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Login to post Forums » Grassroots Motorsports » Overflow my brain with knowledge concerning building a wiring harness from scratch

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Dec. 3, 2011 10:27 a.m. 92CelicaHalfTrac SuperDork

I'm trying to plan out the Celica build a bit better than i did the Escort. That being, i want to have true "stages" to complete before moving on to the next instead of "BUY ALL THE THINGS, TEAR DOWN AND DESTROY TWO CARS, HOPE IT ALL GOES TOGETHER!!"

I'm still working on the motor side of things (and transmission for that matter, ugh), but something else i've been worrying about is the wiring.

I know i can somewhat easily adapt the harness that's in the car right now to run a "Gen2" 3sgte, which is what i have at the moment. There's only 1 PNP EMS at the moment that i know of that would work for this, though, and i REALLY don't like that option. (Hydra)

On top of this, there's a good chance that i'll be ending up with a weird hybrid motor of sorts. (Camry block and crank, "gen3" head and fuel system) This makes converting the existing harness a bit harder.

So why not just make the harness from scratch, and use whatever EMS i want? (Probably AEM because it's so well supported)

Motor will be completely stripped of all unnecessary accessories, so i imagine that would make things easier to deal with.

Anyone done this? Is this too big of an undertaking for someone like me that has very basic knowledge of electrical stuff at best? The plus side is that i do solder pretty well.

My biggest concern is making things worth with the factory connectors at the motor end, but i imagine i could take a try with re-pinning connectors off the Celica harness?
Dec. 3, 2011 10:30 a.m. 92CelicaHalfTrac SuperDork

I gotta be honest, though.... i kindof really want to run this thing on an FCon.
Dec. 3, 2011 11:41 a.m. Taiden Dork

Check out the various wire tucking threads on Honda-Tech. Those guys have figured out where to get just about every plug, wire, crimper, distribution block, fuse box, loom, molex connectors etc you could ever need to wire tuck an f16 jet to military aerospace standards with oem honda components. I know that makes no sense but it's true.

See Chase Bays for inspiration
Dec. 3, 2011 11:52 a.m. 92CelicaHalfTrac SuperDork

Heh, i've read the huge one over there a few times, actually.

That thread is what sortof prompted me to start thinking about this.
Dec. 3, 2011 12:26 p.m. Taiden Dork

I think you could probably spend the most money on colored high quality automotive wire. I put out a feeler on an automotive site a year or so ago that I wanted any junk wiring harnesses people had laying around. I use these harnesses to make modifications to other harnesses. I have been able to color and gauge match every single change I've made to a harness in the last year or so. I only really paid $15 or so in shipping.

The issue with this, is if you are making one from scratch, you will find yourself extending wires by soldering many times. This is not necessarily bad, but it's certainly not good either.

I used to be totally anti distribution block, but that is starting to change.
Dec. 3, 2011 12:45 p.m. pres589 Dork

Here's what I would do if I were in your shoes (and I hope to be within the next year or so).

Get full definition of the engine management system you want to work with. Pins, connectors, etc etc. Make sure the sensors and related accessories that you plan on using are compliant with the EMS. Like the idle air control system, if the EMS only outputs DC and not a pulse width modulated signal, make sure your IAC matches.

Learn about the OEM connectors used, how to de-pin and re-pin them and what gauge all contacts accept. Find a crimper and source of contacts that work with those connectors. This may be a bigger task than you expect it to be.

Draw the entire system out. There was a thread here recently about using software to accomplish that task with some examples. Something I saw there that I didn't like; drawings that look great until you realize the contacts weren't named. The drawing controls everything and makes your life hell or heaven.

I'd try to do as much as I could on a bench in the garage next to the vehicle, laying out the harness over and over again. It will take time but worth it in the end.

That's probably a good start.
Dec. 3, 2011 12:49 p.m. Taiden Dork

Also make sure you double, triple, quadruple check your layout. Then pick an origin and work from there out, checking fitment along the way.

One thing I've found that always bites me in the ass is 90* bends. If you have a bundle of wires and you make them go around the bend, some of them can end up being up to an inch shorter in length (inside corner vs outside corner). This can cause problems if you aren't prepared for it.
Dec. 3, 2011 12:55 p.m. 92CelicaHalfTrac SuperDork

Taiden wrote:

I think you could probably spend the most money on colored high quality automotive wire. I put out a feeler on an automotive site a year or so ago that I wanted any junk wiring harnesses people had laying around. I use these harnesses to make modifications to other harnesses. I have been able to color and gauge match every single change I've made to a harness in the last year or so. I only really paid $15 or so in shipping.

The issue with this, is if you are making one from scratch, you will find yourself extending wires by soldering many times. This is not necessarily bad, but it's certainly not good either.

I used to be totally anti distribution block, but that is starting to change.

I was thinking of actually getting the main ECU plug end (well, there will be a big amphenol connector at the firewall) done, then laying everything else out at least a couple feet longer than i'll truly need.

