sup guys i have a 86 f250 with the 6.9idi.
put 2 new batterys in it, wasnt charging. so i just put a new alt on it and now stays around 12.9v at idle but shoots up to 13ish volts as soon as you hit the gas but wont go any higher. if turn on the lights or anything it wont go below 12v but as soon as i turn the lights off it will go back to 12.9v.
it should be charging more then that, right??
it has a internal reg on it.
i did look at the gas ver. wiring diagram and the only diff i seen compared to my wirng is on the gas wiring the white/black wire comes from the 3 wire reg plug and gos to the 3 wire alt plug.
but on mine the white/black wire comes from the 3 wire reg plug and thats it. the alt. plug doesnt have the hole in the plug for the white/black wire. just 2 wires. could this be my problem?
besides that everything else looks right.

oh forgot i check all this reading at the battery with my multimeter.
the battery guage in the dash doesnt move at all.
and i hooked up a volt gauge today and get the same readings as with the multimeter. when driving with the lights on doesnt go anyhigher then 13v, at idle 12ish.
before i put the new alt on it would drop to 11ish volts when running so i know the alt is chargin somewhat

 

try checking with a multi meter.your gauge could be wrong

 

i must have edit my post as you were typing yours, haha.
read my first edited post. haha

 

o another thing. when i went to autozone to get the new alt. i bought the for for my truck without a/c which i just relized is only a 65 amp alt. the one for a/c is a 100 amp alt. should i have gottin the 100 amp one or is the 65 amp ok??
they both require a pully swap because the pullys on them are for the serpentine belt but my truck takes 2 v belts.

 

Look behind the passenger side battery and check the condition of the connector circled in yellow.



Common for that connector to get corrosion and overheat.
When that happens, you loose power to the cab.

Spliced the yellow wire in mine years ago.

Also the stock wiring is not really big enough to the cab.
So with a volt meter hooked into the cab wiring, when you are running the heater motor or headlights the volt meter always shows a lower voltage.

I did the headlight relay mod that draws power straight from the battery and that eliminated the headlight load from the cab wiring and gave me brighter headlights as a side benefit.

As far as alternator size, bigger is always better.
I went with a 150 amp aftermarket, but I do plow snow which uses a lot of electricity running the blade.

 

thanx for the info.
ill double check the wire plug.
but i did forget to post i took the 65 amp alt back and swapped it out for a 80 amp and it seems to be working alot better.