In addition to checking the voltage output of the alt, you should check to see if you're getting a drain from the alt when the car is off. Bad diodes in the alt can drain a battery.

With the car off, you can use an ammeter between a battery post and the corresponding cable (while the other cable is connected, of course) to check for drain. More than 20 to 50 mA is a definite drain (although on your older car, you may not even have 20 mA normally like I do with my 93).

With my bad diode, though, I found that I could just put my voltmeter between the battery posts and watch the voltage drop quickly over time with the car off. That stopped when I disconnected the alternator.
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1993 945 NA