GM Service Manual Online
For 1990-2009 cars only

Charging Circuit Operation

The Generator provides voltage to operate the vehicle's electrical system and to charge the Battery. The Generator operates as an alternator to generate Alternating Current (AC) electricity which is then rectified to Direct Current (DC) electricity. The Generator is driven by belt and rotates as the engine rotates. An excitation magnetic field is induced in the Generator's rotor by the Regulator. The Generator's rotation causes the excitation field to move through the stator windings. The motion of the field through the stator conductors induces an alternating magnetic field to be generated in the stator. Diodes in the Rectifier Bridge allow stator current to flow as a result of this alternating magnetic field, but only in the one direction, which charges the battery. The Generator's internal solid state Voltage Regulator regulates the pulse width of the excitation field so as to regulate the voltage generated at the BAT Terminal to be 14 volts, nominally. The excitation field is present in the Generator only when the Ignition Switch is in RUN or START. The Generator does not contain permanent magnetism of sufficient strength to produce a usable excitation magnetic field. The Generator's Turn On Input is present from the BRN (25) wire which connects the Generator to the Instrument Cluster at terminal B16. Voltage is provided to the Instrument Cluster by GAUGES fuse 4 on the PNK (39) wire at terminal B17. The Battery indicator lamp is only illuminated when there is sufficient current flow to cause the lamp filament to emit light. The slight current flow used by the Generator in normal operations is allowed by the resistor which bypasses the indicator lamp Thus, an open filament in the lamp will not prevent generator operations. The Generator controls the Battery indicator lamp in the Instrument Cluster. Internal logic at the Generator closes an internal contact to ground the indicator lamp circuit's BRN (25) wire when the Generator output voltage is either too high or too low. The indicator lamp circuit is only powered when the Ignition Switch is in the RUN and START position.