How To Assemble Your Own Harness
...or at least this is how I like to do it.
Plan your Harness ...
This may be the most important part of the whole process. Make a general plan of how the harness will be routed, where major Trunks will be. How will you secure the Trunks to the engine or body. How will the Trunks merge together.
Make sure you consult your documentation and double check to include all the devices near a Trunks route.
Plan Your Trunks and Branches...
There are many, many ways to route wiring on the engine, here are three examples that i have used on a 2JZ fuel injector rail Trunk and Branch section. Feel free to get creative. Be mindful of moving parts like throttle cams or accessory belts. Also be aware of high heat zones around turbos and exhausts, try to keep the wiring in these areas to an absolute minimum, and use extra heat protection on the wiring loom to prevent the wire from melting.
Plug the connectors to the devices in this main branch you have planned.
Starting with the shortest section, in this case Injector 2 of this 2JZ fuel rail branch, measure aprox. 100mm of loom and apply it to the leads of the connector.
Then measure out the appropriate amount of loom to reach the next longest device, in this case Injector 1, cut the loom to the correct length and apply the loom to the leads of the connector.
Apply heat shrink tube to the end of the loom, you want 50% of the heat shrink on the loom and %50 on the wire getting the heat shrink as close to the connector as possible. If the connector is willing, putting the tub over the connector is even better, and will seal out more contaminants from your loom, and making a very nice strain relief for the wire as well.
[instruction on id labels]
Gather up all the elements that will branch out at this point on the harness.
Wrap lacing cord around the branch once.
Tie a single Surgeons Knot loosely to hold everything together.
line up all the branch elements, line up the lacing cord as close to the end as possible, and pull tight.
Tie a second Surgeons Knot to hold the entire branch in place, pull tight.
Your Knot should look like this.
Trim the tag ends neatly.
Cover the ends of the branch elements, with the main branch loom.
Secure with a Double Surgeons Knot, just as before.
Apply Heat Shrink Tube to the joint.
REPEAT and VOILA
Once you complete each of the planned branches, you can merge them together into a complete harness, starting with the furthest branch from the firewall opening, working your way backwards to the firewall and then finally to the computer.
REMEMBER: Always make your measurements with the the plugs plugged in, and double triple check every length to make sure that each will reach once the loom has been applied. Harnesses tend to shrink once they are fully assembled, you don't want to be caught needing to extend a plug or two once its completely assembled.