My voltage reading never drops under 13.6V while running, as measured from the lighter outlet with no load. This voltage is only that low for less than a minute after start-up, and typically reads 13.9-14.1V.
A regulator with a different set-point might be slightly different, but under 13V indicates that the alternator is not working.
The usual advice: remove the alternator and measure the brush length. New spec is around 13mm, replace under 5mm. Wear is usually slow and linear unless grit enters the well-sealed brush chamber.
If the brushes are short, and the alternator isn't heavily corroded, I recommend replacing only the voltage regulator. Rebuilt alternators have a significant risk of being bad. There are probably only a handful of bad alternators in the system, but they each go out to a new customer every day and are returned the next.
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