F-150 alternator problems

try a voltage meter,the altenator should be charging around 14.4 volts,if its not your altenator is on its way out. or disconect your baterrie when its running,if the altenetor's done your truck will stall.

Start the truck then unhook the pos. Cable if it dies it is the alternator

I clean the battery real good then I start the car and it runs.but before I turn the key on nothing else was working. So is it the battery or the alternator.