SpeedTECH The definitve guide to EFI transplants


How to Wire Your Car 

(When all you know about electricity is that it can kill you or put your car on fire)

Page Contents 

Methods of Wiring a Car Planning Car Electrical Circuits Power Supply Car Electric Circuits
Engine Electric Circuits Selection of System Features Electrical Component Selection Identification of Component Operation Wiring Diagram
A Little Necessary Theory Wire Current Ratings Wire Battery Earths
#Alternator #Fuses Switches and Relays Engine Systems Lights - General
#Headlights Tail and Stop Lights Hazard and Indicator System Reverse Lights Wipers and Washer
Fuel Pump/ Oil Cut Out Multi Speed Fans Radiator Fan Warning Lights and Instruments Tools and Equipment
Tool Shopping List Soldering Terminals Physical Wiring Conclusion


I never cease to be amazed at the number of enthusiasts I meet who are quite happy to undertake any of the mechanical aspects of a car build or restoration and yet they run a mile at the prospect of wiring the car themselves. Most of us take the practical approach when confronted by something we know nothing about: we press gang a friend who can help!

This is all well and good, but if you canít find a victim then your options rapidly reduce to having to pay someone to do it. While this is not a bad thing in itself - after all, some things are best left to the experts (wheel alignment leaps to mind) - the problem is that any wiring job, especially one like custom wiring a car from scratch takes a fair bit of time, particularly if you want a neat and tidy job. Unfortunately this will cost a lot of money, assuming you can find an auto electrician who is willing and capable of tackling the job...

Wiring a car yourself is one of those jobs that looks impossible at first glance, however, further examination will reveal that in reality it is NOT beyond the average enthusiast. The key is to think about what you are doing, plan thoroughly and take your time. Physical wiring is merely a case of stripping wires and soldering terminals on, which takes a lot less skill than welding, panel beating or painting!

I have completely wired three cars now, including my own car twice (due to a total rebuild and upgrade). The last thing I am is an electrical engineer, so if I can do it there is honestly no reason why you canít. The three jobs I have done from scratch have all included EFI engines and I am about to embark on another complete car with EFI. With the acquisition of a few specialised but relatively inexpensive tools you can wire your car as good as if not better than any professional, and save yourself money while you are at it.

I have read a number of articles over the years in magazines on how to do car wiring but they have always struck me as being somewhat superficial. My intention is to provide a step by step guide to wiring a car yourself, and I have endeavoured to fully cover all aspects of the subject in sufficient detail to enable you to successfully complete the job. Whilst I have endeavoured to ensure a high degree of accuracy I accept no liability if your car catches fire!

Methods of Wiring a Car 

There are four main ways of wiring a car I can think of:
using a complete loom from a donor car 
creating your own wiring loom from scratch 
a combination of a and b 
paying someone to do it all 
This article deals with wiring from scratch as it is the most complicated but the principles apply to any major wiring job. Although the last option is always a fall back if things go horribly wrong, they shouldnít if you plan carefully!


I canít 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, wonít 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 didnít 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 engineerís 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 - donít 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.

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 wreckerís 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 Ďrightí 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.

Wiring Diagram 

This is where the diagrams you have begged, borrowed and stolen come in, however, any bits you have not been able to get diagrams for you will now have to work out. You can do this by using the continuity function of your multi-meter, which allows you to figure out which connections go where under various switch positions etc.

This is actually quite easy if you take your time. Just make a drawing of the terminals on the item and see which ones are joined together. Obviously an idea of how the device works is useful and once again the wiring diagrams or parts catalogue drawings will come in handy, as there are generally only a couple of ways of wiring, say, windscreen wiper motors. Some components you can carefully remove covers from to see how the internal connections are made. It is a bad move to try hooking power up to things and seeing what happens...

Once you have worked out how each bit works you need to integrate the lot into a wiring diagram. I try to think of the car as three main systems - power supply, car electrics and engine electrics/electronics. I draw out the wiring diagram with the power at one end supplying a common Ďmainsí with car systems off one side and engine off the other. This results in a tidy diagram overall and a logical map to find your way around. Design one circuit at a time, and when you have it done, transfer it to the overall diagram. It is a good idea to have a large overall diagram - it makes it a lot easier to understand - A1 size graph paper is good and only costs a couple of dollars a sheet. The Fraser was all put on one sheet - including the entire EFI loom.

Obviously there will be links between some systems but try to keep these as neat as possible so you donít end up with a tangled mess. Most modern cars switch the earths for many circuits - this method provides the most fused protection, however, there is no reason to complicate matters more than you have to, so apply a little common sense to prevent running wires to and from a remote item - switch the power and earth at the component. Be aware of how each switch operates - does a two position switch turn off position one when position two is turned on, or are they both on?

Remember, it is your car, so you can do what you like. You may want to have it so the radiator fan runs whenever the engine is hot, even when the car is turned off - it is up to you. The thing to remember is that you donít want to turn the car off and have indicators or something else still working that will flatten your battery. Make a complete diagram and wire the car to it. Remember to modify your diagram as you go so that it accurately reflects the finished job. This makes it a whole lot easier to fault find on later.

