The glow plugs are like heaters for each cylinder. When you turn the key on you see the wait to start light which means the glow plugs are (supposed to be) on. After a time the light goes out and you can start the truck because the glowplugs have warmed the air/cylinder enough for the fuel to combust under compression. The glowplugs do NOT go off just because the wait to start light is off. They can stay on for up to 2 minutes after the engine starts. On single alternator trucks the alternator doesn't charge the battery until the glowplugs go off. That basically is the glow plug cycle.