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Story by Matt Smith • Photos As Credited
We’ve all been spooked by electrons in one form or another, from harmless static shocks to threatening lightning. However, electrons also have the ability to …
Thank you. I needed that. 
Pro tip: don’t just buy a spool of 12 gauge red and use it for everything including grounds. 
Patrick said:Pro tip: don’t just buy a spool of 12 gauge red and use it for everything including grounds.
This was written for our first Lemons car, back when they would literally penalize you for spending money on different colors of wire.
In reply to Patrick :
Worse would be what my old boss did in a chemical plant (he thought he could fix anything)
He ran out of red, and decided green was just as good to use for hot legs. 
Tom Suddard said:Patrick said:Pro tip: don’t just buy a spool of 12 gauge red and use it for everything including grounds.
This was written for our first Lemons car, back when they would literally penalize you for spending money on different colors of wire.
It wasn’t a shot at you guys, it was a shot at the guy that wired my first WRX. Not even black tape at the end denoting a ground, just berking red everywhere
Patrick said:Pro tip: don’t just buy a spool of 12 gauge red and use it for everything including grounds.
I have read - and I'm not sure of the truthfulness, or of the age from when it came, if it is true - that every wire on F1 cars is the same color, the rational being that it forced those working on the systems to refer to the wiring diagram, and not assume "oh, that red must be a constant +12V." I'm not sure I believe this, but in thinking about some wiring I have coming up in the hopefully near future, I think I might adopt a variation on this. That is, something like all constant 12V are red, all power grounds are black, all sensor signals wires are blue, all computer outputs / commands are purple, etc.
When you're finished with all that, also put together two binders with full wiring diagrams of each system, fuse reference chart, one chart alphabetized by color for wire function, and another chart alphabetized by function for wire color. Strap one into the car and keep the other on the shelf or in your hauler. The amount of time that you can save by simply looking up "black-red" in a chart to find out what the heck that wire goes to is surprising.
In reply to kabohm :
Can I use that to wire fishing lights on my canoe?
NorseDave said:Patrick said:Pro tip: don’t just buy a spool of 12 gauge red and use it for everything including grounds.
I have read - and I'm not sure of the truthfulness, or of the age from when it came, if it is true - that every wire on F1 cars is the same color, the rational being that it forced those working on the systems to refer to the wiring diagram, and not assume "oh, that red must be a constant +12V." I'm not sure I believe this, but in thinking about some wiring I have coming up in the hopefully near future, I think I might adopt a variation on this. That is, something like all constant 12V are red, all power grounds are black, all sensor signals wires are blue, all computer outputs / commands are purple, etc.
FWIW, a lot of those wires have labels stamped on each wire. So you can see what it is when you look closely at it.
We had some test cars built that way, it was was horrible until someone figured that out. Even then, you can tell the lack of skill of the engineering company that did it (who I have bitched about more than once on this board, but that was 20 years ago now).
I've done more than one major wiring harness...
One thing missing- a peg board or a large piece of plywood where you can lay out the harness. On that, you use screws to go around corners, and map out exactly where stuff goes. That way the entire harness is laid out in an easy location. This is how it's done in real prototypes, and is pretty easy.
That does require a good drawing with dimensions of where stuff is and where wires go. From a hole in the bulkhead to corners to where one wire comes off the harness to it's plug. Repeat.
Doing that helps a lot.
Also, it's a LOT easier to NOT design your own harness. Which means if you can use the diagram of the car you have, and just take out what's not needed- do that. Let some other engineer do the hard work. If you have a simple digital copy- then load it into paint brush, and carefully use the eraser to take out circuits you don't need.
For the actual harness- for those of us who have old cars, you can contact Molex https://www.molex.com/molex/home and on that page, find the exact PINS you will use, and ask for an engineering sample of them. They will happily send you 100 of each. I did my GTV race car that way (using the original connector housings).
And while soldering should help, do bear in mind that no OEM solders wires. A good crimp connection is superior. So spend your money on a good crimping tool vs. soldering connectors. BTW, a crappy crimp sucks, and soldering is superior.
Lastly- don't be afraid. It's not as bad as it looks.
Buddy of mine buys these on ebay for as little as $80 and installs them for $200 or so. Each wire has its function written on it along the full length. Takes him about 4 hours to do a car.
I cant buy the wire and connectors for that $$$ .
Canoe deleted
If you are going to solder, make sure to use leaded solder, and have appropriate ventilation. Non-leaded solder has a tendency to crack under vibration. At least we aren’t flying helicopters where manufacturers demand RoHS...
I decided to re-wire my whole car (well, nearly all of it). The main reasons I made this decision were 1) 35+ year old wiring 
, along with relays, that were very costly, and likely no longer available. I ended up with an EZ, 20 circuit, wire harness kit.
Patrick said:Tom Suddard said:Patrick said:Pro tip: don’t just buy a spool of 12 gauge red and use it for everything including grounds.
This was written for our first Lemons car, back when they would literally penalize you for spending money on different colors of wire.
