Powerstroke charging problem

[Alta]

I bought the truck new, it has 170K on it and yesterday the battery light started comming on and at the same time the dash alt gauge drops to about the bottow 1/4 of the guage (10 volts?). this only happens above 2000 rpm. When I let it return to idle, the light goes off and the guage returns to normal - about 2/3 up the scale (13 - 14v?). Around town it operates normally but if I get on the freeway, get it up over 2000 rpm (60 mph in 5th) it happens again. I dont know if this thinkg has an external regulator, and Im assuming its an alternator problem, but dont like swapping parts to try and figger out a problem. the belt and tensioner are new, batteries are good. Since its electronically controlled, I figger the thing will leave me stranded if i let this go too long. normally I wait til something fails to get all the miilage out of it i can, but I will probably make an exception in this case. Any guidence would be appreciated - Im figgering this is not the first time this has happened on a powerstroke charging system and Im as curious as anything else. Thanks.

Also last night 5 pannels blew off my shop roof so I have enough to deal with without getting stranded. The fun never stops....

[Franklin2]

There have been several instances on here where guys have had rpm related charging problems, and it has been the alternator that is bad.

If you get on the interstate at night and drive for awhile, you probably will have a problem.

[cangim]

The alternator has an internal regulator, I agree your alternator is going out. If I was replacing mine, I would go for a rebuilt one with a little more amperage capability.

[Alta]

I tend to agree. Also my son's input is that the rotor is out of round (maybe got hot?)and the brush springs are too weak to keep the brushes in contact at higher speeds but are up to the task at lower speeds. And there is a shop that does this kinda work next door to the alignment shop Im takning my bronco to tomorrow morning (just finished installing a 6" lift and a detroit tru-trac) when I get off work so I'll take in the alternator too and have it rebuilt and beefed up a bit. thanks for the info