Wiring harness knowledge

I have a 1976 f100 custom stepside that needs all the wires replaced, and I’m wanting to do it myself. How do you go about connecting the wires into the fuse box? What are the basic wires to starting a engine and how would you wire them? I just really need any tips on wiring.

Order a Haynes repair manual online.


ALL the wires? that would be a first. Don’t even know if it is possible.

Will cost a fortune and take months. For example, you have to disassemble the dash, a long process. All the doors have to be taken apart. and much more.

one internet source:
Average car has about 1000 wires, totaling almost a mile in length and 40 pounds in weight.

If I already have a harness but it has to many wires coming out of the fuse box could I just remove those wires and it be fine.?

Impossible to say without knowing a lot more details. But the odds are against it. Where did you get the new harness?

Attempting to rewire the fuse box will likely result in donating the truck to a charity just to get it towed away.

The Haynes for my pickup has only general wiring diagrams. When I had a wiring problem I had the good fortune to know someone with the shop manual (unfortunately he died recently). For a job this scale it may be worth getting the exact diagram, even if one has to purchase the shop manual, which probably costs > $100. Perhaps one can obtain just the diagram.

Perhaps not a 1976. There are no wires in my '87’s doors. When I had a short in my harness I had to remove the dash and test the wires in the harness individually. It turned out to be the wire for the glovebox light, which my pickup doesn’t have; only the Forerunner, which uses the same harness, has it.

The wiring harness for my '87 Toyota pickup has wires for more circuits than my pickup has because the same harness was used for other vehicles: a harness may have more wires than your vehicle needs. You have to be really careful to get it right.

I would strongly suggest a complete new harness including a new fuse box. Like this;


While rather pricey, it makes the task possible. Tedious, but possible.


This is what I love about working on vintage cars and trucks- 21 circuits, 12 fuses and two relays. It doesn’t get much simpler than that…

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If you want to see what is involved in blending together a modern fuel injected engine wiring harness into a car it was never installed in, watch this:

This is from an Australian YouTube series called the Skid Factory - skid means burnout down under. Engines turbo-charged that never were, engines installed in cars that never had them and the amount of work required to bring them together and make them work.

By comparison, that '76 Ford truck is easy-peasy!

Please know that I’m not trying to insult you, but based on the questions you have asked, you do not have the right skillset to do this on your own. That’s not to say you can’t acquire it, but you need to start smaller.

We start kids in math learning how to add and subtract - we don’t hit them with calculus in first grade. :wink: Similarly, rewiring an entire car as a first foray into automotive wiring is a really good way to create a lot of headaches for yourself. Start with something much simpler, like installing a new radio.


Hat’s off to that guy for accomplishing that on a bench! First of all, he has most of that stored in his head (as noted when he rattles off what certain combinations are used for) and that would be necessary to keep it straight as it devolves into a jumbled mess. Personally, I would lay it out on the floor and use tape to hold stuff in position along with some velcro loops to keep the bundles together as it was de-loomed. A bit harder on the back but easier on the sanity.

I cut up my brother’s Wrangler to install HEI ignition and throttle body fuel injection. Pretty good pile of “spare parts” left over after that exercise but what an improvement in performance, especially at altitude (combined with attitude) that used to make the old system give fits.

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I checked out a book from the public library with good basic information about getting started building a kit car. It was a Haynes Kit Car Manual.

I really don’t know about the other services but my factory manuals have complete wiring diagrams plus illustrations of the plugs and where the wire harnesses are strung throughout the vehicle. I’ve never seen that kind of detail in an after-market general manual. Second question is how in the heck you ever would get new harnesses strung throughout a vehicle without tearing the whole thing apart.

Generally speaking, you don’t

I’ve replaced my fair share of complete wiring harnesses, and it always requires removing a lot of stuff

Might be different with a Model T, but I’m talking about modern vehicles

Back in the GOOD OLE DAYS I acquired a set of Mitchell wiring diagrams covering about 20 years for all domestic and Asian imports. The pages opened out to be 4 feet wide and I can’t recall what they cost but they were very expensive. And while current digital manuals provide component diagrams which are often preferable to the full schematic the full picture is sometimes needed.

And as for Haynes… It’s better than nothing but only barely.

Can you repair only failing wires? If it was me I would go wire by wire from old to new.

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My 70’s Ford truck’s entire wiring diagram is very legible on just three 8 1/2 x 11 inch pages … lol . .

OP: replacing the entire wiring harness will be a big, time-consuming job, but definitely do-able on a 76 F100. You’ll need a complete wiring diagram. Fortunately yours will be about the same size as mine, 3 or 4 pages total. By contrast, more than 100 pages on my Corolla. The wiring to the fuse box is the same as the wiring to anything else, just hook them up. Likely some soldering involved. I think the fuse box on my truck only has 4 or 5 fuses, so connecting the wiring harness to it can’t be that big of job.

When you get the wiring diagram, post a page and you’ll get some guidance here what the symbols and nomenclature mean. The wire colors and wire size annotations will be huge clues how to hook up the new harness. I wouldn’t attempt that job myself except by purchasing a complete harness, new connectors, new wires, and which would probably include a new fuse box. If you are moving a used harness from another F100 to you F100, that will be more challenging. But still doable.

If there are just a few wires causing you a problem, but the connectors were still ok, you could probably just replace those wires, cut them off near the connectors and solder new wires on, heat shrink tubing, done.

The main wires for running the engine are from the battery to the ignition switch to the starter solenoid to the coil + terminal, and from the points to the coil negative. All are shown on the schematics.