there is also an earth (brown) which I'm not convinced was connected in the first place, but i have now attached it to earth.
What could be the problem?

E30 316 1985

[mark_i]

Double check the earth connection of the alternator.

Measure the resistance btween alternator body and battery negative, should be close to 0 ohm.

I had this before when the alternator diode pack failed, had to replace complete alternator.

[Grrrmachine]

For one, do a multimeter test to make sure the battery is receiving charge, so that you know the main cables are wired correctly.

The idea of the warning light is potential differences between voltages. A wire from the battery goes to the bulb, and from there it is grounded to the alternator rather than the battery earth. When the alternator is not working, that side of the circuit is 0volts, and the battery glows. When the alternator starts turning, the alternator raises to 12V (or more) and voltage difference across the bulb becomes 0V, so the light goes out.

If you've connected the bulb to the alternator and earth, or to the battery and earth, rather than alternator and battery, then it will glow with the circuits turned off, I would have thought.

Sorry for the lengthy response

[Chuffy]

Thanks, will have a go with the meter and let you know what happens.

[mark_i]

Not 100% true.

Battery negative, engine, and alternator body are connected together and at the same potential.

If there is a bad connection or missing cable, then you will have charging problems.
In this case, the charging light on when the ignition is off is a symptom that the diode bridge assembly in the alternator is defective.
The damage could be caused by an incorrectly earthed alternator.

[Grrrmachine]

Not 100% true.

Battery negative, engine, and alternator body are connected together and at the same potential.

Yeah, but when I say "bulb connected to the alternator" I mean to the specific plug on the back allocated to the warning light, not the alternator housing.

[Chuffy]

Well, checked with the meter today and the battery seemed to be charging ok.

Took the blue lead off the alternator and started the car, everything fine apart from the red battery light stopped working altogether of course!

Stopped the engine, re-connected the blue lead, suddenly everything working as normal.

Strange eh?

[Grrrmachine]

Must be a small short somewhere then, but as long as the battery's getting a charge, there's nothing to worry about

[monkey]

looks like the fault is in the alternator as mark said , if its a new one get it changed on warranty , if not then it is charging but you wont know if it stops charging until the car stops .