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I went through this a few years back with one of my vehicles. It was painful and slow but well worth it in the end. I found about 3 melted wires from historic almost fires in there. I also found a whole heap of bastardry where previous dealers and owners had spliced in wires using whatever colours and sizes they had handy.

I was able to clean up a massive amount of unused wire. Repair all the firewall grommets and now all the warning lights and starter signals do exactly what they should.

My tips:
1. Break it into sub sections which are all loomed and labelled.
2. Buy a heap of wire in the appropriate colours and sizes. Label it all by size and amp rating.
3. Solder seal connectors are fantastic.
4. Add more earths and fused power points now. Run leads to power points that are fused but always live and well as key activated so you have options later.
5. Buy a really nice to use set of crimpers.
6. Use coloured heat-shrink to further identify wires.
7. Move all the relays to one point and stick them in a dedicated fuse-box. I got one off aliexpress that is from some Toyota. It worked great.
8. Diagrams so you can work faster but also retrace your work later. You will not remember all the details.

I've attached a pdf of a diagram that I had to do for my extra relay/fuse box and fan speed control. As you can see it's just not possible to wing-it.
 

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Dougal,
All good points. All things I'm planning on except #4. That's a good idea.
Those hybrid shrink wrap solder butt connectors are awesome. I thought it was a gimmick or they'd be super expensive, but they're just plain sweet.

I've got most of it worked out in my head except where to put connectors and what style connectors. Like, should I put a bulkhead 26+ pin connector on the firewall or run all wires to my central fuse box in the engine compartment? Or put connectors on the fuse box. These are the things I need to work out in my head.

I'd like to be able to take the body off some day and not have to cut a million wires. Same with the motor. I'd like to just unplug stuff and pull the motor. But I also don't want an excessive amount of connectors all around the fuse box that won't tidy up because they're 5" in diameter.

I know it sounds weird, but I really need to set some rules (a mission statement) and stick to it.
I've got the majority of my engine on a (maybe 12 pin) deutsch plug by the thermostat housing. That's oil pressure, water temperature, fan switch and alternator wires (energise, tacho feed) in one loom. The main alternator feed is a single anderson powerpole which connects right beside it and I have three bundled bullet connectors (big and small) for the fans.
So apart from starter wires (hot wire and solenoid feed) and AC clutch which come from the other side it's all in one place that can be unclipped in seconds.

I didn't worry about a big firewall connector. Instead I had looms come through the firewall (maybe 4-5 different places) that are obvious in direction and can be unclipped and laid back. So if you need to disconnect say the gearbox loom (handbrake, reverse and diff-lock light) it all unclips and folds back in one piece with each connector being obvious placement to reconnect later.

Also. Put in feeds for a reversing camera with display.
 

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Had my first run-in with one of those heat-to-solder-&-shrink connectors about 2 months ago. NOT recommended in any area that will see heat. Like under the hood. Nearly lost an engine to over-heating because one of those connectors in the electric fan circuit failed. The under-hood heat was enough to partly disconnect it. I'm not impressed, I'll keep using the crimp and heat to shrink Ancor terminals.

Unless you're going to place those marker heat-shrinks every foot or so I think it's a bad idea to reply on them exclusively. Sometimes you need to know which wire is which in the middle of a long run and if they're all green then which one is the one that you need? (I used green as an example because IH did their Scout II's this way. All green wire with numbers ink marked on them. Which comes off if you happen to use lacquer thinner to clean the loom..... )

Research it because I know that they're out there somewhere, there's a spiffy tool made that allows you to use a Dullie® (that's a Sharpie® after a couple uses) to put a stripe on any wire. I have one that a friend printed. The bonus is that you can use any color Dullie® that will contrast and show up on the base color.

Del City Wire was recently running a special on what looks like it might be a quality crimper that uses interchangeable dies, and that has a wide range of available dies.
Indeed. I used I think several hundred of those crimp-seal connectors too. They have glue lined heat-shrink which seals against the wire insulation and heals the cuts from the cirmper. Different colours for different wire sizes. Here's a pic of an audio loom I had to repair after some clown cut out a head-unit instead of unplugging it!

130770
 

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I was planning on putting anything with over 5 amps on a relay in my new design. An engineer at my work said I was being ridiculous, and needlessly adding more complexity than necessary.
After looking at factory fuse boxes in my other trucks, it's seems like only relays are used over 20 amps. The original Wagoneer had one relay for the starter. (it also was a rolling fire hazard).

The way I was designing this was going to need 12-14 relays. I think I'm taking this idea of keeping all high amps firewall forward might be a little stupid. I spent a couple hours trying to find a general consensus on when to use a relay, but couldn't find a good answer. Too many variables. What's the switch rated for? Is it a high amp spike start, then low amp run circuit? How far is the run?

Bearing in mind that I'm using all the 1972 original switches on the dash, what is a general, or rule of thumb amp load to be considered safe without employing a relay?
Depends how voltage sensitive your components are. For headlights and radiator fans IMO relays are a must as power output drops fast with voltage loss. For most other things it doesn't matter if it gets 11v instead of 13v.

One of my british vehicles had the headlights running from about 4m of cable through the fuse-box, steering colum switch and then back to the headlights. Relays made an enormous difference.

Using relays can also mean a bundle of smaller trigger wires to your switches instead of big fat ones. Which can help cable routing a lot. I have a table of wire size and max current that I use: But I can't find it right now!
 
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