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More follow up - I took the bad fuse box to an electronics repair hole-in-the-wall in town and my only instructions were "it's garbage unless you find an obvious way to fix it." I picked it up today and they said they found cracks at a bunch of solders and re-soldered them. I put it in and fired right up! Just gave it a half hour test drive around town and all seems to be well and good. Again, just wild how a few cracked solders can totally kill a hulking beast of an off roader.
So now I have a spare cabin fuse box and would probably encourage a lot of you to do the same.
On the PCBs in my trucks, there have been failures at solder joints. I noticed the points of failure all had very thin solders, barely tenting up on the wires, and usually higher current circuits. So, while vibration might play a role, it seems the root cause is a manufacturing error. The thin solder gets heated up by high current, then melts/cracks, perhaps made worse by vibration. During repairs, be on the lookout for very thin and weak solders joints, in addition to the little black melted circles.
Little end of summer update - After driving it for about 5000 miles, the slightest of symptoms started to creep back - The situation where it's driven, sits while we run into the grocery store, and then when we come back it runs a little rough. Not anything crazy, wife didn't notice anything, but of course I did. Very subtle. Anyway, I figured that perhaps the solder repair on the cabin fuse box maybe started wearing out. I took it out, installed a replacement I had on hand, and as I was plugging connectors back in noticed corrosion on the terminal plug that was associated with the terminal that had all that nasty corrosion on it originally. I don't know why I didn't think to clean up that side of the corrosion issue, but obviously that needs to be dealt with just like the pins on the fuse box were. So to fix, this is what I did -
Disconnect battery (should already be if you're swapping out the fuse box).
1. In one glass jar I made an imprecise solution of vinegar and salt, let the connector sit in the solution for about 20 minutes. Used a small toothbrush to gently scrub inside the connectors. Rinse the connectors, let it sit a little longer, brush, repeat.
2. After a couple cycles of this I used another container and made a solution of baking soda and filtered water and rinsed the connector out a bunch.
3. Let it all air dry over night.
That took care of a bunch of the corrosion. Reconnected everything and it's been smooth sailing for about 500 miles so far over the last couple weeks.
BTW - highly HIGHLY recommend getting Dioxit D5 spray contact cleaner. I spray it on every electrical connector when I remove / replace / clean. It’s brilliant. It dissolves anything on there.
You must let it dry before any current passes thru because the stuff I think conducts electricity -- btw and it takes a minute to dry.
I have an older M-B too and those guys recommend it and it’s basically the only reason why the ECU’s and electrical systems on 90’s M-B’s work at this point.
