Alternator voltage isn't controlled in any way by idle rpm unless it goes so low that the alternator doesn't produce any current at all, and is totally useless as an indication of idle speed. Avoid any mechanic who tells you otherwise! 14.7 volts at idle could indicate a charging problem -- you may need a new voltage regulator/brush set.
Idle speed is 700 out of gear, 500 in, and is controlled electronically. Idle speed variations, rough idle, surging at idle, or stalling when coming to a stop can be caused by a bad or dirty idle valve, or vacuum leaks. Minor variations that don't cause the engine to "thump" or vibrate can be ignored.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!