Many people shy away from vehicle wiring, with a little knowledge it is relatively easy to correctly install wiring for accessories, or even replace damaged wiring to professional standards, or even higher standards in many cases. Original vehicle wiring is designed down to a price rather than up to a quality, manufacturers need to save money on all a vehicles aspects, wiring being one; this is where we can improve these manufacturer compromises.

Before we move on to wiring we need to understand some of the terminology and the components we will use to design and manufacture our accessory circuits, and the differences and technicalities of differing components.

Wiring and Cables

Wire is wire right? No it is not right, wire is of two main types, they both have a conducting core or centre, and an insulating outer covering, some conductors are a solid wire and this wiring is installed in our homes. This is not suitable for vehicles, in the home it is fixed and does not suffer from constant vibration and movement as it does in a vehicle.
Vehicle wiring conductors contain a number of very fine conducting wires; these have a slight twist along its length to ensure a good electrical contact between these individual wire cores and the ability to remain flexible across a wide temperature range. This prevents cracking of the cores when it is subject to the constant vibrations and movement from an operating vehicle, and ensures a good working life without deterioration.

Most cable reels contain something most people do not understand, this is the technical specification, and this may be:

14/0.30 1mm 8.75A

This can be broken down as follows, the 14 is the number of conducting cores there are to make the conductor in our wire.
0.30 is the cross sectional area (CSA) of the individual core.
1MM is the cross sectional area of the total conductors in the cable.
8.75A means its load rating is a maximum of 8.75 amperes.

We can now tell that our cable is made up from 14 separate conducting cores, each has a CSA of 0.30MM, this gives the total conductor a CSA if 1MM, and this will carry a maximum load of 8.75 amperes.

Insulation is an important element for vehicle usage; it too has to withstand the normal vibrations encountered by vehicular use, and be able to insulate while being subjected to abrasion and penetration as well as remaining flexible at all temperatures.
Wiring comes in two main types, this is the normal wiring common, and the higher performing thin wall cabling which is currently being fitted to most new vehicles. Thin wall cables are higher performing cables for a given size, in simple terms they carry a larger load (amps) for an equal size of the older standard wiring, in addition their insulation is thinner and lighter while being more abrasion resistant. Modern vehicles need this additional performance due to the much higher electrical loads, and the reduction in vehicle weight due to the additional wiring. Insulation colours are important as they identify the cable and the component it is connected to, most cables insulation is denoted by two colours, this may be:


Green is the predominant colour, it will have one or two stripes, these will be slate and these stripes are called the tracer. Main colours are used to identify the main use of the wire, the tracer is used to identify the individual component to which it attaches, and this is to make tracing wiring through a loom simple and easy.


Vehicle manufacturers use modular wiring systems, this means one type of use will have one predominant colour, in our chosen vehicle green will be for the lighting circuits, any cable with a predominant green will supply a vehicle light somewhere. Tracer colours will identify which individual light the cable runs to; this makes it easy to trace a wire back to its source through the wiring circuit.

In our chosen example we encounter many green wires with different tracer colours, it may have a number of green wires running to the front or rear of the vehicle, these may be:

Green/Black Sidelight/Offside
Green/Blue Sidelight/Nearside
Green/brown Headlight/ Offside
Green/Slate Headlight/ Nearside
Green/Orange Rear light/Offside
Green/Pink Rear light/Nearside
Green/Purple Rear fog lights
Green/Red Brake lights
Green/Light green Interior roof lights
Green/White Dashboard illumination
Green/Yellow Instrument lighting.

From our example we can see that all the vehicles standard lighting modular circuits are green with a different coloured tracer, this predominant green identifies any lighting circuit cable when it is in a wiring loom with many other wires. This modular wiring system ensures it is easier to identify specific segments of the wiring when it is incorporated within a wiring loom, these looms regularly hold in excess of 50 or more cables, it also allows for easy fault identification.

Cables may come as individual or single cables in a variety if predominant and tracer colours, but others are a number of these individual cables in another sheath or insulation, these are called multicore cables. Multicore cables are most popular for automotive applications in 7 core or 13 core multicore cables, these are seen on older or new towing circuits to power trailers or caravans, they do come with a variety of other cores in a number of standard sizes.

