Two problems, alternator/speedometer on 05 6.0
 

Problem #1. My battery light on my dash has been flickering on and off, I just got back from a two hour drive down the interstate and it would flicker on and off every 15 to 20 minutes or so, sometimes up to 5 seconds, My batteries are weak cranking amp wise but not bad yet, could it be the batteries? or is my alternator going south, this current alternator has 49k miles.

Problem #2. This also happened on this two hour drive tonight. My speedometer just died, i was doing about 70 when i hit a large bump and my speedometer immediately dropped to 0, this problem with the speedometer happened this past Wednesday while i was driving in town, on wednesday it started working again after about 2 starts and 25 miles or so.

any help would be appreciated, ill probably go tomorrow to get my alternator tested

Personally I would replace the batteries if you know they are weak. The speedometer may be caused by a bad VSS. Are you losing any other part of the dash when it goes out? Could be just a loose wire, too.

I would say your alternator is bad...hence the weak batteries.

VSS? the speedometer is the only gauge that dies when this happens

If the batteries are failing then replace them NOW. Everytime you start your truck the glowplugs cycle for up to two minutes, and when they're on the current draw can cause the voltage in the FICM to sag, burning out the circuit board and damaging the injectors.

Get the batteries charged and load tested, and if they fail then get new batteries ASAP. The alternator could be bad (we just replaced the one in my dad's 6.0L@61k), but the batteries might be past saving at this point, and failed batteries will cause engine damage to a 6.0.

DC Power is a very popular aftermarket replacement alternator. If you don't do something high-end like this than I'd stay with a FoMoCo/Motorcraft replacement, I've had bad luck with Autozone alts.
https://www.dcpowerinc.com/

Oh i know the dangers of bad alternator/batteries for a 6.0. when the batteries first showed signs of weakness i took the truck in to get them checked and they came back fine... Im gonna go to autozone first and see if they can check the alternator, if not its on to ford i guess

Ok so yesterday i had both batteries tested, one battery tested at 1144 cca and the other 1156 cca. Today when i went to walmart (store i purchased the batteries from, I dont go there for anything else) they read 715 and 776 cca. So walmart says my batteries are good and wont give me new ones, but i know they shouldnt read that high in cca. So im in a dilemma my battery light is still coming off and on moreso than before so i starting to believe my alternator is bad, again.... Might just take it to ford to have them test it because according to autozone it checked out fine.

Oh and my speedometer hasnt acted up anymore

Maybe this link will help you out http://econofix.com/alt.html

thanks for the link, ill test it myself with a volt meter and see how it goes