As this is the race and rally side of the forum its the best place to start a thread on how to build a wiring harness for a circuit/ rally car.
With most builds, people will adapt what they have to make it work but to maximise your weight reduction and simplify the electrics for a car, you really need to start from scratch. Over 20kg can be saved when removing everything and starting again.
You can also save a lot of time in removal and installation of the engine, gearbox, dash with a quick release system.
With the right switches,relays, fuses can reduce the time it takes to find a electrical problem and resolve it.
I do not have the answers to building a basic effective system or one with all the bells and whistles.
I'm hoping that someone will be able to fill in the blanks for people like myself in how and what to do, what not to do.
 
 
 

Building the ultimate wiring loom?  It's not a small job, but it is a satisfying result.  Start with a basic job list, and include what modifications you intend to make to the standard layout.  Will you retain the factory dashboard, aircon system  (seriously, aircon in a tarmac rally car for example is a good thing.  Long events hinge on driver comfort and concentration), what lights (if any) will you need.  Are you running the factory ECU, and is your ECU in the standard location?  Is your battery in the standard location, and will you be fitting an isolator switch?
 
Each job will be subtly different.  Another twist is what are you starting with?  Do you have a complete roadcar that is being stripped out, or are you jumping into somebody else's project?
 
Scenario #1 - (Potentially the easiest) You've got a road going SW, it's going to be a stripped out, caged racetrack only weapon.  You're happy to lose the aircon in this case, and battery and ECU will remain in their factory locations.  First step is to remove EVERYTHING CAREFULLY.  Don't be tempted to rush, cut or hack.  Once ALL of the loom is removed, lay it out on the workshop floor ( or a pingpong table if you have one )  and start carefully removing the wrapping from every section.  Take it slowly, putting a twist of tape every foot or so to stop it all getting tangled.  Once it's unwrapped, and with your wiring diagram folder close at hand, start tracing every circuit you want to remove.  Word of warning, before you remove wires, double-check that you aren't taking a shared earth path out of the circuit.  If you're able to, don't cut wires off the back of a connector - in most cases the pins can be removed from the body of the plug.
 
OK, once you've got the loom pared back to the minimum that you need re-fit it to the car and take the time to check that everything still works.  Once you're happy, THEN you can start re-wrapping it.  Because it's a race car, you really don't need to do a 150% covering.  I wrap the curves (in situ so the loom keeps it's shape around the curve) then use split-sheathing over the longer runs.  This way if you need to modify the loom it's easy to access.
 
 

This is a great thread and should generate some interest. Given the wiring would be 20 or more years old on most of the sdubs and adubs, would you not be better off replacing it rather than refitting the pared down loom?

Ahh why did you post this up? Im going to be here for hours!
 
I am by no means a professional auto elec, altho my family were auto elecs, but as some of you know im building a race car, so just like you, im re wiring the whole car. When i say re wire, i mean everything, there is NO factory electrical component left, no wire, connector, dash, fuse box, anything, exept alternator, starter ect, but even like tail lights have been rewired, led globes to reduce current draw, reduce wire size, reduce weight ect.
 
I am a cabler my trade tho, exept buildings, houses ect.
 
So professional race car wiring is a passion and hobby of mine. But its not cheap! 
 
I could type away for days ha ha but wont, but heres some basic stuff.
 
 
Factory wiring (in modern cars) is not a bad thing, it is very reliable, designed for and routed in a way it won't get damaged, under conditions the vehicle is designed for.However when the vehicle is to be used under different conditions and purpose, for example as a rally car, factory wiring wont stand up, it is driven under much harsher conditions, water from river crossings ect would certainly get into factory connectors and fuse boxes causing failures, so connectors used need to be water proof, wiring can be damaged from collisions or even a fault in a loom or connector and need to be replaced very quickly during a short service break, getting to, removing, and replacing a factory loom in a section of the car could take hours, and hours, with a purpose built loom it may be able to be removed in seconds or minutes. 
 
Connectors
Commonly called mil-spec (military specification) connectors, professional circular connectors used in motorsport (F1, WRC, v8 supercars, ect) are in fact not 'mil-spec' but 'motorsport-spec' or 'autosport-spec'. Made to be much smaller, denser and lighter, using aluminium and have a lip for molded heat shrink boots to shrink over were as 'mil-spec' typically are larger and heavier materials and typically have a threaded section on the end for a back shell support bracket / clamp to screw on to hold the cables.
 
