Gen 1 Issue?

[JDM1008CUCV]

Hey everyone, I'm sure this has been asked before but I can't find anything on this problem and I've never seen this happen before.
I have a 1986 M1008 with 85,000 miles.
Original wiring, 24v/12v system, always worked great or so I thought.
I used to daily this truck until I let it sit for about a month and a half and the batteries of course were sluggish starting the truck. No big deal, run the truck and let it charge. Well I checked the front battery and it's not charging at all, sitting at 12.0 volts while running.
Took both alternators off to have them rebuilt, the other has a rattle so I figured the bearings need replaced.
Went to the shop to pick them up, he said they're both charging well and good to go.
Put them back on the truck, go to start and no start and see smoke from the batteries.
I ended up finding on the ground post on Gen 1, the ground strap for the block is grounding the whole system. I remove the only the ground strap and it fires right up but Gen 1 light is not on.
All the wiring is correct and the same as before.
Even tried switching alternator to the other sides and the exact same thing happened.
Before the rebuilds this didn't happen, now it has.
I'm also measuring voltages at both small posts on both alternators, is this correct?
Measuring voltage on front battery and it's 12.0 volts and the rear battery is around 14.5 volts.
On Gen 1 small post to ground is around 12.0v
On Gen 2 small post to ground is around 26.6 v

Very stumped and just a beginner of the electrical systems on the things.
Any help is greatly appreciated.
Thank y'all and God Bless.

 

[Barrman]

The alternators have what is called “isolated ground.” Basically there are some fiber washers inside the alternator that make the case not the ground. It reads like 1 of yours got those washers installed during the rebuild while the other didn’t. I think you are going to be back at whoever rebuilt them asking for them to make it right. Make sure you remind them about the isolated ground.

 

[WWRD99]

Hey everyone, I'm sure this has been asked before but I can't find anything on this problem and I've never seen this happen before.
I have a 1986 M1008 with 85,000 miles.
Original wiring, 24v/12v system, always worked great or so I thought.
I used to daily this truck until I let it sit for about a month and a half and the batteries of course were sluggish starting the truck. No big deal, run the truck and let it charge. Well I checked the front battery and it's not charging at all, sitting at 12.0 volts while running.
Took both alternators off to have them rebuilt, the other has a rattle so I figured the bearings need replaced.
Went to the shop to pick them up, he said they're both charging well and good to go.
Put them back on the truck, go to start and no start and see smoke from the batteries.
I ended up finding on the ground post on Gen 1, the ground strap for the block is grounding the whole system. I remove the only the ground strap and it fires right up but Gen 1 light is not on.
All the wiring is correct and the same as before.
Even tried switching alternator to the other sides and the exact same thing happened.
Before the rebuilds this didn't happen, now it has.
I'm also measuring voltages at both small posts on both alternators, is this correct?
Measuring voltage on front battery and it's 12.0 volts and the rear battery is around 14.5 volts.
On Gen 1 small post to ground is around 12.0v
On Gen 2 small post to ground is around 26.6 v

Very stumped and just a beginner of the electrical systems on the things.
Any help is greatly appreciated.
Thank y'all and God Bless.

That stinks!! Unfortunately it does sound like the guys rebuilding them didn't know gen 1 had to be an isolated ground. If they're not familiar with the 24 volt system on these trucks it wouldn't be hard to miss. You should be able to test if it was isolated or not by checking resistance at the ground terminal to the case...it should not have continuity. If it does things burn. You might want to check the fusible links now too. Could be part of the smoke you saw back at the firewall.

Sent from my SM-S906U using Tapatalk

 

[87cr250r]

There is an important ground strap to the block. It's behind the fuel filter housing off the passenger cylinder head. I forgot to reconnect it once. My brake lines were smoking during the glow plug cycle.