Batteries not recieving voltage from Alternator

Start where u ended.. you replaced the starter. Your cab grounds run to a lug behind the starter that grounds the cab to the engine block.. if you unhooked that to get at the starter and forgot to hook it back up the cab is not grounded correctly and thats why the voltmeter gauge is reading off... You will also see a voltage drop if u turn on all your accessories like a/c, lights, etc you can confirm this by taking a voltage reading off the main power cable that goes to the cab, check voltage from that wire and ground to the frame then find where it comes into the cab and take a voltage reading when grounded in the cab.. if its lower inside the cab then the cab is not grounded correctly and should check if u loosened that lug by the starter or maybe it is correded just enough that it broke off if u accidently hit it while trying to mangle the starter out of there.. i chased a similar problem like this in a 389 with a cummins and tore my hair out trying to figure it out...

 

Did this problem begin after replacing the starter? If so then connections are likely wrong. Check the voltage at the alternator by placing the red + lead of the meter on the B+ post, the one with the cable that goes to the batteries. Place the black lead on a ground such as the block or alternator mount. See what you get there. On some vehicles the alternator is controlled by the ECM/computer.

 

Look for the 8 gauge wire that never got hooked back up to the positive post of the starter.

 

So are your batteries not acutally charging , or are you just taking that reading off the voltage meter on the dash? The newer trucks all those gauges get their signal from the computers.

Take a voltage at your batteries themselves with engine running and report back what it is and at the alternator as well... Maybe we can page @Goodysnap @Heavyd they might be able to give u some better ideas

 

Use something like this
https://www.amazon.com/TESMEN-Multi...8-4-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&th=1
and test the voltage on the large terminal on the alternator with the engine off and on.
You can then check continuity from the alternator and starter battery lug.
A multimeter is an inexpensive tool that should be in every truckers "toolbox" but not stored with the wrenches.