Attached is the wiring diagram for the single alternator on a 6.0L. As you can see, the Orange w/ Light Blue wire (A circuit) goes directly to the battery through a fusible link, so it should have battery voltage at all times.

The Light Green w/ Red wire (I circuit) goes to the Instrument Cluster. It is what allows the cluster to "turn on" the alternator, and what turns of the battery light if there is a problem with the cluster. On the diesel, the cluster turns on the alternator when the glow plug timer has expired.

The chart below shows what voltage each circuit should have at what times. It is somewhat inaccurate, though, since the I circuit won't be energized until the glow plugs are turned off.