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78 F150 wiring

 
  #1  
Old 05-15-2019, 03:50 PM
Wyguy118
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78 F150 wiring

Hey all. Been lurking on here lately to learn more about my truck. Long story short I inherited the truck from my dad who passed and it sat for 10 years after he passed. Finally got it running not long ago after some issues with the wiring. For some reason there is a wire going from the solenoid and wrapped around a bare spot of another wire. This actually melted at one point when I tried cranking the engine over too much. Picture below to show a better idea.

I was wondering if anyone knows why this is like that and what can I do to fix it? I'm learning as I go with fixing this truck up since I'm not familiar with Fords or wrenching on this older stuff for that matter. Thanks


 
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Old 05-16-2019, 01:09 AM
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That black wire looks to be bypassing a fusible link.
 
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Old 05-16-2019, 09:15 AM
Arktech
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Wiring


The fusible link from the alternator has been bypassed. This needs to be there. Search the boards or google fo wiring diagrams.
 

Last edited by Arktech; 05-16-2019 at 09:16 AM. Reason: Spelling
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Old 05-16-2019, 11:24 AM
sbpbrent
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I am also a lurker and this is my 1st post here. I have a '78 F150 with the Ranger package and I have the wiring diagrams for it. I can't tell much from your pictures, but I just changed the really and starter in mine last weekend. Let me see if I can attach the wiring diagrams. Forgive the large format. I'm not sure how to reduce them. Brent

Index Sheet

Starter motor relay at locationB10
 

Last edited by sbpbrent; 05-16-2019 at 11:29 AM. Reason: Typo
  #5  
Old 05-17-2019, 03:51 PM
Wyguy118
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Thanks guys. Can I just purchase a new harness that goes from the alternator to the starter solenoid and voltage regulator?
 
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Old 05-17-2019, 10:22 PM
1TonBasecamp
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Originally Posted by Wyguy118 View Post
Finally got it running not long ago after some issues with the wiring. For some reason there is a wire going from the solenoid and wrapped around a bare spot of another wire.
Hey Wyguy. As the others have said, it's a PO (previous owner) hack job. And being bare like that, is very dangerous as that wire has constant 12v power directly from the battery.
It's also too small of a gauge for it's duties, which is to charge the battery. After cranking for a long time, the alternator was putting out it's max output most likely, which overheated the wire. It's the same thing that a fusible link does sometimes, but in a more controlled manner, and with no tape to melt off!

Originally Posted by Wyguy118 View Post
I was wondering if anyone knows why this is like that and what can I do to fix it?
Looks like quite a bit of hacking when on previously. Probably a melted charge wire from the alternator heavy gauge (probably 10ga) Black w/yellow wire (edit: sorry, probably Black w/red on these trucks) in that harness along with an Orange Field wire from the voltage regulator, a yellow sensing wire for the regulator, and maybe even the White w/black stator wire for the carburetor's electric choke.
Gonna' have to study your pic a little more before I can make any reasonable recommendations. Not sure if there are pre-made harnesses, but at least the good news here is that it's very simple and straightforward section of the wiring.

Some other things to do right now though, would be replace the main battery and starter cables. The battery positive is rusty to the point that it's building up resistance and heat probably, which may be indicated by the obvious shrinkage of the outer jacket, exposing the conductor strands.
Old age can do that as well though, so it may just be time to replace it anyway. It's cheap insurance. But if you look at the end closes to the relay, you can see that it started to melt.
It's also cheap insurance to give yourself some future head room and go bigger too. Your starter cable from the relay/solenoid is a large gauge quality replacement it looks like, but it also looks like it's had excessive heat in it's life too. So maybe not a bad idea to replace all of them.
And speaking of cheap insurance and going bigger, I think Ford used 6ga because they could get away with it, most upgraded to 4ga because it was better, but nowadays most auto parts stores even carry pre-made 2ga cables (my recommendation at this point) and for only a couple of bucks more you can have that added security.

Some will say never get the store bought ones. It's true that they're the bottom of the barrel when it comes to cabling, but you can get very good service out of them too, and on the cheap. You don't have to, but I always use shrink tubing over the junction to keep moisture from intruding itself into the conducting strands. So far that practice has served me well. Of course, it helps to live in a dry climate too...
You also do not have to use Red for the positive, but it sure helps keep track of things visually. I ALWAYS use red for positive and starter (because it's also positive) and Black only for the negative. Just simple sense and makes it easier later on.
While you're at this whole cable replacement thing (that I'm putting you through should you decide to agree!) of course do the same with the negative cable to the engine block. But get one with the extra small wire (10ga, so not that small) sticking out of the lug with the Yellow crimp connector on it. Add your own piece of wire from that "pigtail" down to the body somewhere convenient. On our trucks that often turns out to be under one of the fasteners for the starter relay. But the body needs a ground too, and that's how Ford did it originally.

Do NOT get rid of that starter relay yet, but I would definitely buy a new one as a spare. The older ones were a hundred times better than the new chaeapo crap, but you can still get good ones if you pay the piper. Your existing one may be the best you can get, but it's probably been put through a lot of heat as well. Which is why I recommend a new (quality) one and keeping the old still-working one as a spare in the glovebox or tool box. In case your new one craps out after only three starts.
And no, I'm not joking...



Hope that helps.
Even though it's more work than just fixing a bad junction, it's worth it for the reliability. Do it right to keep your dad's old truck in good working order. And you and yours safe too!

Paul
 
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Old 05-17-2019, 10:31 PM
1TonBasecamp
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Thinking more about it and looking more at your pic, that relay may be toast due to some rust and heat having corroded the power studs. You can look at it and make that determination, but if you do find this one too far gone, then I recommend buying TWO new relays. One for use, and one for a spare.
And if you have a failure anytime soon, report your findings (brand and model and length of time before failure) to the thread for that purpose here on the forums. Sorry don't have a link, but the failure rate is so bad that some of us prefer buying old used ones from the junkyard over new ones. Unless you can find a good old Made in the USA model still.

Looks like that new black wire that bypasses the fusible link, is actually bypassing TWO fusible links. Maybe the others can correct me if I'm wrong and see something I don't, but there should be one of those Black wires (probably with the yellow stripe?) coming from the alternator's BAT terminal to the relay post to charge the battery and power the truck while the engine is running, and the other one would also be attached to the battery side stud of the relay and run along the fender, across the firewall and then power the entire rest of the truck. I believe this is how your '78 should be laid out. Others may have more input, but you'll need to separate those two functions even though they're basically doing the same thing, just to make them more sturdy and reliable in function.

That one wire is handling it all and should be removed to allow both of the other (new ones you're going to install) handle their jobs.
Might be some un-taping of harness sections in your near future, but it should be a fairly short run and relatively easy compared to re-wiring the whole engine.

If you're not familiar with wiring techniques, or if you are and are just intimidated by it (like most of us) let us know and we can walk you through much of what's needed. And you can get the rest from the bazillions of YouTube videos that are likely covering the subject.

Do it right, and be safe. Above all, have fun!

Paul
 
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Old 05-17-2019, 10:46 PM
Wyguy118
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Thanks for the detailed response. After searching for a alternator harness on Google I found a thread on here from two years ago of a guy selling his wiring harness and alternator harness. I emailed him asking if it's somehow still for sale. If that falls through I'll have to resort to making one myself.

I'll start looking up new wiring/cabling after I get off here. Truck actually starts up surprisingly quick once I got it running good last year so I haven't had issues with the wiring but like you said for safety I'd like to fix it. I greatly appreciate your response and will keep you updated.
 

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