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Well my 86 diesel ain't charging the gauge reads a little below 12 volts the other day it was charging fine and started goin down the road then saw my headlights turn dim, it's been driving me nuts my truck will not charge at all drivin at night and battery's eventually get drained to 8 volts yikes I've eliminated all fusible links and I tested all connections going to alternator and they are all good getting voltage the first thing I tried a couple weeks ago was replacing the alternator and it was doing same thing with the new one, I know that my battery's drain themselves overnight when left hooked up, before it started this charge problem I did run the battery's low starting it with the fuel problem and burned the 0 gauge battery cable off of the starter is it possible that the solonoid burned up and is causing the alternator to not kick on or what, when its running you can tell that the alternator is under a serious load cause the belt is really tight on one side and kinda loose on other so I think it's trying to charge but something is not letting it any help would be awesome

DO you still have the dual alternator 24V setup? If so, there isn't much I can help you with specifically about that, as it's not something I've actually dealt with.

If the trucks been converted to 12V at some point, then what kind of alternator are you using? is it the older 12SI alternators with the 2-prong spade plug or a 4 prong GM weatherpack connector on it?

One thing I can relate to, is that I had the old giant direct drive (DD) starter on my truck, which was very powerful and fast when it was warm, but when it was cold it could barely spin the engine over, but would drain 2 brand new batteries down to nothing within 10 seconds...which was no good in -40 weather LOL I replaced it with the newer style gear reduction starter from the 90's 6.5 diesels and it had no problems cranking the engine and would not drain my batteries in seconds either.

Once I had new batteries, new starter and new bigger cabled, I could crank the engine over easily and it would fire yo sio much easier too. It might be worth checking into.

This is non military 12 bolt set up biggest 108 amp alt factory 2 prong regulator prongs

I did put a 4 gauge positive to starter cable on it cause the old 0 gauge or however thick it is burnt off that day I was trying to get her going like I said if you leave it sit over night it's either shorted out or something cause there is barely any juice, I didn't no if something probably shorted out in the starter solonoid the day I did that or what but ever since then I've been having charging problems old alt checked out good at autozone, and this new one didn't fix the problem however it's weird how the one day I was running it it decided to charge and then quite within 5 minutes goin down the road

What kind of alternator are you using? Is it the newer style CS130 105a alternators or the old style 12Si alternator?

The CS130 needs some form of resistance of an idiot light in the B circuit (IIRC) to begin charging, otherwise it will look fine but no charge going to the batteries. I learned this the hard way when I did a 12si to CS130 swap with my old 6.2 last year.

Look for either a 2-pin connector (12si) or 4 pin style weatherpack (CS130) on the alternator case.

Mines a 2 pin regulator are you saying it could be the gauge cause I've tested the charging with a voltmeter and its not charging but probably 12.3 volts

Have you had your batteries load tested?

These diesels are hard on starters and batteries (especially if either is bad). I know this because I ended up going through 3 sets of expensive batteries trying to figure out all the starting issues. It's mostly because I was running it in arctic winter temperatures and took a while to get it perfect.

I've tried reviving a bunch of batteries with my good charger, but not very often do an old battery hold a strong charge like a new one in good condition. Even with a good alternator it might not be able to keep the batteries going.

If you replace your alternator, I'd switch over to the CS130 style. They were the standard 105a alternator from most of the late 80's to 2000's GM vehicles. They are a cheap upgrade to the old 12Si style in every way. They are more reliable, have much better idle output, a 105a 12si style alternator wouldn't last for very long in a stressful environment, because I went through multiple of these as well.

One CS130 swap and it was a much stronger system overall. But I'm also running welding cable for all my battery cables, which are fairly large, similar to 1/0 IIRC, I upgraded pretty much every part of the starting/charging system so it would start and run for the apocalypse...but didn't get to the beefing up the transmission in time.