A Little Necessary Theory 

Time for a little electrical theory now. In order to size your fuses and select thick enough wire, you need a couple of formulas:
Power (Watts) = Volts x Current (Amps) or Current = Power/Volts

Power = Current x Current x Resistance

Volts = Resistance (Ohms) x Current (Amps) or Current = Volts/Resistance

OK, three formulas!

A 12 Volt car system is not actually 12 Volts, its closer to 14 (13.8 officially). So, to work out the fuses add up the power used by all items in the circuit. Say you are doing park lights. You have two tail lights at 6 Watts (written on the lamps), 2 number plate lamps at 3 Watts, 2 front park lamps at 4 Watts, 6 instrument lamps at 2 Watts and 2 interior lights at 6 Watts. This totals 2x6 + 2x3 + 2x4 + 6x2 + 2x6 = 50 Watts. At 14 Volts this equates to 50/14 = approximately 4 Amps, so use a 5 Amp rated fuse and wire.

If you donít know the power (Watts) a motor uses, say a fan motor, use the resistance (Ohms) function of your multi meter and measure the resistance of the motor. Say it is 3 Ohms, then the current used at 14 Volts is 14/3 = about 5 Amps, so use a 5 Amp fuse. Alternatively if there are a few things in the circuit add up the Watts. To work out the Watts the motor is, Power (Watts) = Current x Current x Resistance = 5 x 5 x 3 =75 Watts. Then you can add it to the others to get a total number of Watts in the circuit, and convert it back to Amps.

Wire Current Ratings 

Conductor Loomed Wires Unloomed Wires
Cross Sectional Area (mm2) Number and Diameter of Wires (mm) Current Rating (Amps) Volt Drop Per Amps Per Metre (mV) Current Rating (Amps) Volt Drop Per Amps Per Metre (mV)
1.0 1 @ 1.13 11 40 13 40
1.5 1 @ 1.38 13 27 16 27
2.5 1 @ 1.78 18 16 23 16
4 7 @ 0.85 24 10 30 10
6 7 @ 1.04 31 6.8 38 6.8
10 7 @ 1.35 42 4.0 51 4.0
16 7 @ 1.70 56 2.6 68 2.6
25 7 @ 2.14 73 1.6 89 1.6
35 19 @ 1.53 90 1.2 109 1.2

The way to interpret this table is as follows - if you are using two 130 Watt head lamps fed from a single wire that is 3 metres long then you will draw (130 x 2)/13.8 = 18.8 Amps (A car is closer to 13.8 Volts than 12). If you use say 2.5 mm2 wire (this rating because the wires are loomed together) then the voltage drop is 16 x 18.8 x 3 = 902.4 millivolts which, since there are 1000 millivolts in a Volt means you are only getting 13.8 - 0.902 = 12.9 Volts to your lights.

As a starter motor is easily rated at 1 kilowatt if the battery is in the boot, say with 4 metres of wire between it and the starter motor, then you have 1000/13.8 = 72 Amps current draw (and more once the motor gets hot, which it will if your car needs a bit of cranking). Say you used 16 mm2 wire Ďcos you bought the closest one to what you needed (and the wire is not loomed up), so now your voltage drop is 2.6 x 72 x 4 = 748.8 mV, so you still get 13 Volts to your starter motor - provided your battery is in perfect health. If you had used 10 mm2 wire you would be down to 12.6 Volts - see how quickly you can get to the point of not being able to start your car?

Basically if you size the wires according to the above table you will be fine. So, if you need a wire to take 7 Amps in a loom then you need only 1 mm2 wire, which is actually the cross sectional area - 1.13 mm is the diameter, which the table also gives. Note that the bigger wires are made up of a number of smaller wires twisted together. The number of strands will often be more in a car - these tables are normally for domestic wiring which has thicker strands. The maximum loadings still apply.


5 and 7 core trailer flex is about the best thing around for doing the wiring runs to the front and rear of the car. Standard configuration is rated at 14 Amps and you can get heavy duty rated at 18 Amps. You will rarely need more than 7 wires to the rear of the car, and you can always run two lots of 5 core. Regardless, it prevents having to loom the wires together and can look very neat and tidy. 14 Amps is quite sufficient for 130 watt head lamps as well, provided left and right are separately fused.


One of the most important components of your electrical system will be your battery. Buy one big enough for the job. If you donít plan on using your car often either buy a battery big enough to start the car after lying for a month or keep it regularly charged. The battery should be placed in a location where it can be securely mounted while placing it as far as possible from hot areas like exhaust pipes - batteries die real quick if you cook them!

It also pays to position it so you can get jumper cables or a battery charger on it without having to remove half the car first - sooner or later your car battery will either go flat or you will be asked to help some one else whose battery has, and it is not much fun if you have to take a wheel off (invariably in the dark and in the rain after you forgot to turn the lights off when you went into the movie theatre three hours ago) (of course you have 90 watt dip bulbs so the battery is really dead!) to get at that battery terminal - fine for a race car but not so hot in a road car... of course you can always add a couple of remote contacts.

