Low voltage

Ive replaced battery and alternator, but underload w no throttle my volts drop significantly. When I'm on throttle the volts go back up. Does anyone know why this is happening?

 

COuld be a wiring issue. Have you checked actual voltage with a multimeter?

 

Yeah volts drop to 12.5

 

So you are saying at idle youre showing 12.5volts, but you rev up you go up to 13.5-14v?

 

Yes

 

replacement alternator could be bad, especially if if was a cheap replacement. Ive had bad rebuilt starters and alternators that were brand new out of the box. Same goes for the battery, I have had new ones with bad cells. Load test will confirm that.
Battery cables or grounds could be bad or corroded as well which will reduce volts.

Are you having start or run issues? Are you measuring volts from back of alternator or on battery? When truck is off, are battery volts within spec?