Tutorial: How to make a wiring harness that will outlast your grandchildren

[darrenct83]

Hello All. I have received a lot of help in working on my old truck from members here. I own an auto repair shop for newer vehicles, and I do a lot of professional modern wiring repairs. I have decided to take the time to post this wiring harness tutorial to help others who may find this task too daunting to do on their own. I am here to tell you that anyone can rebuild a harness, but it will take time and patience. If you plan on doing this in an afternoon than you are in for an unpleasant surprise. Be prepared to spend about 8 hours just to manufacture the harness. Removal and installation not included. As time consuming as it is it is absolutely needed on a lot of these old cars.

The harness I am showing here is off of my '46 1-1/2 ton truck. I chose to use modern wire and materials due to availability, price, and longevity. Using the same technique you could produce an equivalent harness from the old style cloth wire and loom. Additionally I chose to modify my harness to include additional circuit protection(Fuses&Relays). This is not required but may add to the longevity of your electrical system.

First off, you will need to obtain some basic tools and materials. The tools I recommend are:
1)Proper style wire terminal crimp tool
2)Soldering Iron(Approx 35-50 watt recommended)
3)Wire stripper(I use wire cutting pliers)
4)Heat Gun(For shrink tubing)

Materials you will need:
1)A good wiring diagram(This is needed, obtain how you see fit)
2)Wire:
When choosing wire get as many colors as available. You will also need various wire thicknesses(gauges). I bought my wire at Napa in clamshells of 25'. Most of the wire on a 6 volt system should be 14 gauge. I use 12 guage for the headlamp power wires, and 8-10 gauge for the power to the system as well as the generator armature wire. Remember the more current through a wire the lower gauge the wire must be.
3)NON-INSULATED butt connectors and ring terminals.
Do not buy the connectors with the cheesy insulation. It looks unprofessional(and is unprofessional). Those terminals can not be soldered, allow corrosion, and do not crimp tightly. Napa stores(or similar) usually have non-insulated connectors available, but they are not stored with the electrical items in the showroom. The parts person will have to look up numbers in catalogs and get the parts off the shelf. Make sure you talk to a knowledgeable counter person.
4)Rosin core solder
5)Heat shrink tubing(Get a few sizes)
6)Wire loom. Various sizes
7)Electrical tape (Use Scotch brand or something UL approved. Cheap tape decays)
8)Masking tape(used as labels)

A need-to-know technique:
Making Two Wire Splices:
There will be places where two or more wires need joined. To join two wires strip a small portion of insulation from the wires being joined. Place a length of heat shrink tubing over one of the wires. Place the stripped ends of the wire in to a non-insulated butt connector and crimp. Use a soldering iron to heat the entire butt connector. Hold solder to the small hole in center of butt connector. Allow solder to melt in to the hole which will solder the wires in to place. Allow to cool. Slide heat tubing around connector and shrink tubing with heat gun.
3-4 Wire Splices:
Same as above. Use a larger butt connector and install two wires in one end of the connector then crimp. Slide your tubing on. Then place wire(s) in other side of butt connector. Make sure to use shrink tubing large enough to slide over two wires.
Ring Terminals:
To install ring terminals cut a short section of heat shrink and slide on to wire. Crimp on ring terminal then solder. slide heat shrink over terminal base and heat with heat gun. Having heat shrink on all your ring terminals will greatly reduce your chances of accidentally shorting terminals against each other or the components they connect to.

Getting Started:
Remove the old wiring harness completely from the vehicle. Label Everything! You may need to cut the wires in places to get it out. Again, Label every wire you cut! If you have to cut wires make sure to cut the wire halfway between the harness and the component you are cutting away from. Leave wire to work with on both ends! For example I cut my wire several inches from the headlamp connector, and used the original headlamp connectors when I put it back together.

Once you have the harness out of the car lay it out on an open surface(ground). Here is where the work begins. You need to cut one of every wire in the harness. On your wiring diagram pick a wire to start with. Find that wire on the harness. Using a wire of the same gauge lay the new wire along the length of the harness where the wire starts and ends. Cut your new wire to that length plus a few extra inches. Label both ends of the new wire the same as you had labeled the original wire on the harness. Now mark the wire you just cut off of the wiring diagram(I use a highlighter). Now do this until every single wire on your wiring diagram has been marked off. This takes a while.

