This could be due to the alternator they sold you is not equipped to match your vehicle. There are many options including ones with internal regulators and other with clockwise rotation.
I think the best thing to do is hook up a meter to your battery when the vehicle is running. We need to see what the voltage is when it is running at idle and then at 2,000 RPM's.
Let me know what you find and we can go from there.
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Friday, November 1st, 2019 AT 5:21 PM