Planning 

I can’t emphasise enough the importance of initial planning. No doubt you have spent a lot of time thinking about car type, body style, engine and gearbox choice, wheel style and the fun stuff like paint colours and trim, but probably very little about electrics. Yes, you know you need wiring, but its just like brake lines - you fit it around everything else and it will work, won’t it?

Using a complete loom is not a silly idea if there is one that will do the job. If you are building a small car running a 1600 Ford pushrod motor then it may be worth using a complete Escort loom. I initially wired my Leitch in this fashion using the loom from a written off Toyota Levin I bought as a source of the engine etc. With 20/20 hindsight this option was not as good as it could have been. It is very important to label all the wires before you remove them from the car, in some manner that will last until you finish the project. There is nothing worse than trying to figure what went on the end of a plug 18 months after you removed it.... Also label any plugs that do not connect to anything otherwise you will go mad looking for the missing bits!

Because I used a Toyota engine/computer and had the entire loom I decided to adapt the loom to my Leitch. By the time I worked out how some of the bits worked, removed those I didn’t need, spliced in the instrument loom (ex-Triumph), wired in the switches (all aftermarket), hooked up all the lights, wipers etc and shortened or lengthened the loom as required it would have been quicker and easier to do it from scratch, especially if you do not have an EFI system to worry about.

I wired a 20 Valve Fraser 18 months ago and designed the car loom from scratch but used the Toyota engine loom (due to the usefulness of the engine bay fuse/relay box), adapted as necessary, and it has proven a much simpler job. I would only really recommend using a complete loom if you have the space to fit it in the car, it is more or less exactly what you need and will require minimal modification.

Car Electrical Circuits 

Hands up all those who have tried to chase an electrical fault on a car wiring loom diagram. Thought so. It is just about impossible, especially on any car assembled in the UK in the 60s or 70s. Generally the diagram is condensed on to one A4 page and you need an electron microscope to read it. To 'help' even more they only used 6 wire colours. The final straw is mid-production run changes that are not clearly defined, so you don't know which diagram applies to your car. Luckily white or red was power and black was earth.

This is nicely contrasted by the Japanese approach through the '80s and '90s where the car wiring diagrams fold out of the repair manual like some perverted electrical engineer’s centrefold. The experience is heightened by the fact that there tends to be half a dozen of these centrefolds in a row, and naturally the wires disappear off the next but one diagram. They use hundreds of different colours, any one of which can be power, but white is earth. But white is also used for main battery feeds...fortunately your electron microscope will show you that there is a thin black line on the earth wire...

These kinds of experiences certainly did nothing to entice me into a career as an auto electrician (although I unsuccessfully attended a job interview with one when I first left school - don’t let that vote of no confidence add to your skepticism of this article!), and yet I have managed to wire three cars without a mishap. The key is taking one step at a time, and it begins with thinking of a car electrical system as a collection of 3 smaller, easier understandable sub-systems. These three basic systems are the power supply, car electric circuits and engine electric circuits. The first two systems are largely the same irrespective of the engine type, however, the last one varies depending on the use of carbies or injection.

Power Supply 

The power supply comprises the battery, alternator, voltage regulator and fuses. Think of it as the components and wiring that provide electrical power ready to be used everywhere else in the car. The system stops basically at the fuses while the circuits on the other side of the fuses decide what that power will be used to do.