By the way, my car is injected with a standard Toyota EFI system and has been jump started and used to jump start other cars and never suffered a problem through voltage spikes. I always hook up with the engine off, but doubt it makes much difference!

Additionally, if you intend to corner hard it pays to orientate the battery so the filler holes are in a line fore and aft. As they sit in the middle of the battery this means the acid has to slop further to escape and start eating your panel work. I have the plates barely covered as any higher the electrolyte slops out when I scream around corners.

It is a good idea to place them as low as possible too, as they are obviously heavy, so you may as well have them contribute to lowering the centre of gravity as opposed to raising it! Some prefer to place them in the boot due to under bonnet clearance or appearance considerations, or to use its weight to counter balance the driver etc. If you decide to put it in the boot or car interior etc it should really be placed in an enclosed battery box that is externally vented. You need to think about where the acid will go if you roll too - I for one would not like to be trapped upside down with what little remaining hair I have lying in a pool of battery acid!

If you have a long run between the battery and the engine or dash etc it is a very good idea to use a single very heavy cable between the battery and the starter motor, and donít forget a good earth either. As shown above a really heavy wire is not a bad idea. The last thing you want is a sluggish starter motor because there is only 10 Volts getting to it. You must also ensure that the wire is heavily insulated. This is because it will not be fused and the last thing you want is the insulation wearing through and the wire contacting the body... Some people use arc welder hand piece wire as it is heavy duty and well insulated.

Now, since you have power permanently connected to the positive terminal on the starter motor, why not use this point as the starting point for the power to the rest of the carís electrics? Saves having heaps of wires trundling all the way back to the battery and face it, this big heavy cable is just sitting there unused all of the time once the motor is running.

A battery master off switch (as per racing regulations) with the power off and the key removed makes a pretty effective immobiliser - especially if you have it mounted under the car so the only evidence is a small hole in the inside carpet and floor pan to put a key through. It is hard to get better or cheaper protection than that. Only thing is, remember to have a bypass for those computer memories - it is easy enough to have a little switch that will allow them to be isolated with the rest of the car via the main power switch if you want to go racing and be safe.


A word about earths - a bad earth tends to cause an awful lot of intermittent electrical problems. Combining earths eliminates a lot of hassle - bunch all the earths in an area together and attach to a really good chassis earth. Remember that electricity has to go round in a circuit - the wires into a component must be the same size as the ones coming out of it. Since your motor sits on rubber mounts it is most important that the engine to chassis earth is at least as big as the wire going to the starter motor. In some ways two earths is a good idea, in case one breaks - you really donít want to have your starter motor earthing thruí your accelerator cable or temperature gauge! Donít forget that starter motors arenít fused and are often rated at over 1 kilowatt - that is about 1.3 horsepower - and about 75 Amps...

One word of caution about shared earths though - if the connection to the chassis is not good you will get weird things happening due to partial voltages feeding back up one circuit from another sharing the same earth. Keep that chassis earth clean and bright.


It is a good idea to put a fusible link on the line between the alternator and battery, and another on the feed from the battery to the car electrics. A fusible link is just like a big fuse. Do not fuse the starter motor power lead. A Ďbigí alternator is not a silly idea - if you use a Japanese Import engine it will probably be supplied with quite a hefty alternator at no extra cost to cater for all those electric gadgets that the Japanese consumer demands but we, as true sports car enthusiasts, scorn at. Hell, I donít even have doors, let alone central locking, power windows or mirrors! So although your carís total electrical demand is probably considerably less than the car the alternator was designed for it is quite possible you wonít use the car all that regularly (shame on you) so you may as well have it able to fully charge the battery up in record time when you do use it.


You can buy fuse boxes in varying sizes that are a much neater solution than having dozens of in-line fuses. Also it means that you can use the same style of fuse throughout your car. Hella make relay mounting blocks that mini relays plug into, so all the wiring is locked tidily in place. The blocks slot together to form banks of relays.

You can buy aftermarket integrated fuse/relay blocks which look quite smart but are fairly expensive. I have never considered using one, however, they are not a silly idea, especially if you will use most of the relays and fuses. I have always taken the following approach, which stems from having the complete EFI wiring loom including the factory fuse/relay box. If you are running an injected engine grab the under bonnet fuse/relay unit which will have all the relays you need for the engine. The one we used in the Fraser catered for the alternator fusible links, fuses and relays for the engine, electric fan, horn, headlights and a few incidental fuses, leaving only six fuses and five relays to go under the dash. Remember to place fuses and relays where they will stay dry and be accessible in case they fail.

