I've got a 1985 Coupe Deville that wont charge a battery. I've replaced belts battery and alternator all to no availle. I've recently noticed the the check alternator light never comes on during cranking. I read somewhere that the light is actually part of the circuit and acts as a resistor to turn on the diodes in the alternator or something. I can't find this light..... I've gotten most of the instrument cluster taken out with the exception of the speedometor (not digital) this is where the guage is but I don't see any bulbs.

So..... THe alternator has a three pronged L shaped plug. The wires are white, orange, and two brown. THe brown share a spot. The orange wire has constant battery voltage. The other two never change whether off, cranking, or running.
As a test I connected the browns to battery and cranked the car then disconnected them. At this point the alternater seems to be working fine. Turned the car off and cranked again without doing this and no charge again...... unless the browns have voltage to start with.

Anyone have the wiring diagram that can tell me where these wires go or what they're for.

Thanks

The brown wire is the wire that connects to the dash light and beyond to the fuse. If the bulb is out, the alt. doesn't charge.

Thanks.... more tracing for me I guess.

On my GN I had the same problem. The brown wire is the one on the GN, I am assuming that it would be the same for your Caddy.
I had problems after replacing the bulb even getting it to charge, so I jury rigged it.
I wired a relay from a known good +12v to the relay, and then to the brown wire on the alt, with the trigger for the relay from the fuse box IGN terminal.
You might not want to do this, but I did it because I needed a stronger field for the alt. I boosted my system voltage to 15vdc for a hotter spark, so I need the field to be bigger.

Shane Smith
87 Buick Grand National