I'd note that these SI alternators are easy to take apart and repair. Hardly anybody messes with them today, since the McParts store replacements are so cheap - possibly indicating the quality of the parts within. You can buy the regulator and replace it. SMP/Standard VR111 might be worth a try -
https://www.rockauto.com/en/moreinfo.ph ... 84&jsn=396
I recall there is a zener diode inside the voltage regulator that is compared with the charging voltage to determine the alternator output current. The higher the output current, the more voltage across the battery due to its internal resistance (plus electrochemical potential), and the higher the charging voltage. Not much you can do to adjust this voltage set point. Likely there is a voltage divider inside the regulator that is compared to the zener, but that's not accessible.
To reduce the charge voltage at the battery, you'd need to increase the voltage that the regulator sees at its sense terminal. There's no reserve of positive voltage to draw from - the charge voltage is the highest voltage present in the vehicle. One could imagine adding an additional supply (switching? a battery?) to provide the extra positive voltage, but that's really out there compared to just making the system work as designed.
I'd note that the Jeep SI alternator with a voltmeter picks the sense voltage right at the alternator. The sense wire wraps back on a very short wire to the big wire to the battery. With an ammeter, the charge wire goes up through the dash and back to the battery (yellow 10 ga wire), where the sense wire connects. In both cases, I believe any resistance along the charge path (times the charge current) will be added to the sense voltage the alternator sees. The zener does not change, but the alternator has to work against the path resistance to bring the voltage up to the set point at the sense wire.
Normally the wire and connector resistance is effectively nothing, and you get whatever voltage the zener calls for. I could imagine some bad connections or burnt wire or something could make that resistance a factor. One way to test this is to take the red voltage sense wire and run a large new clean wire directly from the alternator sense terminal to the positive battery post. You should then measure exactly the same voltage at the alternator sense connection, the sense wire battery connection, and the alternator charge wire connection to rule out the wiring contributing.
D2 is the zener diode.
S!AlternatorInternalSchematic (358 x 576).jpg
Note the inner dotted line is within the regulator. Connection 2 is the sense wire. The diagram shows R2 as adjustable, which would change the set voltage by the voltage divider between R2 and R3. Pretty sure there is no access to this divider on regular production SI regulators.
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