The above circuits make up the basic groups that you must fuse so your car won't catch fire if something shorts out. The number of fuses is up to you, but the less you have the more systems you lose when one fuse goes. Not counting engine EFI system I recommend the following fuses:
Stop (Permanent Power) - stop lights 
Park (Permanent Power) - tail lights, front park lights, interior lights, number plate light, dash lights (via dimmer if so desired) 
Hazard/Horn/Indicators (Permanent Power) - obvious, but you need a cunning trick to prevent indicators going when ignition is off - I will explain a little later (although it depends a bit on the hazard switch and flasher unit). 
Left/Right Headlights (Permanent Power) - Strongly recommend separate fuses left and right. 
Wash/Wipe (Run) - self explanatory 
Heater (Run) 
Gauge (Run) - gauges, warning lights, reverse lights 
Engine (Run) - ignition system, fuel pump, radiator fan? (could be on separate fuse) 
Accessory (Accessory) - radio 

The starter motor solenoid circuit is not fused as it is only momentarily in use. That amounts to ten fuses. The next trick is to figure out where to put them. I have my headlight fuses under the bonnet to reduce the length of wire required (which makes things tidier and reduces voltage drop), but it is up to you. You need to place the fuses as close as possible in the circuit to the battery to minimise the length of unfused wire. This depends on the battery location and it may be better to run one big feed from the battery to the dash area that can then split into the power feeds to the various fuses via the ignition switch.

Switches and Relays 

If you are buying new components shop around until you find the switch gear etc you like the look of. There are a lot of different companies marketing different styles and colours. Look at other kit cars and see what they use, and ask where they source them from. Remember that switches must be rocker type and not toggle unless they are Ďbehindí the steering wheel. Check the certification regulations.

Switches and relays by convention are drawn in the Ďpower offí position. A normal switch or relay that you turn on to make something happen is termed Ďnormally opení ie the contact is open (unmade). Some switches and relays are normally closed ie the contact is closed (made). You turn them Ďoffí to stop something that will happen automatically otherwise. An oil pressure switch is like this - with no oil pressure the switch is turned on, therefore the contacts are Ďclosedí to make a circuit so the warning light comes on when the engine is stopped. As soon as there is oil pressure the switch opens, breaking the circuit so the warning light goes off.

When you buy your switch gear make sure you find out what its rated capacity is, expressed in Amps. If your circuit totals too many Amps for the switch capacity you will have to use a relay. Most switches will handle about 15 Amps - but make sure you check. Mini Relays are made by a variety of companies and come in a variety of types which generally look the same - a big trap for the uninitiated. The trouble is that they all have the same size and shape pins in the same location - but they are connected to different parts internally.

Mini Relays will generally handle 30 Amps and are also useful where you want an electrically operated switch, say as a cut-out. If you are unsure about a switchís capacity use a relay. Mini Relays come in two basic types - switching and change over. A switching relay merely turns a circuit on when the operating solenoid is powered up (normally open) while a change over relay will put power down one circuit with no power on the operating solenoid (the normally closed leg), and power down the other circuit when there is power on the operating solenoid (normally open). A change over relay can be used as a switching relay if you donít connect anything to the normally closed leg. A change over relay can also be used to turn something on when the power to the solenoid is turned off, or conversely turn something off when the power to the solenoid is tuned on. This can be useful when making interlocked circuits, say for an oil pressure cut out that shuts the engine down if you lose oil pressure.

Care must be taken to ensure that all relays have no power going to the operating solenoid coils when the ignition is turned off as they will drain a battery flat over a week. Additionally some switching relays have the same terminal layout as change over relays, however, the extra connection is just a double of the switched terminal. Put it in where a change over is required and youíll wonder why both circuits are going. Also some relays have the solenoid terminals where the switched terminals are, which also causes problems. Check the physical layout of the terminals. Relays should also be mounted with the terminals down to prevent them filling with water should any drops land on them - condensation can cause droplets of water in the most unlikely of places.

Engine Systems 

Presumably you know the type of engine you want to use. Odds are you probably made that decision before you decided on the car! You will ordinarily have two options - carburettors or electronic fuel injection. The principles are the same regardless of engine choice - you will either use the factory ignition/EFI system or you will use an aftermarket electronic ignition/computer. I say the same because both need one power feed to the ignition system for the engine to run, another for the starter motor, wires to and from the alternator and the various sensors/senders for the instruments.

All an EFI motor has in addition is a bunch of wires hooked up between some additional sensors or injectors and the computer brain. What these wires do is take the place of the passages in the carburettor and linkages between extra throats etc. This is why an intact EFI loom is such a bonus - plug it on the engine one end, into the computer on the other, and there is only about half a dozen extra wires you need to worry about. Half of these are for the gauges/warning lights while the rest are there to keep the computer's memory alive (a factory computer constantly adjusts the engineís tune to get absolute peak performance and needs to remember where it is at otherwise it has to start again), tell it when to use the 'choke' to help start the engine and to let it know to keep the engine running. It really isnít much more complicated than that.

If your loom has been cut or a chunk is missing (ie you have the engine loom to the fire wall and just the wire stubs from the computer) all is not lost - all you have to do is splice in the missing bits. Naturally a diagram helps. Hereís a tip - if you donít know which plug wire is which, take the top off the computer (and whatever you do, do not touch the circuit inside - static electricity on you that you cannot even feel is enough to fry the electronics) and look at where the plug terminals mate with the circuit board - they should have a little abbreviation for each connection that will correspond with the wiring diagram. This can be made easier by getting a diagram that identifies each wire on every plug, but you can work around it.

