4 Wheel Drive/SUVs: 4 wheel drive, 4 wd, vacuum line

Question
after changing the "box"by the drive shaft and the dash board switch the fwd won't engage any i dea why?

Answer
Micheal,

What box, what kind of vehicle, what year, and what was the original problem?  This is not nearly enough information.
Depending on the type of transfer case there are numerous components that can stop if from engaging into 4-wd.  This can range from the switch, wiring, bad ground, shift motor, vacuum line to a internal components in the transfer case.

You might consider purchasing a repair manual and follow the trouble shooting section for the transfer case.

Sorry for the poor answer.

Carl