Sudden catastrophic power failure at 13 months 20,000 miles.

[ShadowsPapa]

Here' my hypothesis on what happened.
Since there were no warning lights leading up to the initial failure, the CEL for the charging system must have been caused by the failure. The negative battery cables may have been bumped and come loose from the IBS during the dealer maintenance, preventing the main battery from charging and leading to the failure of the aux battery over the next few days.

I wouldn't be surprised if the dealer techs accidentally shorted the positive battery cable while working on the batteries and blew the N3 fuse, further complicating their troubleshooting. Blaming the IBS could be their way of recovering some cost and obfuscating their incompetence.

As others have stated, the IBS has nothing to do with the aux battery. Here's my diagram that shows how the aux battery current does not interact with the IBS.

Nailed it. The aux battery ground is pretty much direct to the body ground. The IBS is simply a terminal block, a junction point, where the aux battery cable and chassis ground cable meet. The IBS is basically just a point where they meet. (technically, the IBS isn't even the junction, it's only holding the connection - the connection is the other, larger nut)

I hate it when shops "make something up" to explain things away - knowing that most people will accept it because it sounds feasible, and they walk away, satisfied they got at least some explanation.
How many people really know what the IBS is or does, or does NOT do? 99.9994% of all owners of vehicles that have one, and it could be even closer to 100% because they are used in millions of vehicles around the globe.

I thought dealer tech incompetence right away. Followed by a cover up story.

For the initial problem to occur, who knows. If we could only have been there. Maybe there were other signs of an eminent failure that the OP did not catch.

Many ways "stuff could have happened" like Jerry suggested.
If that top connection that joins the two ground cables to the top of the IBS was loose, definitely could run the main battery down - further, heat could have actually damaged the IBS. Resistance, heat, arcing, whatever.
Doesn't matter really at this point - they made up a story, someone screwed up.
If you can't lay out the facts and explain correct what really happened then you screwed up, or you don't know what the @#$% you are doing which is just as bad, and is a screwup in itself.

It really "gets me" how people who work on these in a shop that's trusted, and they are supposed to know what they are doing, know so very little about a system that in reality isn't all that complex.
I wonder how many of them spent even 1 quarter, let alone a semester, in college in automotive electric systems - and yet they are working on them.

Like the man said - aux battery negative cable clamps directly to the chassis/body ground cable. If that small nut (on the right in this pic) is loose, you have problems.

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[jebiruph]

Here' my hypothesis on what happened.
Since there were no warning lights leading up to the initial failure, the CEL for the charging system must have been caused by the failure. The negative battery cables may have been bumped and come loose from the IBS during the dealer maintenance, preventing the main battery from charging and leading to the failure of the aux battery over the next few days.

I wouldn't be surprised if the dealer techs accidentally shorted the positive battery cable while working on the batteries and blew the N3 fuse, further complicating their troubleshooting. Blaming the IBS could be their way of recovering some cost and obfuscating their incompetence.

As others have stated, the IBS has nothing to do with the aux battery. Here's my diagram that shows how the aux battery current does not interact with the IBS.

Upon further review, I may need a new theory. As long as the alternator is working, a discharged main battery should not cause a power failure while the engine is running. Either the alternator failed first and the charging CEL was not noticed or the N3 fuse was already blown and the aux battery ran down causing the power failure.

I like the N3 theory, that fuse blowing seems to confound a lot of mechanics. Could they have mistakenly thought the fuse array was an IBS for the aux battery? @BearFootSam do you have a part number for what was replaced?

 

[ShadowsPapa]

As long as the alternator is working, a discharged main battery should not cause a power failure while the engine is running.

Yeah, didn't consider that.
And I should have since the battery symbol was lit - and that's a charging system malfunction/issue.
That's a wide open thing in a way because I'm not sure how or what they determine to be "charging system issues".

From FCA IBS documentation:

if the alternator is working and there's an intact path to both batteries (through the high current fuse) then both would receive charging current.
A blown N7 in the high current fuse array would mean no charging current to either battery, and nothing from the alternator to the bus. So the truck would be running on battery only.

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