Larger sizes of insulated cables are available, these are mainly for starter motor and alternator applications, or as un-insulated braids for main earth cables to run from the battery negative terminal to the engine or vehicle body.

Wiring Looms

Wiring looms are where a number of cables are grouped together in a large bundle, they will nearly all terminate at, or from the fusebox and run to a number of applications around a vehicle to cover all electrical applications.
Early wiring on older vehicles was not so complex so fewer cables are needed to power fewer electrical items, they followed what was called the “highway routing system”, this was basically one loom to the front of the vehicle, and one to the rear of the vehicle. Highway routing meant one large bundle of wires running down one side of the vehicle, from this loom wires exited to specific items to power them, these were called loom termination points, and these termination points housed a group of cables to a specific vehicle area.

Modern vehicles have considerably more cables due to the ever increasing quantities of electrical equipment now fitted as standard; the old highway system has been superceded by a newer variant of this system. This new system breaks the wiring loom down to generic applications, this means all engine management cables may be grouped together in their own dedicated wiring loom, lighting cables in another, and safety systems in another. These looms may be run through the vehicle as separate wiring looms; it may be that a modern vehicle has three or four wiring looms running the any part of the vehicle, some may be inside the vehicle under the carpet, and some may be outside the vehicle.

One other variant of wiring is found, this is called zone wiring, this is where a group of cables run to a particular or specific area of the vehicle, this is often used in conjunction the newer multiple loom variant of highway routing.
Zone wiring forms a wiring loom by selecting a number of cables from differing looms, so may incorporate some lighting cables, engine management cables, and accessory cables.

As an example we will look at a modern vehicle, it contains a fuel tank mounted lift pump, rear interior lights above each rear door, and electric rear windows, the main wiring loom runs to the rear of the vehicle as two looms, one on each side of the car.

The offside loom has a termination point behind the drivers seat, from this termination point are three wires to the fuel tank, two are to power the in tank lift pump, one is for the fuel gauge. Two wires run to the rear door to power its electric window while two more run with them to the doors low level light, one wire runs up the “B” post to the off side rear light in the hatchbacks boot.
We can now see we have 9 cables exiting from the termination point, this is done by using a 9 way multi plug connector, it contains one brown power cable for the fuel pump, one Yellow/Black for the fuel gauge. The loom sweeps around the floor to the “B” pillar and runs up it, two power cables feed the door for the integral light and electric window, and the loom continues to the roof and across to the roof mounted light. In addition the termination points have three earth cables, one to the fuel tank lift pump, and two more to the doors to earth the electric window and the integral door light.
This is a prime example of zoning, all the wiring for, or around the offside rear passenger seat terminates the main wiring loom at this one point, it is mirrored on the nearside, obviously minus the fuel tank wiring. Zoning often categorises the front near side seat as one zone, front off side seat as another zone, rear nearside seat as another zone, rear offside passenger seat as another zone, these are the main singular zones.
Engine compartments are often divided into as many as four zones, depending upon its wiring complexity, dashboards as many as six zones, depending upon complexity, centre consoles as one zone, and boots/hatches as one zone.
Zoning has one other considerable benefit; it allows easy diagnosis of a particular wiring defect, or the replacement of one small segment of wiring loom if the vehicle is involved in a minor accident which causes the wiring loom damage. It can be considered the automotive equivalent of plug and play, a very useful feature on some of the more complex wiring systems on complex vehicles.
Many of these zoned wiring systems have each wiring loom section individually listed under its own part number; these are often attached to the original loom as stickers.

Multiple wiring looms have considerable advantages, mainly it is the reduction of heat generated by so many wires grouped together to form a single wiring loom, an obvious safety benefit, and more critical wiring running inside the vehicle is better protected. If the wiring was incorporated into one bundle it would be huge, then, if a fault occurred in one cable it could destroy a considerable proportion of the wiring loom, a considerable expense to diagnose and repair. In addition it could disable a number of the vehicles safety features and even cause an accident if something like an airbag deployed, or seat belt pre-tensioners activated.

One type of wiring system is rarely fitted to a vehicle, it is often a combination of these systems, a modern vehicle may incorporate a multiple highway routing system with zoned systems, it is important we realise this before wiring.


