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To quote the 700/900 FAQ:
http://www.brickboard.com/FAQ/700-900/ElectricalStarting.htm
Exciter Current Through the Warning Lamp on the Panel. The alternator (charging) warning light in the instrument cluster feeds a small amount of electrical current to the field coils in the alternator when you turn the key on and the engine isn't turning. This excites the coils (electromagnet) and the alternator gets the magnetic field it needs to start charging as soon as the engine runs. When the alternator isn't charging at all (engine stopped) that wire to the alternator also provides a ground path for certain warning bulbs in the cluster, so they light up. Diodes in the network keep current from backfeeding into unwanted circuits. So if you have a bad connection for the alternator warning wire, the alternator won't start charging right away. But most alternators will self-excite once the revs get high enough (say 2000 engine RPM) and then they stay excited and charging, even at idle RPM.
[Editor] To initiate charging upon startup, the small wire going into the rear of the voltage regulator must be at 12 volts. If this is grounded or disconnected (as, for example, through a loose instrument panel or chassis connector), your alternator will not charge. The idiot lights may or may not go on, but if they do and they remain on, then suspect this wire or the panel connector for faults. You can also rarely experience a flexible circuit board failure on the back of the panel. And make sure the alternator warning lamp bulb is not burned out.
[John Randstrom] If you pull the wire off the alternator that comes from the dash board charging lamp and ground it, the idiot light should light with the ignition in the run position. If the lamp does not light then there is a problem with the bulb, the ignition switch, or the wiring/instrument cluster circuit. An easy way to see if this circuit is causing your no charge situation would be to connect a wire from the battery positive terminal through a small spare idiot light bulb(or any bulb with the same wattage and voltage specification) to the idiot lamp circuit connector on the alternator. The alternator should charge and the lamp should light when the engine is off, and go out when the alt. starts charging just like the original dash lamp should do.
[Tip from Paul Golden] I removed my instrument cluster and did the continuity checks, sure enough the solder joints had come loose from the wiring. I fired up the soldering iorn and hit each solder joint with some fresh 50/50 flux core solder. Installed the instument panel and presto, all the lights work once again. Since then I have repaired that same problem in 6 different cars, 760, 3-960, 2-940 so it seems to be a regular problem.
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