I went to fire up the 68 today and when I cranked it, I managed to release some magic smoke. The ground from the battery negative to the core support to be exact. Completely melted the insulation.

The wire in question was 14ga, from the terminal to the eyelet on the core support. That sort of failure would have indicated around ~20 amps going through it. If I'm not mistaken that one is primarily for the headlights, which were off at the time. It would have to indicate that something is shorted to the chassis, though no fuses or fusible links have failed. The primary positive cable to the starter doesn't appear to have any shorts, and even if it did short I would expect the ground cable from the engine to the terminal to carry that current.

All the other grounds were in place; the heavy 2 ga cable from the driver's head to the battery terminal, and the braided cable from the passenger head to the firewall. Both are in good shape and free of corrosion.

Short of any suggestions my next thought is connecting the positive side wiring, connecting one probe of a multimeter to the negative terminal, and checking to see where is showing +12V.

 

Check the starter cable, wiring runs very close to exhaust.

 

You may have a bad connection in the battery neg cable to the block forcing that ground to take the load.

 

 

Another ground failed and used/overloaded the 14 gauge wire. Posts 3 and 4 nailed it, look very closely at the other grounds, especially the negative cable. Look for burns, corroded or loose connections and tug on the negative cable where it connects to the engine and at the battery.

Good luck!!!

 

Thanks for the suggestions. Unfortunately it doesn't appear that simple.

I hooked up the positive cable, leaving the ground unhooked. I put the one lead on the negative post and measured several spots on the chassis and engine.

First suspect was the starter. Unhooked the cable. +12V chassis/engine.

Next suspect was the cable to the starter shorting. Removed it from the chassis. +12V chassis/engine.

Now that means it has to be on the other wire from the positive terminal. On a hunch I disconnected the two prong connector to the alternator. It decays from 12V down to ~4V. Disconnected the battery hookup to the back of the alternator. Down to 0.8V. Now we're getting somewhere.

For posterity, of the remaining wires at the junction on the fender, the two with eyelets make up about 0.15V, with the other two connectors making up the remainder of the 0.8V.

Now with all that information, it would point me to the alternator being the issue, correct?

 

You will measure 12v on the chassis with the negative disconnected because of lamp filaments, relay coils, resisters, etc that look for a ground to complete a circuit. Again check the large ground cable.

 

Good point.

Although the ground cable is like new. I even disassembled it at the terminal to check for corrosion. I checked the connection at the engine as well; clean as a whistle, no surprise since I cleaned it when I swapped engines last year.

No signs of broken insulation or the sort on the starter cable either.

 

There are only a couple parts on a car that can cause a large amp draw, starter and maybe a power seat.

 

Voltage and current follow the path of least resistance. When you fired the starter, the load should have went through the heavy negative cable from the battery to the engine. Apparently that route was unavailable and it tried to go through the only other ground wire available to/from the battery. That wire could not accommodate the load and you know the end result.

 

X2 on other Eric. Bad ground of heavy battery neg cable. Small wire is trying to carry everything.

 

Not only does the connection point need to be clean, it needs to be bare metal with no paint.