Originally Posted by trees
OK, I have read these links and it seems straight forward enough, BUT just wiring in an idiot light to terminal L (no such label on my alternator, but looks just like the CS120/130 in the links) confuses me. I can understand the light coming on but do not understand the light going out when the alternator is operating properly. If I wire the alternator as depicted, then the only time I am feeding voltage to the L terminal is before engine start. When the alternator starts charging, then there is no current flow through the the light so it goes out and for this to happen, then the alternator is self sustaining when charging as the way it is designed. Weird!!!
You are still applying power to the bulb from the ignition hot side, just when the alt starts making power it puts power to the bulb from the ground side, effectively "removing" the ground path for the bulb, making it go out. When the alt is not turning, this field wire is the ground for the bulb, through the alternator to ground, and the light comes on. If the alt is turning but still not making voltage(bad alt), no power is put into the field wire from the alt side, so the bulb stays grounded and lit.