OK here is what i got going.
in Park / idling i have 13.8 volts
in park / idling with high beams on 13.78 volts
in park / idling with high beams and MAX AC on 13.59 volts
in park / idling with high beams and MAX AC with vehicle in drive 12.92 volts - And this is when its starts to run the worst.

So is my alternator going out? battery reads 12+ volts when vehicle is not on.

 

you have plenty of voltage. i think you have something else going on. you need to figure out why putting the truck in gear is causing such a load on the engine. i would check for vacuum leaks.

 

What the above says, by the sounds of your readings your alternator and battery are fine. Check elsewhere to find where your loss of power is.

 

Neither.

 

While you checking things, check all of the connections is the charging system including the grounds.

What is your idle rpm ?
What is the rpm when in drive and your get the 12.9 voltage reading ?

 

12.9 volts at idle in gear is actually pretty good. i say this because you are now putting a load on the engine (torque converter trying to turn). you have something else going on

 

Idle rpm and in drive are at 850, but when in drive she runs rough

 

have you checked for vacuum leaks, check and clean your iac valve.

 

New iac, found no leaks..

 

did you clean the maf? how do the spark plugs look? how do the wires look? are the wires arcing?

 

New maf, new plugs, new wires, new throttle pos sensor, PCV valve, NEW COIL PACK.

 

its not throwing any codes?

 

how is your oil level? do you have any way of reading oil pressure? i highly doubt it is any major or internal problem but you never know lol...



give me more info on the truck.

 

Idk about the idle voltage, with my truck at idle in gear with headlights, foglights, highbeams, and blower motor on max my voltage is 14-14.2v. The brushes might be getting worn in the alternator if it has a decent amount of miles, just a thought.

 

No oil issues, 90k miles, no codes at all.

 

With you say it's idling rough I'm going to say it's likely a vacuum leak somewhere. Perhaps do the old carb cleaner routine looking around the engine bay?

 

i still think the most likely cause is going to be a vacuum leak.




and 14.2V when the system is loaded like that is pretty high...

 

Well that was before my upgrades, now its 14.1-14.4v at idle i have a bigger alt and big 3 upgrade to accommodate the my sub setup, and besides at 850 rpm the alt should keep the voltage higher like around 14v, my stock alt with 165k mi kept the voltage at 14v give or take .2v.
By the sound of it the OP might have two different issues and we need more info like year and motor and 2wd or 4wd, because if he has a 4wd setup with pvh like my truck there are many points for vacuum leaks.

 

a higher amp alternator does not create higher voltage too. your voltages are definitely on the high side. safe working voltage is normally no higher than 13.8V

 

Not saying anyone is wrong but from my observations i have checked many cars including rangers, s10s, silverados, g6's, Volkswagens, and f150's and all read on the DMM right around14.2v
But back to the op, we ideally need some more info

 

2003 xlt 4x4 ... Looked for and never found any leaks

 

Does your truck have the 3.0 or 4.0?

 

how are you looking for leaks?