Aftermarket systems are simple - just use the wiring loom and diagram supplied. If you use a factory system, especially EFI, then it is absolutely essential that you get your engine supplied with the complete engine loom (or at least both halves if it has been snipped at the fire wall), computer, sensors and wiring diagram otherwise you could be in for a lot of money to get it to go. It is unwise to mix and match any of the EFI system components - some automatic engines run different computers which also control transmissions and things, and an EFI computer programmed to run an airflow meter system will not run a Manifold Absolute Pressure (MAP) sensor system. All these do is tell the computer how much air the engine is sucking in, so it knows how much fuel to give it. Of course there are always ways around this, the easiest being with an aftermarket computer, but these do not come cheap. As always it is a lot easier if you get the right bits to start with.

Lights - General 

Lights are a potential minefield. To start with they must be standards approved with the respective ĎEí standards numbers for the use they will be put to. What this means is that you canít use an indicator designated as a repeater as a main indicator on the back of the car. Light placement is pretty heavily regulated as well so check the regulations. Make sure the lamps you use are the correct wattage - too high and you can melt plastic lens covers with the excess heat.

I find that I go through a lot of tail light, license number plate and dash switch bulbs - they seem to just shake to death!


As far as Iím aware headlights must be halogen. I like to be able to see in the dark so I have 100 watt main bulbs and 90 watt dips, plus 2 x 100 watt driving lights. Properly aimed 90 watt dip beams will not dazzle oncoming drivers. In total I have 400 Watts on main beam. Most cars have 60 watt mains and 55 watt dips. The Fraser has been wired for 130 watt mains and 90 watt dips. This is some big current - 400 Watts draws almost 30 Amps, which is half an average alternatorís output.

The heat from big bulbs is considerable and actually melted the low quality lamp terminal connectors supplied with my head lamp bowls. Spend a few dollars and buy quality lamps connectors like Utilux. It is also hot enough to melt insulation so make sure the wires are kept clear otherwise you will have problems. You can either incorporate front park lamps in the headlight bowls, which certainly makes them visible, or use separate units.

If you want to use fog/spot/driving lights you may as well incorporate them into the loom. I had my driving lights connected so that they always came on with main beam. The relay for them had an external waterproof switch located close to the driving lights so that when I removed the covers I just flicked the switch and they would operate with the headlights - saved on wire and reduced the need for an additional switch on the dash.

With a small car it may not be a silly idea to have the headlights on low beam turn on whenever the engine is running (park lights arenít really visible enough). This is easily done via a relay that turns the lights on whenever the ignition is on. It is wise to have the headlights (but not the park lights) cut out on the start position or else you may find that there is insufficient power available to the starter motor. You will also need to manually over-ride the park lights.

Tail and Stop Lights 

Excessively bright tail lamps are not a good idea. If you have ever driven behind one of those morons who drive European ego inflators with their tail fog lights on when there is no fog you will know what I mean. Most cars use 6 watt tail and 25 watt brake lamps. Donít forget your hi stop light, which can be very challenging to mount successfully and legally. I have never been a fan of hi stop lights on convertible roll bars, as if you are driving at night with the roof up every time you use the brakes the inside of the car lights up and destroys you night vision and concentration. Additionally, have you ever tried backing down a hill at night with a bright red light reflecting off the inside of the back window? It makes it rather hard seeing what is behind you...

Hazard and Indicator System 

You may not believe it when you first read this, but an easy way to do this is to get a three terminal flasher unit. These are very common and have power (via a fuse) and earth permanently connected. The other lead has power on it from within the flasher unit and should be connected to earth via the hazard/indicator switches and indicator lights. The flasher activates in one of three ways - the hazard switch is turned on (all indicators flash) or else left or right indicators only are earthed. Trouble is this system will result in there permanently being power available to the indicator system - how to prevent the indicators remaining on if you donít centre the stalk when you take the key out?

The answer is to hook in a normally open (open when power is off) relay so that only when the ignition is turned on (which also closes the relay) is the circuit between the flasher unit and earth via the lights made. This way if you take the keys out with the indicator still on nothing will happen yet the hazard switch will still work. If you use only one indicator dash light you will need some diodes too, to prevent one set of indicators back feeding the others via the common connection to the dash light - just put the diodes in the lines between each indicator feed to the warning light. If you get them in the wrong way around the opposite indicators will flash when you turn the indicators on! If you run two warning lights this is not a problem.

Indicators must also flash at the correct rate. Generally, if they flash too fast put bigger wattage lamps in (without melting lenses). Flasher units come in a variety of configurations and are designed to work with varying loads. Think about where you will place the lights and whether or not to use repeaters. If your indicators rely on a non self-canceling switch for operation a warning buzzer which sounds whenever the indicators are going may also be a good idea, especially in a convertible where sunlight may make the warning light difficult to notice.

Of course, if you have warning buzzers for oil pressure, headlights, indicators and reverse you may find yourself going mad trying to work out what you have left on!

