DTC P0621: Generator "L" Terminal Circuit: Description
WARNING: This page is about a different car, the 2002 Saturn Vue. However, it is still accessible from the selected car via links, so may be relevant.
"L" terminal is a 2-way link between ECM and generator. ECM supplies generator with 5 volts whenever ignition is on. ECM will not supply 5 volts to generator during engine cranking if battery voltage is less than 9.6 volts. Once engine is running, generator must have 5-volt supply voltage to charge. If generator stops charging while engine is running, it will ground 5-volt supply from ECM. When ECM detects this ground, it will send a message to instrument panel cluster over Class II link to illuminate charge light.