Speedbird
I'm not sure what you mean by "just NAV engaged". Nor am I familiar with the 747 FMC/autopilot setup, however I expect it's similar to that on the 757/767.
The autopilot does not have a mode, which tracks navaids without FMC 'prompting'. Its lateral modes are such that you are able to either select a heading, hold a heading or follow the FMC (called LNAV - Lateral Navigation).
The FMC uses any known navaids within range to constantly update its known position. Even if you manually restricted it to a particular VOR as you approached it, it still would not slavishly follow the signals, but would merely use them as position confirmation.
I suspect you have been misled by MS Flightsim, which has a realistic-looking autopilot panel, which functions very differently from the real thing.