Battery light on and alternator not charging battery

I am not familiar with your truck, but the charging system looks exactly like a Ford system, even with the same wire colors. I have repaired lots of those!

The yellow/white wire gets battery voltage through fuse 14, a 30-amp under the hood. If that is a spade-type fuse, it will have two test points to take voltage readings on. Those will tell us where we need to look next.

Where are you finding that 1.5 volts? If that is on the light green/red wire, that is normal because the regulator has grounded it to turn on the dash warning light.

Originally you said you had no voltage on the yellow/white wire too, and now on the output wire. Both of those are fed from the 175-amp mega-fuse. A fuse that size is going to bolted into the fuse box. Those nuts are a real good place to find bad connections, so be sure those are clean and tight. Before you touch them, to verify if that is the problem, measure the voltage at every tiny step along the way to see where it drops to 0 volts, (1.5 volts for our story is 0 volts). This means measuring on the tab that is the fuse's terminal, then on the stud or bolt head that terminal is bolted to. If you find twelve volts on one of those points, and 0 volts on the mating point 1/4" away, that is where the bad connection is.

Also, look for fuse number 14, a 30-amp, in the fuse box under the hood. That is fed right from that 175-amp fuse. If that is a spade-type fuse, it will have two test points on top. Check for voltage on those. If you find twelve volts there, the connections at the 175-amp fuse are okay.

If you do have twelve volts on both fuses but not on the output stud on the generator, there has to be a break in that output wire. Most commonly that would occur right at the terminal. The additional clue to a break in the output wire is you would find too much voltage on that stud when the engine was running, but it would not make it back to the battery. You might see 16 to 18 volts at the generator, which is excessively-high, but 12.6 volts or less at the battery, which is unacceptably-low. Both points should always have the same voltage because they are connected directly together with just a fuse in between. If output voltage is indeed too high, that proves the generator is working, and that means the white/yellow wire has to have twelve volts.

By "master fuse", do you mean the huge fuse that's bolted in? If so, follow the fat wire from the generators output terminal to see if it has rubbed through somewhere and is grounding out. Also inspect that terminal to see if it look charred or overheated. If it is burned, the diodes inside the generator are shorting out. It is extremely uncommon for that to be an intermittent problem. Normally when they short, they're done for. There's two groups of three diodes. Any one in each group has to short at the same time to have a dead short that will blow that fuse.