The +12V is there, so you are missing the Key On ground for those bulbs...

Those warning lights (ground side) are connected to the alternator D+ terminal via a thinnish red wire, which does two things:

1- 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- Also 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

[The BF sensor deals only with Low beams, Brake lights, Tail lights.]


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