Alternator Field Coil


The alternator field coil is the magnetic coil inside the alternator that controls its output current.  The more current passed through the field coil, the more current the alternator produces.  Since the coil is resistive, this appears as a voltage across the coil terminals.  The current through the coil is regulated by the power module, which is controlled by the logic module.  The logic module uses its voltage sense input to monitor the charging system voltage and adjust it accordingly.  One side of the coil is connected to the ignition key switch "on" terminal (for 12V) and the other side is switched to ground by the power module's switching control circuit.  The logic module cycles this end of the coil on and off at a fixed frequency and varies the amount of time the terminal is grounded, which is called the duty cycle.  At 0% duty cycle, the terminal is always open and no current is produced by the alternator.  At 100% duty cycle, the terminal is always grounded and the alternator produces maximum output current.  The duty cycle is varied by the logic module, depending on the electrical demand placed on the charging system, and varies the target charging voltage, based on battery temperature.  The greater the demand (more accessories on), the greater the duty cycle.

Fault Codes

The switching control circuit is diagnosed by a special sensor transistor inside the logic module.  If the logic module detects that there is no current through the switching control circuit when the circuit is turned on, a fault code 41 is stored.  There is no "limp-in" mode for this code, but it will eventually result in undercharging or overcharging which will generate a fault code 46 or 47.

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Updated 12/17/2003.

Copyright © 1996-2003 Russ W. Knize.