Once you have all your wires cut you will begin to put it all together. Lay your new wires over your old wiring harness following the wires they are replacing. Now pick a place to begin taping wires together. I started at the starter solenoid as that is where the power starts. I did the interior of the truck portion of the harness last. Holding your new wires together at your starting point use some electrical tape to tie the wires together leaving enough of a "pigtail" out the end to add the ring terminals later. Keeping both harnesses, new and old, close together follow up the harness to the first "break-out" where wires come out of the harness. Using electrical tape bind the wires breaking out together. Then bind the wires of the main harness on both sides of the break-out. Continue down the harness using the old harness as a template. Bind the wires coming out of every break-out. If there are long sections of wire between break-outs then bind the wires of the harness in between as well. Do this until every break-out is bound and all wires are held in sections with electrical tape. Be sure to leave an inch or two of extra wire every place a wire leaves the harness. You can cut it to the correct length when installing the ring terminals.

Now you are ready to install ring terminals. Holding your new harness up to the old one cut your wires that break out to a matching length as the old ones and install ring terminals where they were on the old harness. I like to make my wires slightly longer to make sure there is enough. Where you are attaching to old wire just leave the wire how it is and connect once the harness is installed on the car.

Once your harness is all bound together it is time for loom. I used plastic split loom. It is available in many sizes. I used three sizes. 7/8", 1/2", and 3/8". Your harness may be different. I measured the diameter of my wire sections, and their lengths, and had Napa order me lengths of loom to match. You must also bind split loom along the way periodically with electrical tape. If you are using original style materials you may need to get a different material to wrap as loom.

Now you are ready to reinstall.

Other Notes:
My wiring diagram varied slightly from the actual wiring on the truck. You can use a multimeter to verify two ends on the harness are the same wire. Small differences are easy to figure out once you get to working on it.

Fuses and Relays:
Fuses: I added a 50 amp in-line "maxi" fuse where my power wire comes off of the starter solenoid. This could save you a lot of trouble. These old cars were not well protected from short circuits which can cause electrical fires. I wanted a fuse large enough to power anything the truck might want to power. I chose 50 amps because 14 gauge wires run off of this fuse in places further down the circuit. A 14 gauge wire can blow a 50 amp fuse if shorted to ground, but if I used a fuse larger than 50 amps the wire could burn up without blowing that fuse.

Relays:
I added three 6 volt relays in to my system. An electric fuel pump relay, a tail light relay, and a headlamp relay. The purpose of this is to allow these three items to get power directly off the battery, and not require the extra current running through the aging switches of the original system. An electric fuel pump requires a large amount of current, and could potentially wear out an old ignition switch easily. Additionally an old headlamp switch can have resistance dimming headlamps as well as being prone to wearing out. Each of my relays has its own 20 amp fuse to power the "High Current" Side.

[darrenct83]

A Few more pictures. Sorry about the disorganization of pictures. Trying to show the tools used, how to use them, end product, and problems with the original harness.

[Lawson Cox]

All I can say about that is that you live in a beautiful section of the country. lol

[Lawrie]

Thats great information,
12 yrs ago I made a harness for my model A this way, its in use every day by my son as a go to work vehicle.
Thanks Lawrie

[Royal Ryser]

What a grest contribution to the Fordbarn! We need a lot more like this. Thanks for the great article.
Royal Ryser

[Tinker]

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Great write up.

Might I suggest a less department style stock.

I wired some of my A this way. Hey I'm a poet and didn't even know it. :]

http://www.thehotrodcompany.com/shop...uct_id=12PW-yg
http://www.thehotrodcompany.com/shop...&filterkey=prc

Looms

http://www.thehotrodcompany.com/shop...roduct_id=4272

It'll give it a swell look.

I do still like the wiring setups offered by venders. They are well done. Great buy for the dollars you might save.

Nice write up!

[40fordpu]

sweet

[darrenct83]

Thanks for the compliments.

