Inline fuse in place of fusible link @ junction block pops 40amp fuses

Hello. Got this '86 305 truck a couple weeks ago and am sorting through its problems. I've got 2 wires coming off the junction box near the master cylinder, both with home made splices on them. One is a constant power wire coming from the loom behind the engine, hooked to starter I think. The other has the inline fuse holder that's eating big fuses and then splices into two directions, through the bulkhead connector into the cab and into the loom behind the engine to an unknown place. Neither of these are obviously shorted to ground with a test light.
It's kind of peculiar when it pops them, it's not never stopped mid-crank because the fuse is blown, but after the truck is running you go under the hood and the fuse is popped. All cab power is gone then, turn it off and it no cranks (because no ignition power I assume). I know the distributor is keyed power so the truck still running with this fuse popped is kind of a head scratcher to me.
Anyone with more experience able to fill in some blanks for me possibly?
Also is it fair to say it's not anything the cab fuse block protects because those smaller fuses would pop first right? Appreciate the help thanks

First you're going to have to find out where those wires go and what circuit(s) they feed.
Don't 'think' they feed this or that; KNOW.
If necessary, disconnect the battery ground and use an ohmmeter to check the wires.

Easier said than done lol, do you guys climb in the engine bay with a step ladder to reach this stuff?

I did climb in with a ladder, the wire with the fuse holder goes to a rubber boot on the alternator and into the bulkhead connector. It's the wire with my finger on it in the photo

I figured it out, it was the alternator charge wire popping it, the original link must have been able to take more than 40amps

Damn, from the looks of that first picture directly above, I think you need a few more pounds of gunk and grime on the back of that box where all the wiring comes out!!!JM2

Gene King said:

I figured it out, it was the alternator charge wire popping it, the original link must have been able to take more than 40amps

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The original lowest 10SI alternator's output was 37 amps. Upgrade to 61 amps with AC. Also available higher to 87 amps I think in the early years.

Aftermarket replacement alternators are usually all the higher amp version, less part numbers to keep in stock.

Glad to hear you've got it figured out!

You need to replace the fuse with a fusible link or it will keep blowing. Don't ask me how I know this.

when working under the hood of my k5 for any length of time i remove the tires and let it down till the brakes almost touch the ground. much better that way.

Definitely replace all wire nuts with solder and heat shrink or environmental splices(butt connectors with integrated solder/heat shrink). Replace the inline fuse holder with the appropriately sized fusible link.

Gene King said:

Easier said than done lol, do you guys climb in the engine bay with a step ladder to reach this stuff?

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With spinal stenosis, osteoarthritis, and vertebral osteomyelitis I finally overcame my 'just walk it off' mind set - got smart(ish), and purchased a "Top Creeper" - silly name but it permits me to get all the way back to the firewall without weeping.