Reverse Lights 

Reverse lights are a really good idea for two reasons - they let you see what is behind you, and let other people know you are reversing. In NZ you do not have to have reverse lights, but if you decide to fit them they must have compliance. If you have a convertible, a really bright reverse lights (ie 25 watt) are a good idea, especially once the hood gets a little old and the plastic windows loose a little clarity, or when they fog up a little! Fitting a 55 watt halogen light may not be silly, if you can do it legally.

If your gearbox does not have a connection for a reverse lamp switch then you will have to attach a micro-switch to the gear lever that is made in the reverse position. In some cars, especially where reverse is above first gear, with a dogleg change to second gear (like Italian exotica and 1970s Datsuns), a warning buzzer that activates whenever you are in reverse may prevent the driver from running into the car behind when doing drag race starts in the wrong gear...! Obviously wire the reverse lights so they operate only when the key is on, otherwise if you leave the car in reverse the battery will go flat...

Wipers and Washer 

Wipers must be two speed and have to cover certain areas of the screen. Check the current regulations. Washers must have a minimum of one litre capacity. You can buy aftermarket washer kits or raid wrecking yards. If you have a custom built car often the wiper mechanisms used in British Leyland products are useful as they are cable driven and by mixing wheel boxes, arms, crown wheels and cable lengths you can run almost any combination of arms and arcs sweeping left to right or vice versa without trying to modify lever arms and rods.

My Lotus 7 replica uses the complete wiper mechanism from a Triumph 2500 saloon but the arms are cut down dramatically (by some 10 inches) and the wiper blades are Land Rover 10 inch units. The cable and drive tubes have been cut to length to suit the pedestal locations. I also have a 120o gear wheel fitted in the wiper motor (opposed to the 90o one standard) to increase the sweep.

The radical shortening of the arms has resulted in spring tension somewhat in excess of what would normally be used which is a real advantage in that even at 115 mph in rain the wipers do not lift off...the downside is that any contact the metal arms have with glass leaves scratches very quickly! I also use twin blade wiper refills which have two blades but fit on a single wiper blade mounting.

In NZ wipers must be two speed but some older motors may be single speed, so be careful. Additionally they may not incorporate self park or intermittent operation.

Electric Fuel Pump/Engine Low Oil Pressure Cut Out 

This cut out is used to prevent electric fuel pumps from turning into flame-throwers when it all goes horribly wrong and you crash: the engine stalls when the tree you hit embeds in the crank (on its way thruí the sump) but now you have a ruptured a fuel line with an electric pump still humming away... If you run EFI you can just have the engine circuits not come alive before oil pressure comes up - this reduces start up wear considerably - basically all you do is turn the motor over until the oil pressure comes up then the fuel pump, computer, injectors etc come alive and away you go. You canít do this on carbs Ďcos the engine cranking (whilst you build up oil pressure) will still suck fuel from the carbs (due to the amount that is still sitting in the float bowls) and by the time you energise the fuel pump and ignition system you are already flooded... Mind you it is easy to bypass this on start up with a relay.

If you run this cut out you may not need an oil warning light, especially if you run EFI, as the cut out is checked every time you start the car - the delay in start and the noise of the fuel pump wiring into life will indicate the point where cut out pressure is exceeded. This system is an excellent idea if you have minimal sump ground clearance, an oil cooler or a dry sump system where there is increased chance of losing oil pressure due to sump/hose damage and leakage or dry sump scavenge/pressure pump failure caused by belt drive failure.

Radiator Fan 

Electric fans are good but work best when triggered by a thermostatic switch, although some people prefer a manual over ride. You can buy aftermarket fans or hunt through a wrecking yard. Most aftermarket thermostatic switches are placed either through the radiator core fins near the inlet from the engine or are placed in the top radiator tank, and the connections are sandwiched between the radiator inlet hose and the top tank pipe. Most factory switches are located in the engine inlet housing. Adjustable aftermarket switches are available, as are preset non-adjustable.

Ensure the thermostatic switch is placed correctly - one designed for the radiator outlet should not be placed in the radiator inlet as it will be designed to operate at a different temperature. Care must be taken to establish whether the thermostatic switch is normally open or normally closed, and the fan relay configured accordingly.

Fans should be test run first - the first one I used was very noisy which was a feature of the fan blade design. Additionally some fans are designed to run in one direction only - ensure this direction is correct for the side of the radiator/mounting position you will put the fan in.

For maximum efficiency the fan should be shrouded and as large as possible. The fan should be located as close to the top of the radiator as possible and hard against the fins (but not in such a manner that it will move and wear the fins through and cause leaks). The decision to place the fan in front of or behind the radiator and so have it pushing or pulling the air through is based largely on where it will fit best. ideally the fan will e more efficient pulling air through the radiator but you may not have the space to mount it this way. 

Remember the old maxim that it is easy to get air into an engine bay, but harder to get it out - donít automatically assume you have cooling problems due to not enough air getting to the radiator - it may be a case of air pressure building up in the engine bay and not letting any more air enter.

