I have a 76 F100 with a 390. I've done engine swaps and currently am using an MSD 6A ignition triggered by a Magnetic trigger FE distributor. The last few years I've been experiencing some random ignition problems. The last time I got stranded, I parked it overnight and tried again the next day. Started up immediately. I let it warm for a couple of minutes and proceeded to drive. I turned on my wipers and the truck promptly died as if I had flipped a power switch. I noticed that my volt gauge showed no dashboard power. I mechanic at work helped me. We determined that there was absolutely no current at the ignition switch. To get the truck home, we bypassed the dashboard/ignition switch by tapping the switched power line into the MSD box directly to battery power, and using a screw-driver the activate the solenoid. Truck started right up and I drove it home without any electricity lights, wipers, signal and brake lights ect. The next free day, I had I probed about. I was able to start the truck again with the screwdriver. I then began probing wires that I guessed deliver current the ignition switch. A Black wire with a green stripe about 10 gauge is both in my ignition switch harness, and coming from a branch near the alternator/regulator area heading to the firewall. Looks to me that that wire delivers current from the alternator or battery to the ignition switch to be distributed about. When I pushed on it, I heard my seat belt buzzer go on and noticed I had current at my volt gauge again. I restored my MSD wiring to key on power and was able to start the truck fine. I also could kill the truck by pressing the wire. So now it seems obvious to me that there is a bad spot in this area of the wire. My question would be, short of replacing the whole wiring harness (seems daunting and a PIMA to me) could I replace point to point this wire with an appropriate gauge wire? It appears as if there is a fusible link on it. I don't think my MSD requires a fusible link. I also have an aftermarket one wire alternator that I had used in my 69 F100 sitting around. Would using this one wire alternator help simplify my wiring as well?

 

I would just cut the wire AFTER the fusible link, solder a new 10 gauge piece in place, and run it over to the ignition switch and cut the old wire again and solder the new one in place. Just abandon the old wire and leave it in the harness.

I would try to keep the fusible link. If something bad happens and that wire gets against some metal and shorts out, it will keep the truck from catching fire.

 

Trouble is, I'm not positive that the break isn't before the fusible link. I'ts near the link I believe. I'm suspicious that it is near the junction that branches between the alternator and the regulator. What if I replace the entire length and put a fuse insert inline?

I know that wire ends up at my ignition switch. Where is the other end? The alternator? the regulator, does it branch to both?

 

It branches to both, from the alternator to both the battery and the trucks elec system to charge the battery and run the truck at the same time.

About the fusible link; I would get the numbers and or color of the fusible link, and go to the store and buy a replacement and put it in your new wire. It will work better than a fuse. If you want to use a fuse, it will have to be very large, at least 50 or 60 amp.

Also be aware, that some trucks used this same wire to feed the ignition switch and the part of the fuse box that has power all the time with the key off. If you cut this wire at the ignition switch, and suddenly some stuff doesn't work, then they probably have a splice back from the switch were it splits off and feeds part of the fuse box.