Commonly used connectors in motorsport are Deutsch DT and DTM series connectors, which are quite cheap and are made from thermoplastic and use pins that are crimped onto the end of the wire with a specific crimper that crimps 8 indents into the crimp barrel to firmly attach it to the wire.
 
Then you go up to the best of the best, the Deutsch Autosport range, these are aluminium shells, with lips for molded heat shrink boots, high density, can get in flange mount (for wire wall mount ect) and use the same style pins as the DT and DTM series, but requires better tooling, you will find these connectors used thought the best race cars out there, from v8 supercars, WRC, ALMS, F1 ect.
 
Helpful note: DT uses size 16 pins and DTM uses size 20 pins. Autosport can have big size 4 for battery power, 16, 20, 22, 23, 24 and smaller and a combination, refer to brochures.
The smaller the number, the smaller the wire its designed for.
Size 16 (DT) is rated to about 14Amps (20A for autosport pins) and size 20 is around 7A, autosport 22 is 5A and then you can get really small.
 
For joining wire and splices mechanical crimp is preferred over soldering. Soldering has no place on a race car, every high end motorsport loom, aviation, military and even factory looms will have no solder! Vibration and flexing can cause solder to fail. However this required tooling, experience and crimp connectors, however solder will be better then a poor crimp.
 
Tooling
For crimping pins, specific tooling is required. For Deutsch DT and DTM cheap 'economy' crimpers work ok and can be found on ebay for around $50 each, the DT series uses size 16 gauge pins so a 16G crimper is required, and for DTM size 20. The next best tool is the Deutsch HDT crimper which allows you to adjust to different wire sizes, for example if you used a 20g wire in a 16g pin, if you crimped it with a standard 16 crimper the wire would be loose.
For Deutsch Autosport the tooling is much more expensive! For the smaller pins size 20 and down you need a DMC AFM8 crimper tool and then specific ' positioner' to suit the different size pins, these basically position the pin and the crimp bucket so the crimp is made in the correct spot.
For size 16, 14 ect a DMC AF8 tool is required, with positioners or multi positioner called a turret.AFM8 and AF8 crimpers cost around $500! each new from supplier in Australia, or from ebay around $350 or old aviation ones pop up alot. Positioner are around $50 - 100each.
 
For mil spec wire listed below such as Raychem Spec 55, 44, due to the hard insulation, it is very hard to strip with pliers, side cutters ect and most wire strippers, they will not cut the insulation properly or will nick or scrape the wire. Specific strippers can be bought with correct dies to suit the spec 55 or 44 wire, i would highly recommend these however they cost around $400
 
Wire
Standard automotive PVC covered wiring is heavy, thick, burns well and produces alot of smoke and insulation can melt when heated due to covering with heatshrink or especially molded heatshrink boots, which requires alot of heat to full shrink to shape.
The most common wire used is aviation wire, Raychem Spec 55 or Spec 44. Spec 55 is insulated with radiation cross linked ETFE polymer, which basically has a higher heat rating and is a much stronger material over pvc, meaning higher abrasion resistance, resulting in a MUCH thinner insulation wall, resulting in a wire much lighter and smaller in size then the equivalent PVC covered with the same amperage rating.
Raychem Spec 55 and 44 current ratings are similar to the pin ratings as i wrote above.
 
Coverings
Motorsport spec looms will be covered in heatshrink, the most common and the best is Raychem DR-25, have a look on a F1 car, v8supercar, wrc ect, you will see the words Raychem DR-25 printed on it. DR stands for Diesel Resistant, and is resistant to pretty much all fuels and oils, is flame retardant and is very soft and flexible when shrunk and has a very good abrasion resistance.
Raychem also make molded boots that when shrunk form a shape, typically used to seal the wiring onto a circular connector, either straight or 90 bend, but also T and Y boots for spices in looms ect.
 
A potting compound or epoxy is typically used in or at the end of boots and transitions to ensure its fully waterproof.
 
If your rewiring the whole car i would ditch the fuses and relays, i would use circuit breakers (panel mount ones that can be used as a switch) or even better a PDM (Power Distribution Module)Have a search for PDMs, most can be standalone or connect to your ecu or other sensors.Most common here are Motec PDMs and Racepak Smartwire, but theres also Pectel, Bosch, Life Racing and others.
A PDM has a main battery connector on top and then a whole heap of inputs and outputs, it is solid state, no moving parts!
The number of inputs and outputs will vary depending on model and brand, but each output an input is fully configurable. Some outputs may be rated to 20A and others 8A for example, most PDMS you can join 2 20A together and have a 40A output, or you can program a 20A output to be 12A, 14A whatever you like, its just programing.