Multi Speed Fans 

Heaters tend to have multiple speed fans. High speed is achieved by putting full system voltage on the fan, lower speeds use resistors to drop the voltage so that the fan runs slower. It makes sense to use a heater fan resistor. My car has a two speed fan switch, but a four stage resistor. I just experimented until I found the connections which provided the best speeds.

Warning Lights and Instruments 

Instruments and warning lights are similar to switch gear in that they have a huge impact on the finished Ďlookí of the vehicle. Once again the choice varies from second hand to new and depends on budget, styles available and the type of look you are after. Aftermarket instruments will come with instructions and wiring diagrams, liberated ones you will have to figure out - go and visit the library again.

Before you leap into buying instruments from any source consider what you will be using the car for and what information you want or need. Is oil pressure better served by a gauge or warning light? Do you need to worry about boost pressure in a turbo or supercharged car? Is fuel pressure or oil temperature an issue? Do you want a rev counter that tells you the maximum revs you reached? You may well find that the features you want start to limit the choices available to you. Once you have selected the types of instruments there are considerations such as size, colour and number of instruments (some styles feature two gauges in one housing), and metric or imperial scales. If you want to be able to control the instrument illumination you will need a dimmer as well.

You must buy the correct senders to match the instruments as adapting orphan parts to fit and work correctly can get expensive. Instruments removed from wrecked cars should include plugs as well. Also ensure your rev counter is compatible with your ignition system. A good instrument service can sort this all out for you - at a cost. It will probably save you money if you can get the speedo cable (both inner and outer) from both the car your gearbox is from and the car your speedo is from so they can be joined together.

Calibration of speedo for both distance covered and speed will be required unless you work from a complete donor car or donít care what your speedo reads. If you intend to race your car or drive it hard getting the rev counter calibrated may save you over-revving the engine without realising it due to an inaccurate instrument. Most instruments require a stabilised voltage so make sure you have a stabiliser connected. Additionally things like brake fluid level warning lights are fed from this stabilised voltage also.

With warning lights you need to realise that some (like alternator warning lights) need diodes to limit the current flow to one way only. Also it pays to have the seat and steering wheel in place (or at least have the normal driving position mocked up) so that you can locate everything where you will be able to see it. An instrument or gauge you canít see with your hands on the steering wheel in a normal field of vision isnít really worth having!

If you run a low oil pressure cut out then you probably donít need a warning light as well - over to you. I donít like having only an oil gauge as I once had an engine pump all the oil out through a leak and seize - how often do you scan gauges when driving fast on twisty road or when you are in heavy traffic? At least a warning light is a little more attention grabbing. My brother in lawís rally car had a turn signal light (ie 50 mm diameter) serving as the oil warning light that was also attached to a buzzer...no way you could miss that one!

You can get high pressure oil pressure switches that will set the light off at say 15 PSI as opposed to normal ones at 5 PSI. These are not a good idea if you have an oil pressure cut out - I found that once the oil got hot it thinned out (even using full synthetic and an oil cooler) to the point that the starter motor couldnít turn the engine fast enough to build up sufficient oil pressure to overcome the cut out! The easy fix was to pop the wire off the oil pressure switch - back to instant starting, which is also handy on an EFI car if the battery is a little flat.

Tools and Equipment 

Its about now you need to buy or loan the tools you will need. A decision needs to be made on the physical method of wiring a car. I am pretty one eyed on this front and regard solder and heat shrink as the only solution. Sure, it takes a lot longer than crimping fittings but it doesnít fall apart after six months. I know car manufacturers and aircraft companies crimp, but they use far better tools and fittings than you or I are likely to buy. Remember that you will have some wires that connect directly to the battery without fuses - do you really want to risk that wire falling out of a fitting and shorting to earth?

Tool Shopping List 

Soldering Iron - donít mess about, buy a 60 watt one. 20 Watt irons are great for building circuit boards but you want some serious heat. About $60 from Dick Smith etc. 
Multi-meter - either digital or analogue (needle on a dial or digital readout). Get one that measures resistance, continuity and Volts. These get expensive very quickly so get the cheapest one that will do from Dick Smith or the like. Probably about $60. 
Solder - resin cored electrical solder is the only way to go. It is quite surprisingly expensive and you use a fair bit. Buy at least 100 grams if doing a whole car from scratch and be prepared to buy more if you waste as much as I do. 
Heat Shrink - great stuff, comes in a range of colours and sizes. Shop around electrical wholesalers. 
Side Cutters - for cutting wire. 
Heat Shrink Gun - great if you have one, or else use a hot air paint stripping gun. Buy a cigarette lighter if you donít. A hairdryer may work, if you are allowed to use it in the shed. 
Wire Strippers - the $ 10 pliers type work fine if you donít have access to a fancy one, however, a professional type one is a really sound investment. Donít mess around using side cutters - it is a waste of time. 
Long Nose Pliers - used for crimping terminals on before you solder them. I will explain later. 
Bowl of Water - for cooling insulation that starts to melt after you heat the wires too much when you get it wrong! Good for burns to fingers too... 
Insulation Tape - buy good quality - 3M etc. 
Cable Ties - far better solution than taping wire together. More on this later. 
Wire/Terminals - as required. You will need varying lengths of wire in different colours and thicknesses, so find somewhere with a good selection that is willing to sell by the metre. Electrical Wholesalers like Ideal or J.R. Russell can be good, and will often sell heat shrink etc way cheaper than hobby shops. 

