Fusible link diagnostics:
It helps to have a DC clamp ammeter like this: https://www.amazon.com/KAIWEETS-Multimeter-Auto-ranging-Temperature-Capacitance/dp/B07Z398YWF
Test the voltage at the battery with the car off. Record the result.
Test the voltage at the battery with the car running. Record the result. Note: If it is above 15V at idle - there is your problem.
Test the current through the fusible link with the key in ON but car not running. Record the result. Note: If the meter shows a negative value, remove the clamp, face the meter in the opposite direction, and put it back over the fusible link.
Leaving the ammeter around the fusible link, test the current through the fusible link with the car running. Record the result. Note: The polarity of the current should change.
From this we should get a better idea of whether or not @siteunseen is putting you on the right path. If I have time, I'll make a short video to demonstrate.
Setting idle speed: You can buy a multimeter like this: https://www.amazon.com/gp/product/B0002LZU7K or a fancy timing light like this: https://www.amazon.com/dp/B000EVU8J8 (Yes, I have both.) You can even find a handheld tachometer, but I haven't used one myself.