I'm trying to figure out what the posts go to internally in my alternator. I know the post marked BAT is the post that charges the battery, but im trying to figure out what the two terminals that the little plastic connector plugs into do. On the MFG plate on the alternator, it says the top one is "S" and the bottom one is "L". I'm guessing S is for Stator but I havent a clue what L means. 

L is for Charge lamp. 12v to this terminal MUST come through a load or you will fry the ALT.

 

S is for "signal" volts.  12v switched on older, 12v constant from Batt on newer.

S is for Sense, and L is for Lamp. It's a standard GM remote sense internal regulator. In practice this means the S goes to about a 12 AWG wire that merges back into the BAT lead about 10" down the harness. The L is a +12v that runs through a dash lamp and then to the alternator. It's the battery indicator on the cluster and is also the field flash that starts up the alternator. 

 

GD

S is for Sense, and L is for Lamp. It's a standard GM remote sense internal regulator. In practice this means the S goes to about a 12 AWG wire that merges back into the BAT lead about 10" down the harness.

 

Sense or signal.....whatever.

 

But on older cars it went to switched 12v.

 

it's not until around 95 that they tied it straight to the battery leads.  FWIW, Either way works fine.  It's the L post that really needs to see the 12v through a load.

I posted at the same time as you - didn't see your reply. It wasn't a correction. Either term is fine really. But the line is used for the VR to sense the voltage output at the "main junction" - which, in practice, is not actually the main junction, it's just a ways down from the BAT terminal - I really have no explanation for why this was done other than to save money on wire because they really didn't care to do it the right way.... If you want to get into semantics though a "signal" is generally going to be something generated externally....

 

GD

 But the line is used for the VR to sense the voltage output at the "main junction" - which, in practice, is not actually the main junction, it's just a ways down from the BAT terminal - I really have no explanation for why this was done other than to save money on wire because they really didn't care to do it the right way....

 

On older cars the S wire is not connected directly to the battery output wire.

 

It is connected to the IG. circuit "BW wire on EA, Yellow wire on 90-94 EJ"

I checked the schematic in my 86 FSM for the SPFI version. 

 

The S terminal is wired like a regulator remote sense line.  This is done to control the voltage pushed to the battery  [thus the current for charging] more accurately.  Remote sense for voltage regulators is used when the designer wants to have more accurate regulation at the point where the sense wire is connected.  Without it, the voltage the battery would see would vary a lot more depending on what accessories were switched on.

 

It would take a much heavier wire run directly from the alternator to the battery + terminal, and a separate fuseable link to get the same regulation accuracy.