This is how I tested two internally regulated Nissan alternators. The alternators with "L" and "S" marks by the "T" plug.
First, connect negative from a battery to to the case, or frame of the alternator. Then connect a test light to the positive of the battery, and touch the positive output post on the alternator.
If the light lights up, the diodes in the alternator are bad. Get another alternator.
If the alternator passes this test, go on.
Disconnect the negative battery cable from the battery.
Put the alternator on an engine. Again, connect a ground wire to the frame of the alternator.
Connect the positive output post to an amp gauge, and connect the other post on the amp gauge to the positive battery terminal, or reconnect the wire you took off the alternator positive post. The engine was running when I took this photo.
Connect the "L" connection in series with a test light. Connect the other end of the test light to the positive battery terminal.
Connect the "S" connection directly to the positive battery terminal.
I made this test wire harness with a light.
The dark red terminal goes to the positive battery cable clamp. The white plug goes into the alternator
Now, reconnect the negative battery cable. The light should light up.
This is a picture with a voltmeter, and the engine not running.
Start the engine. The light should go out. Measure the voltage at the battery terminals.
Charging voltage should be 13.5 to 14.5. I know the second picture up had the light on, and the battery voltage is 13.59. If you just recently charged a car battery, it is normal for the voltage to be slightly high, with the engine off. the voltage will quickly decay down to less than 13.5 volts.
It is also normal for the amp gauge to show a high current for a little while, as the alternator replaces the electricity that was just taken out of the battery by cranking the engine. I know that is not a good scientific explanation, but it gets the general idea across.
This was done with this alternator, from a Nissan pathfinder, on a L-18, in a 521 pickup.
Belt alignment view
The pathfinder alternator has 10MM holes on the bottom mount. I used 8MM ID, 10MM OD sleeves to make up the difference.