Here is one of my infamously bad videos.
I test the batteries with the engine off, and at the alternators.
Then I test them all with the truck running.
I don't usually unplug the regulator to check from there because I figure the reading at the posts should tell me what I need to know.
Mistakes everywhere, but it's hot out and I got stuff to do.
Your front battery negative may be on the other side of the battery. I flipped mine around from how it originally was because it made more sense and I'm not going to accidentally touch a ratchet to the front battery+ to the body by accident.
I also know I'm supposed to have post covers on those alternators, but I like living dangerously, and I haven't gotten around to it.
In this case I actually have a battery reading low with the truck running, but it's good at the alternator posts, so I probably have a dirty corroded connector someplace....or I need to pull the regulator plug and test per
@Tow4's great instructions, but this should at least get you started in the right direction. I'm not too worried about it right now.
Take off all your jewelry when you’re playing with 24v system, unless you want a nice tattoo of your jewelry on your body, like the guy from Raiders of the Lost Ark that picked up the hot Staff of Ra top.
The most important tip is … don’t die.