I have a 2002 F-350 4x4, single rear tires with the 7.2 L Diesel. I recently had the battery light start coming on when my RPM's went above about 2300, so I tested the battery and alternator and determined I needed a new alternator. Replaced the alternator along with the serpentine belt and all the pulley's.
PROBLEM SOLVED. For 2 weeks. Now, if driving at LOW RPM'S (below say 1800 - which is about where I'm at when rolling along at 45 MPH), the battery light is coming on and the lights go dim, even had all the gauges shoot to the top and then to the bottom a couple times this morning.
What the heck is wrong now???

 

I guess no one has any answers for this? It is intermittent.

 

How old are your current batteries. How many batteries?

Between the battery and the alternator, sometimes the failure of one lead to the failure of the other. For instance, if a battery is dying, it might require so much charging that it blows the alternator, etc.

I'd take the battery(ies) in for a load test, bnecause they are incapable of providing adequate current at idle....

 

Wacky electrical behavior like you describe with the gages is quite often bad battery(ies). I had a car once that the battery went bad on (fairly new battery - then got it replaced under warranty). The draw from the brake lights from stepping on the brakes caused the radio to switch on and off and reset the auto climate ctrl. For example. I've seen similar types issues other times too and it was always the battery.