Battery light on is that your exciter circuit to the dash light isn't making connectivity.
If the light is on you are running off the battery.
When the light is out, you are charging and your voltage will come up to 13.75.


If you google alternator exciter circuit you can by-pass it...by putting a 12v light bulb between the
connector (black I think but refer to the Bentley) and ground... but google and refer to the Bentley or Haynes schematics.
I am too tired to think straight.


If I remember correctly, you have to push the hell out of that connector to get it to seat....

 

I had something like this happen to me recently. My battery light stayed on for months. Turns out the little board in the alternator was cracked, and over time finally broke all the way and left me stranded. I was getting enough charge to keep driving those many months, but not enough to turn off the light. Swapped alternators, and it now works like it should.

 

Yeah, I had one similar. The light sometimes would not go out, but most of the time it just burned at about 1/2 brightness. It was charging though, just barely enough to keep the battery charged, but not if I was using the headlights for any amount of time. Bad circuit board too.

 

The alternator passed all the tests... It seems that the blue(?) wire on the 3 wire plug is the giving me trouble. I also think the blue wire is for the dash light?

 

Not saying that you are having the same problem as we did, but my Alt passed every test with flying colors until the board finally broke.

 

Where does the exciter circuit (blue wire) get fed from?
I have looked a few wire diagrams, I think it's either the bulb test relay? Or the ignition switch?

With the Key on, the blue wire has no power at the alternator?

 

The exciter voltage is from the alternator, that is if you start the car, then that wire will have voltage on it between 8-12V usually. It feeds the battery light.
The battery light feeds it back to the alternator.


If the exciter isn't there, then the alternator never turns on...


ok Key on, but not started.


There on the dash the battery lamp is lit. Why because there is current flow from the battery through a diode and a resistor to light the lamp.
The current flow through the lamp is a sense voltage that tells the regulator that the alternator is semi-functional and bias's the voltage regulator to turn on.
When the voltage regulator turns on, it starts producing 13.75 volts and reverse bias's the diode that turns on the lamp, so the lamp turns off.


This is why when the exciter circuit is missing so the battery is powering the lamp, no matter what the alternator is doing it will never be allowed to produce power at the voltage regulator to charge the battery.. It is a simple cost effective way to tell the driver that you have a alternator / battery issue.


That is the simple way to describe it. Google Exciter circuits alternator and you can read more about it to the nth.

 

Thanks for the explanation!! I followed the circuit on paper, l did google it and nothing pointing to how fix it?
It looked like it goes through the bulb check relay? The bulbs don't appear to check, as described in the manual.

I'm kind of lost...

 

Sounds like it's not grounding properly.

 

I put the test light on the alternator, and it has a good ground. What other grounds could I check? I also did install a new body ground from the battery.

Can any one give me some items to check, as a process of elimination? To try and narrow this down?

 

Are you sure that the alternator is fine?
You need to ohm out the wire between the exciter circuit on the alternator and the back of the cluster.
Once you fix that you may be good. You have a break in the connectivity between the alternator and the cluster.
So look at the cluster connector ...

If you haven't replaced the main frame to engine, and frame to battery ground DO-IT.