1970 wiring problem help needed

[TLXregnar07]

Looking for advice on my truck. 1970 Ford F-100. Last thursday my daughter drove it to school, but when she came out that afternoon to go to her work, it wouldn’t start. No interior lights, no radio, nothing. She swears the key was off. A couple of her friends tried to help by jumping it off. It still wouldn’t start. (I found out later they had hooked the cable up backwards, thanks to the PO using a red cable marked POS for the ground, and they didn’t look at the battery markings, or realize the solenoid would be on the hot side) In fact it burned a black wire from the solenoid toward the alternator completely in two. I finally got there late that evening and being unsure, what to do I thought I’d try jumping it off wired properly this time. With the key on and the cables attached, I started it from under the hood, I admit I didn’t think to try the key, just seeing if it would turn over. I rigged a jumper wire from the battery Positive post to the burnt end of the fusible link, and got 13.5 or so volts at the battery. Drove it the 15 miles home and it ran fine.

IMG_3393.jpg

Later I was able to look at the wires. It doesn’t seem to match the wiring diagram on the site. The wiring looks factory, though. There are 3 connections on the battery side of the solenoid, not counting the battery itself. One is a yellow wire to the horn relay, another leads off to an empty fuse holder and off to an underhood light. The third is the Large black wire, probably a fusible link that is burned in two.
With the wire burned in two, the only thing that works is the horn. Fuse box is dead. With it jumpered, all is live and the truck can run, that’s how we got it home.
I’ve unwrapped the tape around the wire to trace them, and see where they go. This is where it is not matching. The big black wire goes into a junction block, where it meets the black with yellow stripe wire from the harness. That wire comes out the other end as a yellow wire that runs to the regulator. I think this one is the main power line, as unplugging the 3 connector and putting hot to the other end gives fuse box power. Out of that junction block, the big black wire goes in a loop almost to, but not attached to the regulator, then loops back toward the alternator, and is taped with the two other alternator wires.
So what do I do to fix this? Should I just try to slice into that one black wire, which is pretty rotten, with either a fusible link or a 40 or 50 amp fuse? If fusible link, are there different gauges for different loads?
Also any ideas what might have drained the battery like that? Maybe the key was able to come out with it not quite off? I know the switch bezel does sometimes turn in the dash, maybe she didn’t really have it off?

[PhantomoftheBumpside]

The key won't come out unless it is off... however, things like the parking lights, and dome light could be left on without noticing them.

I'd say replace the power wire with a new appropriate size wire (since it's crispy) and fusible link, keeping the new fusible link close to the location where it used to be. And while you are at it, replace the battery cables with new ones of the appropriate color to replace them so you don't have inexperienced helpers risking battery explosion and burning wires in the future.

Since it sounds like the battery wasn't really dead, the 'not wanting to start' issue may be the Neutral Safety switch. Wiggling the transmission selector lever may help if this is the case.

[TLXregnar07]

I have a few more pictures that shows what I'm consused about. This first one shows the first of what i believe to be a junction block, where the battery connects to the rest of the system. It appears curretn flows down the black wir, then to the second junction, then back up the black with red stripe to feed the ignition off side of the fuse box. The burned black wire is what I'm worndering if it is a fusible link wire or not. I think All I need to do in peel this back and hook to it with a new link or a wire with a regular fuse and it should be OK.

wiring_first _connection.jpg

This second is more of the alternator to system connection, and as such, should be left alone, right? Just tape all this back up and leave it like it is.

[PhantomoftheBumpside]

There WAS a fusible link there, it did what it was supposed to do. If it did during pretty much normal usage, then there may be an issue with the wire itself as it is 40+ years old, and that adds up to things like corrosion somewhere increasing it draw above what it should be, heating up the wire and eventually burning out the sacrificial link.

This is why I would add new wire and fusible link. The wire could be sized a bit larger (the older wiring was pretty minimum already before it got old) and make your fusible link two sizes smaller. A fuse (even a slo-blow) will not work well, as you'll be blowing them constantly because of the nature of the circuit.

As long as the alternator is charging the battery properly, there is no reason to change anything there, yet. Unless you want to rewire the whole thing.

Good luck with your project.

[TLXregnar07]

Thank you for the help. Usually telling the story, or writing it down helps me organize my thoughts to approach a problem more logically.

I put a new 14 guage fusible link in, hooked up a decently charged battery and it started right up. I had nearly 15 volts at the fusebox. After battery is fully charged I'll make sure that goes down, just to make sure the regulator isn't bad.
Maybe it's back to driving....

[PhantomoftheBumpside]

15V is near the top of the charging range, not over, so you're good there.

[wildcard]

The key won't come out unless it is off... however, things like the parking lights, and dome light could be left on without noticing them.

Glad to see you have it running again. But my on 71, if I'm careful, I can get the key out and it keeps running. Discovered that by accident. And I thought the ignition had turned off. I was in a hurry and just turned it to the left and pulled the key out. Then realized it was still running and the key was in my hand.
Since then I've played with it and managed to to do it again 2 or 3 times. This ign. switch is about 6 years old.
Just might be something to watch for in the future.

[PhantomoftheBumpside]

The switch, the cylinder or both? And how much weight are you carrying around on your key chain anyway?? That is a lot of wear for 6 years, unless you have a key pattern that is really close to flat or mostly high through the middle to end.

I recently changed out switches and cylinders in my 68 after the key ran off... yeah, that was plural because the first two didn't work and actually caused damage. Gotta love Chinese parts.

[wildcard]

I changed both. I only have 4 keys on a ring. No gizmos or anything else. But to make it happen again requires patience and a steady hand. I know it happened once so I put in that extra effort to make sure it's off now.

[TLXregnar07]

As far as my truck is concerned, I think I may have finally fixed it.
I changed the voltage regulator, and charged the battery fully charged Friday at 9PM, and let the truck sit. It started Saturday, Sunday, and after sitting all day Sunday and Monday (36 hours or so). I still need to replace the radio capacitor on the regulator, which the local dealer should have in later this week.
Maybe it's fixed finally.

[PhantomoftheBumpside]

You know when you start putting new parts on these trucks they just start wanting more, right?