Haaaaaaaaaaaaaaay the Hillbilly is here.
I'm the tech the above poster is talkin' about.
Is this truck a diesel or a gasser?
There is a 2 minute delay on a diesel truck after startup before it hits the alternator to turn it on, since you don't want to feed the glow plugs 14v, they don't care for that too much.
2 wires on the alternator at the top, since you didn't post a year or a driveline, I can only guess at this, but - say we're working on a 6.0 diesel, in say...a 2005-ish Super Duty. I believe 03-07 alternator circuits are all the same.
On the top of the alternator, the little 2-wire plug that fires up the alternator field, you should have 12v at the.... holy for the love of god why is my Ford access so slow... Orange/Light Blue wire with the key off. With the key on, you should have 12v at the LG/RD wire on the opposite pin (looking at the face of the connector that goes onto the alternator, the LG/RD should be on the left, and the OG/LB should be on the right, if you're looking at the pin side, the opposite of where the wires come out.)
Like I said originally though, let the truck run for 120 seconds and see if the voltage ramps up after that. Don't see a whole lot of charging system issues other than alternators on Super Duties.