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....

 

While you most likly have a alternator failing, I had similar trouble in the past the was simply a loose connection on the alternator. I'd check those conections. I'd make sure they test it before you replace it. Most parts stores have a test bench for alt/starters.

 

Just seems wierd that it would charge at low RPMs and not at high RPMs - probably brushes but I was hoping it was a common thing which would save me some time...