It wasn’t a shot at you guys, it was a shot at the guy that wired my first WRX. Not even black tape at the end denoting a ground, just berking red everywhere
Ugh those people need shamed, publicly
mistanfo said:If you are going to solder, make sure to use leaded solder, and have appropriate ventilation. Non-leaded solder has a tendency to crack under vibration. At least we aren’t flying helicopters where manufacturers demand RoHS...
When I rewired our whole chumpcar that's what I used. We cut something like 40lbs out of the car. We also got back working high-beams, working vtec, and running lights that are now so bright people complain about them (we were told they weren't bright enough by Chump staff).
Thank you for posting this article online. I'll be bookmarking it, as this is one of a small few major projects left on the Supra. The wiring in there now is functional, but it's not pretty, and a large amount of it is simply not used anymore. Also, it's tried to light the thing on fire at least once, and I'd like very much to avoid a repeat.
Please, for the love of all that is good in this world folks, "good enough" well, isn't, in the case of wiring. This is one of those things that really should be done with your very best efforts, as the number of headaches wiring shortcuts can cause is no small thing...
I've been down this road a few times before and I'm never satisfied and end up reworking or tweaking something anyways...
fidelity101 said:I've been down this road a few times before and I'm never satisfied and end up reworking or tweaking something anyways...
You sound like you own a Supra haha! "They're never done, just in their current state of modification..."
A critical feature of your new wiring harness is missing from this article ... the wire to use (and what not to use).
You should ideally use SAE spec wire, which is exactly what the OEMs use ... and there is a trick to this as well. SAE grade wire is not AWG sized. Because weight is always a consideration in an auto, SAE specs wire gauge to be slightly lighter than AWG gauge (we're talking about the copper content, not the overall diameter, although that is a bit smaller as well than equivalent number AWG wire). So be sure when you use the usual tables for current carrying capacity (most serious car stereo websites will have a chart) that your SAE wire is adequate for the length and current necessary (current from your calculations in the article). In some cases you might want to increase the SAE Gauge one step versus AWG.
What not to use? There are many appropriate alternatives to SAE grade wire, although they might be more expensive. The more common substitute would be teflon-dielectric* silver-clad copper aircraft / aerospace wire **, which race teams often use. You can buy it in bulk spools, but as in the ac / ao industry they are only allowed one splice per run of wire, (think of a large passenger aircraft and how long a run might be) so they will always have remnant spools and it is found on the surplus market for good prices. But there might be others you could consider. Why that particular wire? It's used in the engine compartment and any other high heat area as it can withstand very high temperatures without melting the dielectric and more importantly, where SAE wire would melt the dielectric (PVC).
Another wire you might encounter in automotive stores is Copper Clad Aluminum (CCA) wire. It is cheap and light, and many China-sourced large gauge cabling is CCA. Look for those letters and avoid at all costs. It "looks like" copper so be wary. Aluminum is brittle and breaks easily and only carries about one quarter the current at equivalent AWG as copper. Very common on cheap jump start cable sets, which you might have bought inadvertently. The rule of thumb is subtract 4 AWG gauge sizes to be equivalent current capacity as copper. So your "4 Gauge" Wall-Mart jump start cables are really equivalent to 8 Gauge copper. If you inadvertently used it for battery / starter / etc circuits, you have made a serious compromise in your electrical system capacity. *** Avoid.
Do not, under any circumstances, use "Primary Automotive Wire". This grade wire is rated only for wiring trailer wiring and connectors attached to the outside of the vehicle frame. It is a very low grade of wire and won't last, and is not appropriate for underhood or interior wiring. You will be setting yourself up for the same failure you're trying to eliminate by going through the re-wiring process in the first place. If it's not part of your trailer wiring, don't use it. Full stop.
The problem with PAW is it's often the only wire you can find at any automotive store. So do your wire order online from a site that specializes in automotive wiring applications. They will often have spool kits where you might choose 10 spools of different colour and the same SAE gauge, which makes a colour coded harness easier.
You can buy makers in little books that are numbered or lettered, and create markers for same-coloured wire, or just do so for ease of maintenance later. There are also ones that just have sequential numbers to mark your wiring diagram and the wire. There will be multiple examples of each number or letter so you can mark your run at specific intervals (every three feet, for example) or at the terminating ends, or both. Electronic supply shops like DigiKey or Mouser will have them. You can also mark single-colour wire with a line along the run with a sharpie to make (for example) a red with black stripe wire. This saves you the trouble of getting too many different coloured spools of wire.
RE: Solder vs Crimp
I actually often use both solder and crimping. "Tinning" a bare wire with solder has certain advantages prior to crimping. In some cases you can solder after crimping as well if you use bare crimp connectors and cover with heat shrink. The reason OEMs don't solder is because it's not conducive to the manufacturing workflow, and also because a soldered wire is stiff and there is a chance a wire will break right at the soldered / bare point. If you are careful and avoid bends at that point (think about maintenance as well if you disconnect a connector) it's fine and may be superior to a crimp alone.
