Chevrolet Sprint or Suzuki 86-89 Nippon Denso alternator-which terminal of the three flat blade terminals should be connected to the ignition circuit to tell the alternator to charge?

On most ND three pin plug style alternators the pins are marked as "L" / "S" / "IG" The only other connection is usually a large single post terminal marked "B" This is the main wire connected to your battery and is hot at all times.

The three others are fairly easy to understand.

"IG" This is the Ignition wire and issues the "wake up" to energise the circuit when ignition is switched on

"L" This terminal is connected through to the charge warning light on your instrument cluster

"S" This terminal is connected to your battery source and is hot at all times, it may even be connected to the main wire on the large B terminal in the loom but more often to the battery through a fusible link

hope this helps