355 What Triggers the Battery Light on the Dash?

1998 355 F1. My battery light started flickering a month or so ago.

Sometimes very faintly lit for long periods. Other times it would be bright then softly flicker then go out. Other times completely out for full drives.

At one moment, with the battery light flickering at idle, I checked voltage and had 13.8.

Last week the battery light was not on at all and the battery died. I charged it up and checked voltage once the car was running and I had nothing but the battery voltage of 11.8.

Couple of things I want to check, but first....what causes the battery light to turn on?

 

Exactly the same here. I start the car and the battery light, among others, is lit.

When car died, the F1 indicator went blank, but the idiot light were all on. Oddly, not the battery light.

Hence my question.

 

The question really is, what triggers the battery light to illuminate while the car is running?

As to your questions, Because of your posts in other threads and your diagrams, I was able to do some things that I think fixed the issue. Thanks for that.

Earlier I thought is was a lose sense wire at the positive terminal. Cut that off and put a terminal on it and attached at battery bolt. In the end, no change.
I pulled the alternator and had it tested and came back healthy. Same for the battery that is 2 years old.
I tested the sense wire and it ohm'd out just fine between the battery and the alternator plug.
Then I cleaned up the yellow "ignition" wire and Yellow/Green "light" wire at the alternator plug.
Then I went into the connector block in the back and cleaned the male terminals and tightened up the female terminals.
After connecting I used a pin to pop through the wire casing to test connectivity post terminals and they both ohm'd out fine.
Then I pulled relays E12 and E2 and tested them as well as cleaning up the spade connectors.
Tighten up the connector fingers as best I could on the fuse block.

Put it all back together and I get the battery light when I go to start it.
The battery light goes out once engine starts and 14.4 volts once running.
I need to drive it for awhile to make sure it's OK, but looks promising

My real question remains, what triggers the battery light while driving the car? It seems damn near random.

BTW, the "ignition" - Yellow and "light" - Yellow/Green are reversed on my car compared with round plug diagram in your document (Fig11a_1997_5.2_Injection_system_ignition_F1). I have a 1998 F1 car.

 

Yes, as noted I cleaned up the connections in the referenced connection block.

I think, and I may be full of **** here, that the "ignition" wire was not connecting and the alternator would not 'engage'. As a result it didn't charge the battery. Does that make sense?

This is interesting.

 

That makes sense then.

 

In the end, you seem to be correct.

When cold, the alternator charge was 14.4+. Once warm it fluctuated all over the place. 12.2 - 14.4.

So rebuild time. I took a chance.... $47....(gulp) https://www.ebay.com/itm/275543738079

Showed up and....
Transpo Regulator
Regitar Rectifier RN-10 which has the 8mm stud.
New brushes.

Rebuilt it this afternoon. You can do it without removing the alternator.....who knew?

One hour of driving and it seems pretty good.

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Back side of voltage regulator:
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Started right up and landed here at idle:
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