Alternator or battery???

WHY are you doing all this?

Because with only a test lamp, clip leads, and a meter, you can test the entire system right on the car, and if done right, you will KNOW what the trouble IS

The reason these tests are as written is because you need to eliminate different "sections" of the problem

Is it a bad connection in the charge wire?

Is it a bad alternator?

Is it getting field current from the blue?

Is the VR wiring OK?

Is the VR grounded?

Is the VR bad?

ABOUT those parts store "testers?"

RARELY is a tester able to tell if a component is "good"

ONLY if the tester is "a correct tester," is set up correctly, is operated correctly, AND BY an honest person, it will tell you "if it's bad"

Very few if any parts store testers can fully load test a "big" alternator. It takes a BIG electric motor to "pull" a 100A alternator to full output. I built a tester once, we had an unused 15 hp 3 phase motor. Some of the big truck units would dog it down some