You will be buying quite a lot of stuff, especially if you are using new components. A good auto electrical supplier or someone like Lucas will get a fair bit of money out of you so see if you can cook up some sort of deal to get a good price on all the bits and pieces you need because you will not buy it all in one trip! Places like REPCO donít stock all the specialised wire and terminal bits you need but can help with the hardware.


The method I use for soldering is a little rough and ready but it works. Once you have cut the wire to length slip on the relevant insulating sleeve or heat shrink and strip the insulation off the wire. If you are joining two wires together strip about 15 mm off each and twist them together. If you are placing a terminal on the end, strip about 5 mm and using the pliers crimp the bare wire to one set of lugs and the insulation to the next. Then hold the wire/terminal against the soldering iron. You need to place some solder on the iron to get the heat to transfer to the bit you are soldering. Then you just apply solder to the wire/terminal and dunk it in the water if the insulation starts to melt. A 60 watt soldering iron provides sufficient heat that tinning is not necessary. Crimping the terminals on first prevents you needing 6 hands to do the job.


Terminals come in a variety of types and sizes which correspond to current rating. The most commonly used ones are 1/4 inch female spades. These come in two types: solder or crimp. As stated I use solder ones. There are also latching and non latching terminals. The latching type have a wee lug that locks them into relay mount blocks, plugs etc, while the non latching ones do not. Non latching tend to be a tighter fit on the male as they do not have the plug to hold them in place. The same is true for male spades.

Bullet connectors come in slightly different types as well, basically English and Japanese design, and they are not compatible so some care must be taken to get the right ones. A sample is recommended to get the right match.

Eyes come in varying wire and eye sizes too, so ensure you have sufficient Ďthicknessí to cater for the current the wires carry.

If you are splicing into another loom you can either join the wires or try to remove the terminals from plugs and change the wires at that point. A jeweler's screwdriver is handy to unlatch the terminal locking lugs to allow removal.

Physical Wiring 

There are a few little tricks that make your wiring loom look a lot tidier and ease the job. Depending on how you want the finished loom to look I am a big fan of looming the individual wires together with thin cable ties and encasing engine bay wiring in Ďcobraflexí split sheathing of suitable size. Insulation tape should be used as little as possible as it makes it difficult to trace wires or do alterations without removing it and ending up with sticky mess everywhere. If you are recreating the original loom use loom tape or self amalgamating tape (not insulation tape) to bind the wires.

Plan where the wires will run - and avoid exhausts! I prefer to wire from the dash outwards and terminate to length at the extremities. You can buy plugs in a variety of sizes that allow you to have joins in the wires. I am a big fan of being able to unplug the engine loom (especially with EFI) from under the dash and remove engine and loom complete without having to remove individual wires from instruments, fuses etc.

I wire a car in three Ďlayersí, starting with the power feeds to the ignition and power sides of the fuses. It makes sense to loom this separately if you have the room. Next is the car side which comprises all the car systems excluding the engine. Again, loomed separately if you can, which is almost impossible in a Seven. Finally I wire the engine, ensuring it can be unplugged and removed as invariably you will have to remove it at some stage, and if you decide to upgrade the motor all you will have to do is change the wiring on the engine side of the plugs to suit the new engine.

It pays to check each circuit as you complete it otherwise you risk having lots of things not working and no idea where to start looking. If a fuse blows when you turn something on check the circuit, check your fuse calculations and see if you have the right fuse. Merely putting in bigger fuses until it stops blowing is asking for trouble.

Care should be taken to protect all panel penetrations with grommets sized as close to the wire as possible. I tightly wrap the wires in insulation tape until they are a good tight fit in the grommets. A dab of sealant is wise if you are really concerned with sealing out moisture, however, a little thought as to where penetrations will be made can result in the most being made of available shelter. If you are using standard EFI it makes sense to re-use the grommets fitted to the loom.

Mount all electrical devices where they are clear of drips from potential leaks (say from wiper post penetrations) or heater hoses if they terminate under the dash - I have had hoses leak. I had to drive through six inches of water once, which is a lot when you only have four inches of ground clearance, so keep things high up. Think about where stones and puddle splashes are likely to fly up, and keep clear of exhausts. Allow enough length for relative movement between engine and chassis. Also keep wires away from fans etc.


Wiring a car can be done by the average enthusiast - but be warned it can soak up a considerable amount of time. Planning is crucial and drawing out the circuits first is a very wise move. It is normally possible to simplify circuits considerably from the initial design to the finished one. Try to amalgamate power feeds etc as much as possible to reduce the amount of wire in the car. Remember that one day you (or the next owner) will have to figure out how the wiring works so a problem can be fixed or an accessory added, so make a good diagram and stick to your colour coding.

Weíll make a Sparkie of you yet! 



Copyright © 2000 SpeedTECH Last modified: January 23, 2000