Welcome to TRS
Yes 14.3volts is OK just after startup
But you also need to test battery voltage before starting engine
12.8v is newer battery
12.5v is 3 year old battery
12.3v 5/6 years old and time to shop for battery sale
12.2v or under is a dead/dying battery, you will get the dreaded "click, click, click" on cold morning
2004 Ranger uses Smart Charging, so an extra layer of "things that can go wrong", lol
In cab fuse box there are 2 fuses you need to pull out and check, MUST pull them out, it cleans the contacts when you pull out and put back in a fuse.
Fuse 9, 5amp, Battery Light circuit(on/off switch for charging)
Fuse 10, 10amp, Smart Charger circuit
Charging systems are very easy to test with a DC volt meter
Key off
Ground(black probe) meter to Battery Negative
Put Red probe of meter on B+ terminal on alternator, it is the larger terminal on back of alternator that uses a stud and nut
Should read Battery Voltage, as measured above, and match it exactly, well within .1
Now leave red probe on B+ and move black probe to alternators case, should see same battery voltage, if not then vehicle Ground wire is loose or corroded.
Unplug the 3 wire connector on alternator, that's the voltage regulator wires
With black probe on alternators case put red probe on Yellow/white stripe wires slot in connector, it should read battery voltage same as B+
Now put red probe in slot with Light Green wire/red stripe wire, with black probe still on alternators case, should be 0 volts
Turn on the key, engine off
Test Green wire again, it should now have battery voltage, this is the ON/OFF voltage for alternator
Check the short loop White/black stripe wire, make sure it is plugged into alternator correctly, unplug it and check, and make sure wires are not frayed, making it a marginal connection
If all this checks out as OK then replace alternator, it is faulty, you can just replace voltage regulator, but that may or may not be the only problem.
The Battery is ONLY USED to start the engine, it is max. 13volts(brand new battery), and once engine is started the alternator produces a minimum of 13.5volts.
So Battery voltage is not used at all, too low, it is kept charged by the slight higher voltage from alternator.
A battery does absorb spikes in amps and voltage while engine is running, but alternator powers ALL the systems in the vehicle once it is active.
An alternator can not start to work unless it has 12volts FIRST, it must have "startup voltage" in order to produce voltage, and that's where the Battery Light circuit comes in.
Battery Light
How does a light bulb work?
If you give a light bulb 12volts does it light up?
No, it must have 12v and a Ground(0 volt)
If you give a light bulb 12v and 12v what happens, nothing, no current is flowing
A light bulbs filament glows because it is heated up by current flowing thru it, so it must have 2 different voltages to light up, like 12v and 0v, current flows between two different voltages.
When you turn on the key fuse #9 sends 12v to Battery Light bulb in dash, current flows thru this bulb to the alternator(light green wire), since alternator is OFF it is 0volts, so Battery Light is ON, and this 12v is used as startup voltage for alternator.
When you start the engine the "working" alternator starts to produce voltage, say 14volts, so now fuse #9 is 14v(all electrics in truck are now 14v) and alternator(green wire) is 14v, so Battery Light bulb has 14v and 14v, no current flowing so Battery Light is OFF
If Battery Light comes on when engine is running, or even Flickers, that means alternator voltage(green wire) is less than Fuse #9 voltage, so 12v(battery) and say 5v(broken alternator), current is flowing and bulb lights up