Full-field testing the alternator

By applying full battery voltage directly to the field windings in the rotor, it can be determined whether or not the regulator is the cause of an undercharging condition. There are two variations of this procedure that apply to alternators with external regulators. If the field circuit is grounded through the regulator (an A circuit), the regulator is disconnected from the field terminal on the alternator and a jumper is connected between the terminal and a ground. If the alternator receives battery voltage through the regulator (a B circuit), the regulator is disconnected from the field (F) terminal and a jumper is connected to the terminal and to the insulated battery terminal. In either case, the regulator circuit is bypassed completely and full battery voltage is available to the rotor.

Now repeat the procedure outlined for testing the alternator output. With the load applied, observe whether or not the alternator output rises to rated amperage. If it does, the alternator is functioning correctly and the regulator must be replaced. If it does not, the alternator must be further tested to determine the cause of undercharging.

CAUTION: When testing the output of a fullfielded alternator, carefully observe the rise in system voltage. Because the current output is not regulated, battery voltage can quickly rise to an excessive level that is high enough to overheat the batteries, causing electrolyte to spew from the vent holes and possibly damage sensitive electronic components. Do not allow system voltage to rise above 15V.

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