Voltage regulator
Most likely resistance in the system wiring.
Measuring the voltage at key locations under the same conditions will narrow down where to look for the resistance.
If its at 15.3 V only while the battery is charging, that's a clue too.
If it goes back to 15.3 with the battery charged but when you turn the lights on, that's a clue too.
The locations to measure voltage will be battery positive terminal, alternator output terminal, and then some or all of the following:
backprobe the 'ignition run' terminal at the bulkhead connector, the engine connector, ballast resistor, alternator field terminal.
Why? If the voltage regulator sense terminal is seeing low voltage, it will drive the alternator to produce at higher voltage. The closest we can get to measuring the voltage at the regulator's sense terminal is to measure at other locations in the ignition run circuit (coded J2 on the diagrams).
Ignition circuit at bulkhead
It is a little high.
Most likely resistance in the system wiring.
Measuring the voltage at key locations under the same conditions will narrow down where to look for the resistance.
If its at 15.3 V only while the battery is charging, that's a clue too.
If it goes back to 15.3 with the battery charged but when you turn the lights on, that's a clue too.
The locations to measure voltage will be battery positive terminal, alternator output terminal, and then some or all of the following:
backprobe the 'ignition run' terminal at the bulkhead connector, the engine connector, ballast resistor, alternator field terminal.
Why? If the voltage regulator sense terminal is seeing low voltage, it will drive the alternator to produce at higher voltage. The closest we can get to measuring the voltage at the regulator's sense terminal is to measure at other locations in the ignition run circuit (coded J2 on the diagrams).
Ignition circuit at bulkhead