This is true for chevrolet style alternators, but if the engine cranked, then it had enough juice to get the alternator started. It is most definately your alternator. 100% positive. Unless your engine is idling very low. Your battery can be DEAD, and a ford alternator will pick it up to normal after somebody jumps you off. Most chevrolets will not do that. With a dead battery, a chevy engine will die about 20 seconds after you remove the cables.