I discovered something about my 40 yesterday: TL;DR I have part of the wiring harness to repair.
The 40 series have Ammeters in the dash. When the system is healthy they will charge about +20 amps after cranking and after a short drive the ammeter will come back to center when the battery is topped off. Additionally, I have a voltmeter in a accessory power panel and I shared recently that I have been blowing the engine fuse on Sunday drives. Coincidentally, I noticed that the voltage has been very high, as much as 16+ V, regardless of charge (amps), which made me suspect the voltage regulator was giving problems. I ordered a regulator and put the rig in the shop while waited for the goodies from City Racer.
When the regulator got here I installed it and even though I didn't blow a fuse right away I noticed A) volts were still high, and B) the fuse was warm. That's the cool thing about old school Buss glass fuses, you can tell a lot by feeling of them. So, I suspected I had another problem and I pulled some insulation back on the wires going to the alternator. Sure enough...
That ain't cool. I have been getting some heat here - and not just recently. The corrosion leads me to believe that this has been going on a while and finally manifested itself when the wires got hot enough to melt through insulation and eventually started shorting. The upside is that I stripped the main harness back until I found the wires in good shape with no signs of melted/deformed insulation and it looks like the wire damage is contained to the length from the fender to the alternator.
My plan of action: First, gather a few more things, A) I ordered a new engine manual to get MY year specific schematic; the engine manual I inherited with the truck is for a later model (81+) and the wiring is clearly different B) I ordered new alternator connector from Vintage teq Parts because the original has some heat damage:
Second, I am inclined to believe I might have a missing ground causing the high voltage - but to be sure, while I am waiting for better wire schematics, I am going to pull the alternator and take it town and have it bench tested. Once I get all this in hand I will start soldering a new alternator circuit to the engine harness. Big Fun!