More info i came across about the alternator not charging
Now if its not charging at all and will not initiate even at high rpm's then of course it will have to come out and be rebuilt or replaced with a new one.
BUT....
If you can rev up the engine and it does start charging there is a fix for this from CAT, they released a service document on this problem that requires moving a wire in the wire harness under the floor plate on the RH side.

Located on the right hand side of the truck, underneath the floor panel, is the 8-pin connector "MTRTWR8". This will be on the main chassis harness "B". It is located just to the right of the transmission. Do not confuse this with the engine harness side of the connection.
On this connector you will see the letter "G". The White 14ga.wire with Red Brown rings in "G" terminal socket will be removed and then reinserted into the empty terminal socket "F". You will then start the engine and idle it only.
Connect a suitable volt meter between the battery terminals Positive (+) and Negative ( -) and check for charging DC voltage. Voltage should be at 14.5 VDC and continue to charge at this rate when accelerating through the high rpm range of the engine.

Something else i learned the other day is that most all charging circuits go through the charge indicator light on the dash believe it or not and if that bulb burns out the truck will not charge, check the bulb and see if it is burned out and replace it if it is.