Originally Posted by Tinker

Might I suggest a less department style stock.

I wired some of my A this way. Hey I'm a poet and didn't even know it. :]

http://www.thehotrodcompany.com/shop...uct_id=12PW-yg
http://www.thehotrodcompany.com/shop...&filterkey=prc

Looms

http://www.thehotrodcompany.com/shop...roduct_id=4272

It'll give it a swell look.

I do like that wire. The main reason I didn't use a wire like that was the time to ship and cost. I missed driving my truck and wanted to get it back on the road quickly without worrying about the electrical system giving me grief.

[felix37]

Great informative post! Thanks for taking the time to put that together for everyone to see!

[Mart]

Nice job.
I like wiring, it's clean and logical. Nice quiet work, that requires thought and skill.

I made a replacement loom for my 57 T-Bird 30 years ago and it's still working great today.

Mart.

[Ralph Moore]

Nice work! I like to solder all my connections, may not be necessary, but it gives me peace of mind. One note on the asphalt coated loom, keep it away from gasoline.
GasLight auto also carries alot of the original style connectors/bullets.

[Bruce Lancaster]

I too like solder and crimp attachment and hate the K-Mart look in terminals! One thing for beginners, on the soldering: Learn when to stop! If you get the terminal and wire assembly up to heat and just keep pushing the solder wire at it, you will fill the first inch or so of wire with solder. This creates a rigid and brittle area, right where the wire exits the barrel of the terminal. Learn to just solder up the length of the barrel easy to see when you are using heatshrink as a separate insulator.

[darrenct83]

Originally Posted by Bruce Lancaster

I too like solder and crimp attachment and hate the K-Mart look in terminals!

Thanks Bruce. That reminds me of something also that I wanted to elaborate on about the "K-Mart" style in terminals and why they won't hold up. When copper is exposed to air it oxidizes. Anywhere copper is exposed to air for long periods of time will eventually corrode away. These insulated terminals do not provide the seal that a quality piece of heat-shrink gives.

They do make terminals insulated with heat-shrink. I don't recommend this for two reasons. You can not solder them for one. Secondly the crimp tool I have shown is for non-insulated terminals and provides a stronger holding crimp. To crimp an insulated terminal you use an oval shaped crimp tool, and it does not provide as strong of a hold on the wire. Combine this with not being able to solder, and it will eventually come apart.

[Bruce Lancaster]

Tool shown is type generally associated with aircraft applications, which says something right there!

[rotorwrench]

I just wish that the wire made these days was as good as aircraft wire. It would last beyond the great grand kids if it was. Much of the wire I've purchased over the last 10 years is questionable. It is a lot better than the old cloth covered crap but it could be much better.

Kerby

[fourtydlx]

Some of the corregated wire loom that comes from overseas melts if close to heat,starter etc.

[Conrad Rossi]

Great "How-to" guide. Thank you for taking the time to write it and sharing. It is much appreciated.

[Rand]

Wow that's really helpful for us electrical challenged Flathead lovers
Can you tell me how you wired in your headlight and taillight relays sir?
Thanks a bunch for the great write up!
Randy

[FRITZ47]

Originally Posted by Rand

Wow that's really helpful for us electrical challenged Flathead lovers
Can you tell me how you wired in your headlight and taillight relays sir?
Thanks a bunch for the great write up!
Randy

This is a great writeup - I want to second the request to explain how to wire headlight and tail light relays and also how to wire a relay into an electric fuel pump circuit. Many of us backyard mechanics do not truly understand the exact way to wire a relay and need some help at "8th grade level".

Also where can we buy a relay that has a mounting bracket suitable for our old cars, most of which use 6-volts?

Thanks, Fritz

[31chevy]

Originally Posted by FRITZ47

This is a great writeup - I want to second the request to explain how to wire headlight and tail light relays and also how to wire a relay into an electric fuel pump circuit. Many of us backyard mechanics do not truly understand the exact way to wire a relay and need some help at "8th grade level".

Also where can we buy a relay that has a mounting bracket suitable for our old cars, most of which use 6-volts?

Thanks, Fritz

newark.com has 6 volt relays & very reasonably priced.
Gary