A nice straight section beyond your solder point in your layout can be used to mitigate the problem, as well as judicious use of shrink tubing as a strain relief (you can use multiple layers of shrink to stiffen a strain relief). You can buy "kits" with double-layer adhesive lined shrink tubing which is excellent for outdoor applications as it's air and water tight, while it's cheaper as single layer without adhesive 4 foot sections or spools. You can add adhesive to plain shrink by using a small blob of silicone sealant before you shrink, but use 3M electronics grade silicone if you do, as ordinary silicone sealant can be corrosive to electrical connections. Up to you. A bad crimp is worse than a soldered connection, but a good crimp is very good electrically. Do what you feel is appropriate at each specific connection.
Someone mentioned lead solder (tin-lead) as the ROHS directive in the EU has pretty much eliminated lead solder in many applications and substituted some form of tin-silver solder. All you need to know is tin-lead is much easier to work with, and for automotive applications ROHS does not apply, even in the EU. Tin-Lead is also mandated for aerospace and military applications under ROHS. Kester 44 is an excellent brand / formula of tin-lead, but there are other brands that are fine. Choose "eutectic" formula which is 63/37 mix, rather than 60/40, as it transitions from solid to liquid better which makes working with it easier. Buy a spool if you intend to solder anything.
To reduce the chance of water ingress at certain points, you can layout a cable with a loop at low points on the run. Water will then drip from the bottom of the loop, instead of into your wiring or connectors. Do not bend too tightly, a nice no-strain loop is preferred. A couple of inches in diameter usually is good. Doing so also helps avoid strain by too tight a run, so if your layout has extra length (and it should) add a loop so your layout is clean but you can still disconnect something and work on the vehicle.
The reader who suggested a sheet of plywood for layout made an excellent point. I sometimes use shelving planks from the usual hardware stores instead, handy for long layouts that might extend beyond 8 feet. Also up to you. Don't be afraid to use your sharpie to mark your layout before you actually build your harness on it.
You can buy inexpensive stiff PVC-coated wire for tying plants to stakes (like your garden tomatoes) from hardware stores or (my favorite) your local Dollar Store. You can use it to lay out a proposed wire run on the vehicle, and then transfer that to your harness layout on the bench. Makes it easy to deal with bends and routing without trying to measure it with a tape. Another cheap tool is a sewing tape, they are fiberglass and last forever, and 60" long (usually). Maybe $1~$2. Makes it easy to measure non-straight layouts.
* Dielecric: The technical name (it describes the electrical properties) of what we mortals call "insulation" on wire.
** The silver cladding on each wire is not there for electrical properties, although silver is slightly better current capacity than copper. It's there for corrosion resistance; bare copper is subject to corrosion mostly due to atmospheric conditions (contaminants and moisture in air). You know ... that green corrosion on bare copper. Low Grade PVC wire might not be gas tight either ( cough - primary wire - cough) so corrosion can creep up well into the covered length, which embrittles the wire and leads to "invisible" breaks inside the coated portion of the wire.
*** Reasonably priced large gauge wire can be found at welding supply stores. Be sure to carefully lay it out if you use it, as the rubber dielectric is not particularly resistant to cuts and cracks. You can coat it with heat-shrink tubing for added durability. Avoid sharp bends and always use grommets and appropriate sized wire clamps that bolt to mounting points, not wire ties, to route it. You can solder & crimp large gauge wire, but will have to use a heat gun to pre-heat that much copper before soldering.
What about the use of resetable breakers instead of fuses? I'm fond of the idea of just flipping a breaker back after a repair instead of digging through a fuse block to find the one dead fuse?
Definitely going to be relevant to me in the hopefully somewhat near future as I plan to largely (if not completely) rewire the DMC when I tear it down to fix the frame rust problems. It will be a lot of work- but so is trying to find where the wiring problem is on 40-year-old British wiring assembled in Ireland by inexperienced workers...
NorseDave said:Patrick said:Pro tip: don’t just buy a spool of 12 gauge red and use it for everything including grounds.
I have read - and I'm not sure of the truthfulness, or of the age from when it came, if it is true - that every wire on F1 cars is the same color, the rational being that it forced those working on the systems to refer to the wiring diagram, and not assume "oh, that red must be a constant +12V." I'm not sure I believe this, but in thinking about some wiring I have coming up in the hopefully near future, I think I might adopt a variation on this. That is, something like all constant 12V are red, all power grounds are black, all sensor signals wires are blue, all computer outputs / commands are purple, etc.
Airplane wires are all white. Yes they have codes Imprinted in them. At least Navy Planes.
I work in aerospace and about the only thing I can add to Johnny2Bad's excellent post is to prevent vibration to wires and harnesses. Vibration work-hardens copper, leading to hard-to-troubleshoot intermittent problems located way up inside harnesses.
Proper mounting means rubber lined points anywhere necessary to keep it away from sharp or abrasive surfaces. We use a lot of carbon fiber and it's very abrasive. More than a few times we've had cable failures where it was allowed to simply rest against the carbon surface. Not helping is that carbon fiber is electrically conductive, further adding to troubleshooting confusion.
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