the regulator ONLY senses voltage on the B+ and modulates the field current so that the alternator puts out 13.8 to 14.2 volts.

the D+ is isolated from the B+ with a couple extra diodes on the diode board inside the alternator. the ALT light comes on when there's current flowing through the D+ (from the ignition key circuit 15). once the alternator is generating voltage, then the D+ voltage is the same as B+ and there's no current from battery + -> alt light -> D+ ...

 

Yes, I've internalized that circle from the D+ post, light, key,fuse on alt like a dog on a chain having wore a sunken path in the dirt! I'm putting a new alternator in tomorrow; if I take the D+ and cut it before it goes into the firewall joint box and INSTEAD lead that D+ wire BACK to the + on the battery, start the engine so the alt is spinning - and say - turn the lights on - the alt should produce current? This seems the most logical way to bypass whatever trouble the D+ wire is getting into in the circuit. Then - if the current does charge the battery, I can try and fathom what is going on with the D+ circuit in the dash?

 

the D+ voltage is actually only needed to get the alternator bootstrapped, once the alternator is producing voltage/current, its no longer required, so you can just temp jumper it to the B+ pin (which is directly connected to the battery) and should still get juice out.

 

OK good. Then if it's a wiring problem inside the dash maybe I can still get a charge from the new alt. Thx.