Then insert motor and do my routing and cutting the way i need to.

In THEORY, resulting in a harness that actually has no soldering within the wires themselves.

The BIG plus to all this is that i believe i can re-use my factory body harness with little to no changes, so i'll probably do that.
Dec. 3, 2011 1:45 p.m. SVreX SuperDork

In reply to 92CelicaHalfTrac:

I've done it like you are describing. The only difference is that I had a full set of OEM connectors from a cut/ damaged harness.

It works, but careful planning is better. If you do it that way, you'll need a lot of patience. Expect to redo virtually everything 4-5 times.

I'm pretty anal about the final appearance. My final product was gorgeous, but I re-did it several times.
Dec. 3, 2011 1:50 p.m. 92CelicaHalfTrac SuperDork

SVreX wrote:

In reply to 92CelicaHalfTrac:

I've done it like you are describing. The only difference is that I had a full set of OEM connectors from a cut/ damaged harness.

It works, but careful planning is better. If you do it that way, you'll need a lot of patience. Expect to redo virtually everything 4-5 times.

I'm pretty anal about the final appearance. My final product was gorgeous, but I re-did it several times.

I should have at least 95% of a full set of connectors from the Celica currently. I believe if i wanted to use the stock wiring harness to run this motor, i'd only have to make 3 small wiring changes.

My harness isn't in the best shape, though (after 253k miles) and there's a lot of stuff on it i don't need.

And of course, a new development, i may be running an ST205 GT4 WRC Gen3 head, so that makes for some other differences i suppose.

You bring up the good point with the "FULL" set of connectors. Sounds like i should go raid another Celica so i've got some doubles for if (when) i break some connectors.

I wish it would be easier to actually change the connectors at the motor themselves. I'd love to make a fully modular harness from the WeatherPak connector set that DIYAutotune sells.
Dec. 3, 2011 1:54 p.m. erohslc Reader

I used to work at an electronics factory that built aero navigation and communications equipment. They used sheets of plywood, with wood blocks that the connectors attached to. The harness layout was drawn on the wood, with all routing and bends in place. The harness lengths had already been measured. Each bend would each have a spool for the wires to route around, and mounting tiedowns also each had a block in the correct position.
The wires could thus be run pulled taught, terminated, and kept arranged in perfectly combed parallel bundles.
Works of art.
Dec. 3, 2011 1:58 p.m. pres589 Dork

In reply to erohslc:

He's not going to have the resources to make a wireboard possible; no engineers estimating routed length, no series production run to really make it a worthwhile endeavor. I would suggest the same thing he's talking about doing; terminate one side of each wire, run out and bundle each conductor together for the various branches of the engine compartment trunk, and only terminate the other end after everything else has been assembled and installed in the engine bay.
Dec. 3, 2011 2:17 p.m. 92CelicaHalfTrac SuperDork

pres589 wrote:

Here's what I would do if I were in your shoes (and I hope to be within the next year or so).

Get full definition of the engine management system you want to work with. Pins, connectors, etc etc. Make sure the sensors and related accessories that you plan on using are compliant with the EMS. Like the idle air control system, if the EMS only outputs DC and not a pulse width modulated signal, make sure your IAC matches.

Learn about the OEM connectors used, how to de-pin and re-pin them and what gauge all contacts accept. Find a crimper and source of contacts that work with those connectors. This may be a bigger task than you expect it to be.

Draw the entire system out. There was a thread here recently about using software to accomplish that task with some examples. Something I saw there that I didn't like; drawings that look great until you realize the contacts weren't named. The drawing controls everything and makes your life hell or heaven.

I'd try to do as much as I could on a bench in the garage next to the vehicle, laying out the harness over and over again. It will take time but worth it in the end.

That's probably a good start.

I appreciate this, i do need to figure out which EMS i'm going to use sooner rather than later, for sure.
Dec. 3, 2011 2:28 p.m. pres589 Dork

In reply to 92CelicaHalfTrac:

Something else I like instead of convoluted tubing is Bentley Harris Expando which isn't very expensive and provides good flexible protection to wiring;

http://www.buyheatshrink.com/braidedsleeving/non-fray-pet-braided-sleeving.htm#pri...

You can cut with scissors and not have a fraying mess on your hands when done. I'd probably go with 3/8th of an inch for underhood since the stuff can deal with varied bundle sizes. And I hate convoluted tubing.
Dec. 3, 2011 2:28 p.m. Taiden Dork

erohslc wrote:

I used to work at an electronics factory that built aero navigation and communications equipment. They used sheets of plywood, with wood blocks that the connectors attached to. The harness layout was drawn on the wood, with all routing and bends in place. The harness lengths had already been measured. Each bend would each have a spool for the wires to route around, and mounting tiedowns also each had a block in the correct position.
The wires could thus be run pulled taught, terminated, and kept arranged in perfectly combed parallel bundles.
Works of art.

