Generally, not having a diode or "idiot lamp" causes the alternator to "backfeed" current into the ignition system, preventing shutdown, but once the engine is stopped, the lack of a diode should not cause the battery to drain. I am not as familiar with the Hitachis as the Delcos, but on the Delcos, the #1 terminal is fed through a lamp or a diode from the SWITCHED side of the ignition switch, and the #2 terminal (voltage sense) is ALWAYS powered up. The Hitachis have terminals with similar functions, but labeled differently... I can't recall the legend used, offhand. On a Delco, reversing those two leads will cause the battery to be drained while the unit is shut down. I assume it's the same with the Hitachi... you may have the 2 small leads reversed, or the "L" (light) wire is not connected properly, so it isn't being shut down by the ignition switch. The other possibility would be that the alternator has a bad diode INSIDE.
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