Ok yall ive had my 02 2500 cummins for 12 years an the other day i was driving an the voltage flat lined and stayed for like 20 mins but when i got to where i was going i shut it off an started it back and it was charging like it should. I was told the regulator is computer controled. So my question is do i need a alt or computer

 

Neither yet, both are expensive parts to throw at an undetermined issue. The FSM states that the needle goes all the way to the left when the system voltage goes below 10.8V and it also should set the 'check gauges' light (although the FSM is somewhat vague in places and describes the light as 'if equipped'). Start by checking all the battery, alternator and ground connections. Also load test the batteries individually (disconnected). You can get the alternator tested at parts stores.