What would cause the 12gauge wire at the Junction box to ground?
 

I moved about 100 miles 3 months ago, and left my 1967 C10 in storage at the old location. Building a new shop at the new location.
Drove up there this week to start and drive the truck, which I do every month.
Dropped the battery cable onto the + post of the battery and went to start the motor. Noticed a lot of smoke coming from the area around the top of radiator before even turning the key. Jumped out of the cab and removed the battery cable.
Most of the 12 gauge red wire (12R in the diagram) going from the junction box to the main wire harness near the regulator was completely melted. Since I didn't have a tool kit or meter with me, it was kinda hard to diagnose. I'll be be going back this week with some tools to have a look.
The truck was just fine one month ago. The storage facility is at my old place. Totally sealed from the elements and from rodents.
So what would cause the main hot wire to ground? Can the alternator do this while sitting? The battery was disconnected while I was gone, so I consider myself lucky.
Ammeter? Ignition switch? Need some ideas here. Thanks.

Re: What would cause the 12gauge wire at the Junction box to ground?
 

Rodents maybe! Look for signs of them eating and crap around the truck. They love the glove box and heater area.
My guess anyway. The short wouldn't happen on it's own with out the wire rubbing on metal. In other words it didn't happen by sitting there.:chevy:

Re: What would cause the 12gauge wire at the Junction box to ground?
 

No rodents, as stated in the original post. The garage is sealed up tight.
You are correct..... things generally don't just change while sitting idle.....but as you know, stranger things happen.
I will be able to test the alternator with a meter when I go to the garage next week.
Anyone else have any theories?

Re: What would cause the 12gauge wire at the Junction box to ground?
 

SMOKE!!! Melted insulation!! That's a hard short that shouldn't be hard to see. Unwrap that area of the harness and take a look..

If that reveals nothing,my first guess (I hate guessing) is the voltage regulator contacts are stuck - shorting that red wire to ground.. Disconnect the Voltage regulator and, using a test light connected between a battery cable terminal and battery post, check again for the short..

Re: What would cause the 12gauge wire at the Junction box to ground?
 

I wonder why the fusible link didn’t melt before that 12g wire? Be sure to add one of the correct gauge if it is missing or replace it when you find the short and rewire.

Re: What would cause the 12gauge wire at the Junction box to ground?
 

Fusible link either bypassed with 8 gauge wire or simply doesnt exist....seen that many times...

In my experience, it usually melts red wire due to Voltage regulator contacts sticking closed....

:chevy::chevy::chevy:

Re: What would cause the 12gauge wire at the Junction box to ground?
 

Had a look yesterday.
First thing I noticed is the battery has REVERSED POLARITY. Yes that is right. With everything disconnected, my meter was showing -12.28 volts. Gotta say in my many years of fooling with vintage cars and trucks, I have never seen this before.
Alternator has no internal ground, so it may be OK.

Only explanation I can think of is that the battery was disconnected with a battery tender hooked up. Perhaps the battery tender reversed itself, if that is possible.

I brought the battery, alternator, and regulator back to my place. One thing I did notice is that the harness is fried all the way to the alternator, which I hadn't seen before. So I am going to need to fabricate a harness before putting a new battery back in and testing everything.

Re: What would cause the 12gauge wire at the Junction box to ground?
 

Suspect the issue will be the alternator itself...

if the battery was connected in reverse, it could have damaged the alternator internals...

Also replace the fusible link as it obviously did not do its job...

:chevy::chevy::chevy:

Re: What would cause the 12gauge wire at the Junction box to ground?
 

The fusible link is a special wire found in the electric section of the car parts store. It is usually in a blister pack hanging on a peg, and is a few inches of grey wire. 6 to 9 inches of 16 gauge fusible link wire is what you use between the positive battery terminal wire and the junction block, that will burn up and save your main wiring harness if you get a short. It should be 2 wire sizes down from the wire you are protecting, so 16 gauge fusible link protects a 12 gauge main wire. Electric wire comes in even sizes, such as 18, 16, 14, 12, 10, etc. The diagram you posted shows 18 gauge fusible link protecting a 12 gauge main wire, which I think is incorrect.