Why is your 1981 ford F150 starting without an alternator?

A 1981 Ford F-150 can start without the alternator because the alternator only *charges* the battery, it doesn't *start* the engine. The starting power comes from the battery. If the battery has sufficient charge, the truck will start even if the alternator is completely failed or disconnected. Once the engine is running, the alternator would normally begin charging the battery to replenish its power. If the truck starts but then dies quickly, the battery is likely low and needs charging.