In a generic sense the things that will melt the insulation are environmental heat (ie engine bay) or overcurrent. Over voltage alone should not necessarily cause over current. I would be inclined to install a new wire with a fusible link for safety and observe the results.

 

Its a guess, but someone could've installed the battery backwards.

 

Look for shorts in the primary charging system. My alternator became loose, and the main charging wire began shorting. It was a dynator, and the mount bolt became loose. The dynator was not the one from England, but an eBay knockoff from China the folks in great Britain told me. I fixed things.
Ideally there would be a big amperage fuse in the system to prevent a hot shorted wire before it melts the system There is a big heavy gauge wire from the battery to the charger, then a smaller but still heavy gauge wire to the fuse box from the starter on my 67 Alfa. The battery positive wire did not short, but the heavier one directly to the fuse box from the starter needed some liquid tape on inspection. Good for now but will need addressing at some point for prettiness.
The dynator keeps the original look of a generator but is an alternator inside. They are expensive and I thought the eBay version would be an equivalent and less expensive. I ended up removing the mount of a generator and tapping some holes in my dynator. I made it work well, but am wiser now. It was no money savings in the long run but that is life.
Look somewheres around there for your short first, in the primary charging system.

 

Voltage does not generate heat. Grab a plug wire firing at 20,000 volts and see how hot it is. Current generates heat. The damage is either environmental or a short. Probably someone left the cable dangling dangling from changing the alternator or starter without disconnecting the battery.

Defective voltage regulator will not cause that damage.

 

Thanks for the replies.. I will dig in more this weekend, but I will for sure put a fuse in the main wire I am replacing.

 

I would start by changing out the alternator for a known good used or new one.

 

Many auto shops will test your alternator for free if you bring it in. Before that, though, do a quick continuity check with a digital voltmeter from the alternator + stud to ground, see if it's shorted internally or something.

 

Grab a plug wire firing at 20,000 volts and see how hot it is

No, don't do that.

 

Also, never open the hood, pump gas, inflate tires, change bulbs or drive on public roads.

 

If your main grounding wire has disconnected then it can divert all the eathing thru a thinner wire which is a large current when say cranking the starter., check engine earths.

 

While faulty earths can cause an array of headaches, it would not have caused this issue in particular. If a major earth strap is missing, the engine would either still crank, crank slow, or not crank at all. Reason being is there are still other pathways for electricity to flow through the chassis and thus the starter could possibly still crank. But not melt the wire from the alternator to the starter, which starter current wouldn't flow through anyway during cranking.

 

Also, never open the hood, pump gas, inflate tires, change bulbs or drive on public roads.

20,000 volts. Just depends on how strong your heart is Edgewood. They did pass a law in Sacto. County that you are very limited by law on what the County allows you to do on your car at home. They can now cite you for pulling your engine or doing a variety of auto repairs I did with my Dad growing up. I didn't have much money and I did most of the repairs myself. Now it's against Sacramento County ordinances if you do what I did with my Alfa rebuild in your driveway.

 

Yes, anyone with a pacemaker presumably knows the risks of certain behaviors and should proceed accordingly. Maybe have the wife or mother-in-law grab some ignition wires first to be sure they're up to snuff.

Unbelievable about Sacramento. I didn't believe you at first and had to look it up myself. Where does it end with California? Are they working on an amendment to change the state flag to include a hammer and sickle? That truly is a poorly crafted law. At the minimum, there should have been carve-outs for classic/antique/collector cars. I empathize with your being stranded in such an environment. Car culture has such a deep heritage and historical significance in California and to eviscerate those liberties from citizens? Shameful.

 

I had this happen on my Sport Sedan.
Last week...
Alternator had failed. The 10 gauge starter to inner fender positive post wire had gotten hot. I saw melted insulation near the starter.
Then noticed the same at the post.
I fileted the harness open from starter to post to find MORE insulation melted, but no other wires inside the harness were damaged.
I was able to tape up the harness and put it back.
As for the 10 gauge wire, it got replaced with a 4 gauge wire which I routed around the front, all by itself, fastened with P-clips.
Probably overkill, but I had that size wire on hand.

Oh yeah, I installed a CS130 AC-Delco 105 amp alternator using a fabricated mount and modified adjusting arm. Internally regulated and dash lamp functions.
I part out Jeeps, so that alternator was in my parts shed.

 

My alternator wire grounded against the exhaust manifold with similar result to yours. Some of the Alfa alternators appear to have a plastic shell on the back that holds the wire away from the exhaust--mine didn't, so that some careless work in the area apparently left the cable dangling... I'm currently working to replace and splice the damaged wiring.