Pulled into the gas station, shut the car off pulled the key out of the ignition and the bat light came on. FIlled up drove home everything was fine. Shut the car of and the light came back on. I did a quick check for loose or coroded wires but turned up empty.
I don't have time to mess with it right now so I pulled the neg cable just in case.
What would cause the battery light to come on with the key off?
Car is a 1980 BMW 320.
does the 1980 320 have the little gauge cluster battery in it like the e30s do?
Don't believe so?
Diode in the alternator has probably failed. Bosch alternators have 12 volts coming from the dash bulb down to a terminal on the alternator, which is grounded through a diode when its not charging. When it commences to charging, it sends 12 volts back up the wire, resulting in a voltage of zero, and turning the light off. You are probably feeding battery power back up the wire to the builb, which is grounded by all the other stuff on that power circuit.
Or something like that.
In reply to Streetwiseguy:
So can or should I replace the diode or the alt?
I speak generically here, about Bosch, not about your car in particular. The Huns may have engineered a much more elegant solution to turning a bulb on and off than the Swedes...
By 1980, it will likely have an internal regulator, which means there will be a main battery power wire, and the small wire from the dash. If so, unplugging the small wire should turn the light off. If it does, chances are a shiny new reman alternator is the way to go. My experience has been the only way to go is with a Bosch rebuild. They are not a pile more money than a cheap one, and they generally work...
I had it happen once in my '86 BMW M535i, it was a bad diode. I don't know about the 320, but on my car the diode pack isn't easily replaced (on some other cars it bolts in, on this one it's soldered in place.) I ended up getting a remanufactured Bosch alternator from the local Auto Zone, it wasn't terribly expensive and it's worked fine for several years now.
x2 on the diode. Will drain your battery as you well know.
You'll need to log in to post.
