Idle on 5.3L Silverado

the voltage should remain at 14.5 volts at all engine speeds above idle 13.9 volts at idle indicates that the alternator is charging but not at set voltage
head lights are the biggest drain that affects charge rate rate and it it maintains and from you description that is not a problem
when you select a gear it indicates a heavy drain which is a high resistance or a dead short in that circuit that is taking out more power than the alternator is charging
There there is nothing in the gear selection that draws such power
the other thing is that when selection a gear the engine rpm is tool low and the alternator stops charging
for auto vehicles , the idle rpm should be at 800-900 rpm dropping to 750-800 in drive

Engines rarely idle at the same rpm all the time. Its normal for changes to occur, especially small ones. Too many variables that can effect such a small change to list. Its no an issue to worry about.

ICM Idle control module it's problemly sticking or going bad

check all vacuum lines very closely for a vacuum leak. might be a small hairline crack that is hard to see.

which car is it?

problem with 1993 Geo Prizm '99 silverado 5.3 engin will not idle runs at 2500 rpm and drops down to 1000 rpm the back up to 2500 rpm -------------------------

The engine does not need to be raced when cold

The vehicles computer will adjust the idle according
to how the manufacture programmed it

After about one mine or so, the idle on most vehicles
comes down to approx 750RPM, generically speaking

When you put the car in gear,assuming it is an automatic,
the rpm will drop a 100 at most & may be adjusted by the
computer, just as it does when you put the A/C on. That is
to compensate for the change in engine load

You should be waiting for the rpm to come down to 650 to
800 & then a couple more minutes for some temp to come
up, before driving off. In cold weather wait at least 5 min or more

I think your vehicle is not working correctly, or you not explaining
it in a way I understand what answer you want

my 98 chevy 1500 with 5.7 and 373 gears idles at 600 rpm, and at 55mph its running 1500 rpm on the dot

Yes, sounds like the idle speed is too high. If you keep driving like that, you will fry the torque converter / turbine. Since idle is computer controlled, you will have to take it for service tune up. There is one possibility that the rich, high rpm idle used for cold start is not disengaging properly after the engine warms up. Air conditioning will raise the idle speed by a couple hundred rpm, so avoid using it until you have the problem fixed. BTW.....warm engine rpm in idle, and neutral should be around 700-800. Anything over 900-1100 will result in a hard shift into 1st gear and dragging at stop lights.

The problem is in the idle air control motor. It is mounted on the throttle body with two bolts, you can pull it off and make sure it is not caked with carbon also check where it mounts on the throttle body and make sure it is not caked with carbon.

That is normal. When you put it in gear, the idle speed drops about 400 rpm.