I've been trying to find photos of this stuff. I've seen it before, here and there. What are some keywords I can use to come up with some examples of this type of work?
Dec. 3, 2011 9:31 p.m. xFactor New Reader

The plywood thing is for mass replication.

I would also suggest starting at the sensor/injector/whatever end and work your way to the other endpoint being a bulkhead connector or the ecu. When I have both the ecu and a body harness, I run them long, then cut and solder them at a point about a foot away from the ecu. There you can make certain things like 5volt supplies and sensor grounds common.

My loom of choice is the expandable nylon chinese finger trap stuff and heatshrink, and I also like dissasembling factory connectors and piggybacking a new connection on the old terminal, then reassembling.

later, matt
Dec. 3, 2011 9:57 p.m. Curmudgeon SuperDork

Proudly hotlinked picture of a harness assembly board:

I've modified a few harnesses, what I have found is this: those things are built like E36 M3 under the tape. When I did the Jensenator's harness (Mazda 13B rotary), the number of splices and just plain weird stuff hidden by the tape really surprised me. Like xfactor, I say hang the ECU and other stuff such as the air flow meter where you want them, then run the wires as needed being sure to allow for engine movement, staying away from hot stuff, etc. Solder and shrink wrap anything that will be inside the harness. The 'dry' tape used by the factories is readily available and is a LOT easier to use than the hardware store stuff. Just one place that stocks it: http://www.pcsconnectors.com/harness_tape.aspx
Dec. 3, 2011 10:51 p.m. unevolved Dork

Here's a good writeup on motorsport wiring harnesses:

http://www.rbracing-rsr.com/wiring_ecu.html
Dec. 3, 2011 11:13 p.m. Taiden Dork

unevolved wrote:

Here's a good writeup on motorsport wiring harnesses:

http://www.rbracing-rsr.com/wiring_ecu.html

I think I love you.
Dec. 4, 2011 12:51 a.m. griffin729 HalfDork

One other thing that may work is a combination of some of the above. DIY harness layout board with a sheet of peg board. What you do is take the factory harness lay it out on the peg board as your template. Use pegs to define the bends, tape to mark out the paths, and label EVERYTHING. Then working off of the wiring diagram for the motor and ecu you can build your own harness, with continuous runs of wire, and with just the wires you want to be in the harness. This also will let you build the harness with whatever connectors you need and/or want.

I knew I had seen something like this before. eyesoreracing did something like this with the Miatabusa. One more reason why I'm a fan of Dave Coleman. Here's the link for the article on MotoIQ.
Dec. 4, 2011 9:27 a.m. 92CelicaHalfTrac SuperDork

^Ohhhh... i like that idea. I do have the factory harness and know which pieces will have to be lengthened.
Dec. 4, 2011 10:24 a.m. njansenv HalfDork

If I'm not mistaken, DIYAutotune has plugs that will work for the factory 3sgte harnesses. If it's like the Gen3 the plugs are the same as many hondas and other toyotas. Then use their MSII or III. :)

Nathan
Dec. 4, 2011 11:26 p.m. 92CelicaHalfTrac SuperDork

njansenv wrote:

If I'm not mistaken, DIYAutotune has plugs that will work for the factory 3sgte harnesses. If it's like the Gen3 the plugs are the same as many hondas and other toyotas. Then use their MSII or III. :)

Nathan

Which plugs? I have the ECU end plugs, already got a set of those from the Celica. I could use new plugs throughout the rest of the harness, though.

Don't think Megasquirt is in the cards for this car, though. Gonna have too much money wrapped up into it for me to want to try to tune it myself and blow it up.
Dec. 5, 2011 6:48 a.m. BigD Reader

92CelicaHalfTrac wrote: The BIG plus to all this is that i believe i can re-use my factory body harness with little to no changes, so i'll probably do that.

If you can, do this for the engine as well. I'm tired of reading threads from standalone installers who made their own harness and spend years chasing noise, heat and other problems. Toyota has already done all of the debugging to properly shield all sensors signals that need it, route the wires in the safest, neatest way etc. If the OE harness is in good shape, figure out what all of the pins do at the stock computer and adapt them to your standalone of choice.
Dec. 5, 2011 8:37 a.m. 92CelicaHalfTrac SuperDork

BigD wrote:

92CelicaHalfTrac wrote: The BIG plus to all this is that i believe i can re-use my factory body harness with little to no changes, so i'll probably do that.

If you can, do this for the engine as well. I'm tired of reading threads from standalone installers who made their own harness and spend years chasing noise, heat and other problems. Toyota has already done all of the debugging to properly shield all sensors signals that need it, route the wires in the safest, neatest way etc. If the OE harness is in good shape, figure out what all of the pins do at the stock computer and adapt them to your standalone of choice.

It's not in great shape. It's 20 years and 253k miles old. About 100k miles spent boosted with high underhood temps. It's pretty crunchy.

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