Most likely corrosion in the wiring, like prior posts have suggested. And green copper wire near battery can cause same issue.

Green copper wire (corroded wire) adds resistance to the wiring, which causes the alternator to increase it's voltage to charge the battery. This extra voltage is "dropped" across the corroded wire (electrons flow on the outside of copper wire)

So check ground (from engine to chassis) and cables at battery, should solve your cars electrical issues.

Since you replaced the alternator, I'm wondering if the bearing was bad, or if it overheated when it produced the excess voltage.

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