Car Electric Circuits
The 'car' electric circuits do everything but run the engine, so they include things like wipers, heater fan, head lights, tail lights, stop lights, indicators, hazard system, reverse lights, cigarette lighter, stereo, alarm, demister etc etc. The car circuit is further broken down into systems that operate irrespective of the key position (headlights), things that operate only when the engine goes (wipers) and items which work with the engine turned off but not when the key is removed (stereo). I refer to these three sub-systems as permanent, run and accessory respectively:
- Permanent - things you want to always work even when the key is removed - lights, stop lights, horn, hazard lights, computer memories, alarms, immobiliser.
- Run - things that only go when the engine runs - like wipers, windscreen washer, heater fan, demister, radiator fan, instruments, fuel pump, indicators, reverse lights etc.
- Accessory - things you want to go without the ignition being on, but that will turn off when you take the key out. This includes things like a car stereo. Remember digital tuners and EFI computers etc additionally require permanent power to keep the memory/engine tune settings alive.
Engine Electric Circuits
As you would expect these circuits power the engine ignition system and are quite simple if you run carburettors, but not quite so simple if you are on EFI!
Selection of System Features
Before you can design your wiring diagram you need to know exactly what systems your car will use. The decision on whether or not to have a heater or a radio or reverse lights is up to you, but it is wise to wire the car once and be done with it! This is another extension of my argument for thinking in some detail about what you want the car for, and the type and frequency of use it is likely to get. This is a hard decision and a few harsh realities must be honestly faced. Obviously a wiring loom for a track only race car will be considerably simpler than that for a luxury GT car, but the point in between that applies to you must be found.
Electrical Component Selection
There are a few things which will influence the type and source of components you select for your car. These include car style, personal preferences, budget and availability. It definitely pays to talk to owners of similar cars and see what they used, where it came from and what it cost. This gives you a good opportunity to see what you like the look of and prevents you from buying several items as you keep finding better ones. The first physical step towards wiring your car will be the acquisition of all the electrical components which includes such things as instruments, switches, head lights, indicators, stop lights, wipers, washer, heater fan, horn, electric fan, stop light switch, engine loom, car stereo, electric windows etc.
Components can obviously be bought new or second hand. Second hand invariably means identifying cars which have the components you want followed by finding an example of that model in a wrecker’s yard with the components in suitable condition. Anything that is sourced from a donor car should include the relevant plugs and as much attached wire as possible. It also pays to note the exact model and year of vehicle.
New components are available from a variety of manufacturers and come in all manner of subtly different designs. You can spend a lot of time visiting different suppliers and comparing products before you find the style you want.
It is important to note that although electrical components, especially lights, switches and instruments can be very expensive they tend make the difference between having a completed car looking ‘right’ and looking like it is a refuge for trailer and derelict car parts. A close look at a range of different kit cars will soon demonstrate what I mean. The reality is that the cost of instruments etc must be viewed in the light of the overall cost or value of the finished car.
Identification of Component Operation
The reason you need to be able to identify the exact model of car any liberated components come from is so you can perform the next step - collect a pile of change and visit your local library. Take with you the plugs and wire stubs and head for the reference section. There you will find Haynes-type repair manuals which have wiring diagrams. Photocopy the sections of the diagrams that relate to your liberated equipment, ensuring the wire colours match as close as possible.
If a lot of your parts are Japanese Import a kind word to the local brand dealer may result in you being able to copy their service manuals, which may be of use. Excluding some engine diagrams most ancillary car electrics are quite simple and common regardless of the country the car is sold in. Often wire colours may change but the schematic will not.
If you are unsure of the terminals on flasher units, alternators etc the Hella parts catalogue (and similar) that places like REPCO have show the terminals on the replacement Hella parts which are in the same layout as factory items.