Now we understand the fundamentals of cables we now need to connect them to each other, or to a particular piece of equipment; to do this we have a number of connectors which come in un-insulated, insulated, and pre-insulated. Connectors require a crimping tool to correctly connect these connectors to our cables, they generally come in two types, these are un-insulated crimping tools and insulated crimping tools, using the correct one is vital. Crimping tools cover differing types of connectors; some are limited in their crimping capabilities so it is important to select one which covers all the differing types of connectors you intend to use.
When selecting a crimping tool it is vital to avoid the cheap tools, many are simply not strong enough to crimp correctly, expensive or good quality crimping tools cost very little more than the cheap ones, but will last a lifetime.

Most people are familiar with the crimp on spade connectors, these simply crimp on to the cable and connect to whatever they connect to, there are far more connectors than spades.
Spade connectors come in a variety of sizes, in both male and female with the standard 6.35mm spade being the most popular, so what are they.

Spade connectors are actually called blade connectors, or female blade connectors; they come in the standard 6.35mm, 4.8mm, and 2.8mm. In addition they also come as piggyback’s, the female blade has an additional tag fitted to it to connect another female blade.

Rings are similar to blades except the working end is round and comes with a hole of 3.2, 4.3, 5.3, 6.4, 8.4, and 10.5mm diameter punched into them; rings allow multiple connections to a single point such as a dedicated vehicle earth point.

Forks are similar to rings except the round end is slotted so when they are connected to a multiple connection point then can be individually removed by undoing the securing nut slightly and the appropriate fork is slid off the connection point.

Bullets are a bullet shaped connector which comes in male, they come in 4, 5, and 6mm diameters; they perform the same functions as blade connectors and are alternative to these. The female ends are called sockets for obvious reasons.

Butt connectors are basically a hollow tube into which two wires are inserted, they go halfway up the butt connector and are crimped, and these are generally for connecting broken or damaged cables.

Pins have a round end of 1.9mm, these are crimped onto a cable where the cable goes into a screw connector, this is to prevent the conductor becoming damaged over time from continual vibrations, and their most common use is in towing sockets.

Scotchlocks are used for connecting a new cable to an existing cable; they should never be used on vehicles as they create many problems. Scotchlocks crimp over an existing cable and cut through the insulation, they have a small metal fork inside them to make contact with the conductor, this creates corrosion inside the cable, and the small contact area does not carry high currents. If these are fitted to larger sized cables they actually cut into the conductor and damage it, this reduces the original cables current carrying capacity, and in some cases can lead to a vehicle fire.
If anyone fitted a scotchlock to any vehicle of mine they had better be prepared for a visit to hospital, this is how much I loathe these hideous items, and the fact I have spent more hours putting right the wiring problems they create, over many years.

Terminals come in three normal colours; these colours denote the size or CSA of cable they are suitable to accept:

Red accepts cables from 0.65-1.5mm CSA.
Blue accepts cables from 1.5-2.5mm CSA.
Yellow accepts cables from 3-6mm CSA.

Now we know how to identify our chosen cable, we can select the appropriate size of electrical crimp connectors to fit the cable correctly, if we refer to our earlier technical specification for our cable we see it is standard 14/0.30 – 1mm. From this we can deduce our cable is 1mm CSA, this means we would use an insulated crimp connector which is red, this accepts cables from 0.65-1.5mm so is the correct connector for our application.

Un-insulated connectors are just that, they are just plain connectors in a variety of metals which are usually coated to prevent corrosion; they are of limited vehicular applications for successful or professional wiring applications.

Pre-insulated terminals come with a proportion of the terminal insulated, this is usually the section where the cable enters and is crimped, the actual working end, or connection is un-insulated. Certain connectors such as bullets or male blade terminals only come as un-insulated or pre insulated as the sockets or female blades come as pre-insulated or fully insulated.

Fully insulated terminals come with the entire terminal insulated; these are the best option for vehicular applications.

Heat shrink terminals are a fairly new addition to vehicle wiring, these come as ring connectors, male blade, female blade, and butt connectors; these are crimped onto the cable in the usual way, then, heated as they have glue inside them. The heat shrink insulation shrinks to up to half its original diameter while sealing around the cable insulation, the glue melts to seal any voids, and these are much better connectors for off road vehicles due to their better sealing capabilities.

