Weak battery and no start

[mistaken1]

The only option I see is to physically trace wires.

There is only one place to get 24V from and that is the positive post of the back battery.

Perhaps a 24V wire and and a 12V have rubbed together until they made conductor to conductor contact and you are back feeding 24V into the rest of the 12V system.

So, what happens if you remove the jumper wire from between the batteries and put the back battery positive wire onto the front battery positive post?

This will let you feed 12V into the 24V part of the wiring, the truck will not start but this test would allow you to user your meter to see if somehow 24V (now 12V in this test) is back-feeding into the normal 12V system.

If there is indeed a back-feed situation this will allow you to take your time and use your meter tracking it down without damaging components during this troubleshooting phase by subjecting them to 12V instead of 24V.

If you make this temporary connection it should remove the feed to the diamond shaped 12V bus on the firewall meaning that if there is no back-feed the only place you should find 12V is on the 24V part of the electrical system.

 

[MrBouncer]

thanks Just started it to see what would happen with the new alts and gen 1 stays on and no gen 2 voltmeter fuse is blown which is what is was doing before I took the alts off

 

[mistaken1]

Fuses open for a reason, mostly to prevent equipment damage and fires.

Are the new alternators isolated ground units?

The drivers side can be case grounded but the passenger side must be isolated ground. Did you check them with ohm meter before connecting them?

A case grounded alternator installed on the passenger side will create a dead short from the front battery positive to the front battery negative via the case ground on the alternator through the grounded engine block. If everything goes well it will open some fusible links rather than become a two alarm blaze.

 

[MrBouncer]

Yeah they are isolated ground I was thinking I could leave things hooked but to do my testing but the headlights burned out before the fuse blew made me nervous

 

[mistaken1]

It does sound like you are getting 24V to places where it should not be.

Use just one battery to feed only the 24V side of things. As long as you do not try to start it 12V won't hurt the 24V items.

First thing check that diamond shaped bus to see if it is getting voltage when that 12V feed is disconnected (key off and key in run).

Trace the positive wire from the battery to the 24V bus and then trace those wires until you find where it is making connection to the 12V side.

Perhaps the fuse block got hot and the plastic melted do the 24V feed in there is making contact with the 12V feed.