Those warning lights mean the Alternator is not "putting out", and you're running off the battery — who is getting sick of it, since he all he's supposed to do is crank the car for starting. Then get charged for the next time. The lights are connected to the alternator D+ terminal via a thinnish red wire, which does two things:

1- It makes a ground path for the Batt light (and those others*, via diode connections) TO and thru the alternator to ground.
The bulb current flowing thru the alternator causes it to produce voltage as it "spins up". Gets it "excited" so it will... (you know?)

2- And carries Alternator voltage back to the bulb circuit to turn the lights OFF once the Alternator is putting out voltage. The D+ terminal (that was a ground) should rise to Alternator B+ level. So with voltage now on both sides of those bulbs, no current flows thru them and they go out.)

* The other bulbs whose current adds to that of the Batt light are:
• Parking Brake
• Brake Warning
• Bulb Failure
• Maybe others, depending on year & model

EDIT: If you have a meter, you should see a nominal +12V on disconnected red wire at alternator with KEY ON.

• If +12V is there , problem is in alternator, regulator/brush holder, or grounding wire.
• If no +12V, problem is in red wire or connections leading back to instrument cluster.




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Bruce Young
'93 940-NA (current), 240s (one V8), 140s, 122s, since '63.