Posted by Peter Florance on October 30, 2003 at 22:40:20 from 65.120.51.217
In Reply to: I'll get a test light tomorrow. (more) posted by Brian in Abilene on October 30, 2003 at 20:18:57:
It's kind of weird. The light provides current to the field coil. That current is required to get the alt runing as the coil provide the magnetic field - there is no permanent magnet. There are also diodes that feed the field coil from the main windings. Once the alt is running and generating current, the diodes provide current to the field coil and the voltage at that point becomes +12V. Now this is where one end of the bulb is connected and this end is basically grounded (through field coil) when the alt is not running. The other end is connected to switched ignition. When the car is running and the alt is working, there is 12V on both sides of the bulb and no current flows through it and it doesn't light.