 If the output exceeds the value you have entered, for example 14A, it will trip, you can program the PDM the number of times it will try to reset the output and how long to wait inbetween tries. No need to replace fuses, it will keep 'replacing the fuse' as often as you like. For critical items like fuel pump you would have it trying quickly to reset, for high current items thats not so important to get back working quickly, such as blower motor (heater fan) you could say wait 20 or 30 sec to retry (allow wiring to cool down)

The inputs are also programmable, and what outputs they control. If its connected via CAN network to your ECU or other sensors it can also use those parameters to control outputs!
It can be basic such as fan switch input turns on fan, start button input turns on starter or complex and smarter such as when i press the start button, turn on the starter motor ONLY if the RPM is less then 1,000 (so wont start if the car is already running) AND to turn off non critical things like headlights, blower fan, W2A pump, wiper motor ect, to reduce the current draw on the battery while starting, then after start button has been released, turn those outputs back on, but stager when they come back on, for example headlights 2 sec after, wiper 4 sec after, blower motor 10 sec after ect, so theres not a big inrush or spike of current draw, everything turning on at once, how smart it its it just as smart as your imagination and what you program it to do!
 
Also with my Racepak smartwire, or the Motec PDM when connected with the logger dash, you can not only connect a laptop and re program, but also view current dray of each item! It will show you how much each output is using, very accurate! Plus you can log each output and input, current draw, status ect.
 
So now with data logging, not only can you see what gear your in, how fast, temps ect, but how many amps the fuel pump is drawing, if it tripped, when the fans turn on, how much load in total eveything is drawing. Very cool.

Some of the tools required for making my looms. Those cheap looking strippers on the right arnt cheap, they have specific mil spec die/blades to suit the Raychem wire.

^^^
 
Yep, I know, this kid HAD too much money...  I always give him **** about doing all this on his '****box mr2' but at the end of the day, it does look very very nice.  Maybe I am just green with envy?

I was hoping for some great responses but that was just amazing! 
I know you love it D & B, so keep on typing as its a fasinating subject.
If people are looking for a budget wiring system with relays and fuses, how do you work out the correct guage wiring to allow for high pressure situations. If you are using 'A' for street application, do you increase guage by 50% for rally/endurance work?
How much time should be the budget for a rewire?
Should all data loggers, digital dash, ecu, gps be on separate circuits? I know of a few people having problems(including me) with theses set ups. Where do you go with insulation with these systems?
 
 
  
 

Haha, definitely something you'd only do if you had a passion for the wiring side of things. The weight savings and reliability gains become fairly negligible at this level, but it does look pretty sweet. We're all wasting money on cars, so why not waste it on the areas you enjoy.
Personally I just use basic crimp connectors, heatshrink, electrical tape, split tubing, etc. Old wiring looms are a good source of multi coloured wire and basic connectors for cheap. Never had an electrical issue from something I've modified.

Our University SAE car had a full custom wiring harness, with laser marking and proper milspec connectors. Though we were lucky enough to have BAE manufacture it for us. I wasn't really involved much in that, but I do remember one of the crimping tools we borrowed was worth $1700!!

I wouldn't think you'd have to decrease gauge at all.  Certain accessories are only going to use a certain amount of current.  There are two things you have to look at with wiring, current carrying capacity and voltage drop.  One is static whereas one will depend on the length of the circuit.  If you get a good quality wire you can likely decrease the gauge of the wiring.  As generally current carrying capacity is greater.  This is generally due to better thermal and conductive properties of the material.  Something that can radiate heat faster will allow for a larger amount of sustained current carrying capacity whilst still having the same nominal cross sectional area. 
 
As a normal road car, the wiring has to be able to carry the current for sustained periods of times, tens of hours at a time, so I am sure the gauge of the wiring is sufficient for the standard accessories installed.  When you go fitting bigger fuel pumps etc that draw more current it is obviously apparent that you may need to install larger wiring.  Manufacturers of wiring will specify the cables current carrying capacity.  The manufacturer of the fuel pump will also supply a current draw chart probably in the form of current vs pressure.    Marry the two up and you should be reasonably safe.

 
You would only need to replace the wire with thicker if the current draw will increase, wire length is increased or i guess the duration of use increases.