My question pertains to my Volvo, but is also kind of a general automotive question. It seems to me that several times over the years, when I was working on my or someone else's car, somebody would authoritatively tell me that if a car's battery went dead and it wasn't due to an obvious problem like headlights being left on, that I should jumpstart the car and then disconnect the positive battery terminal with the engine running. Supposedly, if the car then died, it was the alternator that had failed and caused the battery to run down, but if the car kept running, the problem must be elsewhere. I'm pretty sure have told me that this diagnostic technique doesn't work and can even damage the charging system. So, which group of shadetree mechanics was right, and what's the rationale behind the answer?