Now we have an understanding of basic crimp connectors, how do we connect our wiring loom now it has a considerable number of cables in it, manufacturers use multi plugs, the plastic connectors with numerous connections, we simply do the same.
To make up multi plugs is easy, it gives a professional finish, and automotive manufacturer standards of wiring to any wiring repairs we may undertake, this equally applies to the additional fitting of accessories, or uprating their circuits.

Multi plugs simply use two types of un insulated blade connectors, female blade connectors, bullet connectors, and sockets, they crimp to the cable in exactly the same way as our standard connectors, but contain a small tab pressed into the back. This tab is the locking tab, once our connector is crimped to the cable it is simply pushed into the appropriate plastic moulding, the tab locks it in, and the rest of the connectors are crimped on and inserted into their respective positions in the plastic mouldings. This makes one half of the moulding up, the other half is made in the same way and the male and females are simply plugged into each other to connect all our cables.

We now have a manufacturer quality electrical connection for a number of cables within our wiring loom, it can be disconnected easily, and the individual cables can be removed by slipping something thin down the moulding to release the locking tab. These basic mouldings are open at the rear which makes back probing simple, it means the circuit does not have to be unplugged to diagnose any problems, and they can be tested under electrical loads, all you need is a thin multimeter probe attached.

What about if we want these connectors fitting to an individual component such as a rear light cluster, it contains rear light, brake light, reversing light, fog light, indicator, and an earth cable, but the multi plug will be outside the vehicle and subject do dirt or wet conditions. No problem, waterproof variants are available, they are much more expensive than the ordinary multi plug connectors, but much better for off road vehicles where an exterior connection is necessary due to dirt or moisture ingress.

Where better connections are required our un insulated terminals can be crimped in the normal manner and soldered as well, this gives a much improved connection, and considerably better than the manufacturers original wiring, so an improvement.

Miscellaneous connectors are the varieties of stationary blades, these are simply a number of female or male blades connected to a common base or mounting assembly, they come in many sizes and designs and allow a number of common connections.
Stationary blades are useful for making common earth points around a vehicle, they can be bolted directly to a vehicles metal body to form this earth, or inside a waterproof box with a substantial earth cable to the batteries earth terminal. Common earth connections are particularly useful on older vehicles, they overcome the old, corroded original connections, and mounting them in a waterproof box prevents future corrosion, particularly useful on 4X4’s.

Many applications require connectors to have no insulation fitted as they may often need testing, or removal to test; this leaves us the option of un insulated connectors, a number of slide on insulations are available in hard plastic or much softer PVC

Now we know how to select cables and how to connect them to professional standards, how do we make up a wiring loom to professional standards, and maintain our wiring loom by securing to effectively to the vehicle, well there are numerous methods.

Many people simply wrap insulation tape around the wiring loom, the first time the vehicle gets warm this comes off and looks a terrible mess, it leaves a sticky residue, and makes our loom look like it was manufactured by an amateur or a child.

PVC sleeving is probably the earliest modern form of wiring loom protection, it is a simple sleeve into which individual cables are slid, many variants of this are available, the most popular has a slit running along its length, but does not hold our loom together. This has limited applications and is only really useful for applications where additional wiring loom protection is necessary.

Braided sleeving is another derivative, it looks similar to the older cloth braided wiring looms fitted to older or classic vehicles, and this is now manufactured in plastic and is much more durable. This can have small slits made in it for terminations to be made by pulling individual or multiple cables through, the ends and terminations have a short length of heat shrink tubing slid over them, this is then heated so it shrinks and tightens the wiring loom.

Spiral binding is similar in appearance to a coil spring, it is made from a variety of memory plastics, it can be wound over an existing or new wiring loom which is in situ, and the memory effect pulls the wiring loom tight and compacts it. Having openings means the individual cables can be seen and identified, this is beneficial to identify where individual cables are.

Polyolefin or heat shrink tubing is another recent development; it is a plastic tube which will shrink down to half its original diameter when gently heated with a match, lighter, or hot air gun. It is extremely useful when we construct a wiring loom as separate circuits can be connected into separate sections such as lighting, accessories, or aftermarket fitments, they can then all be combined into one wiring loom.

Flexible convoluted tubing is a fantastic material for wiring looms, but considerably expensive when compared to other systems, this is the system to use for waterproof wiring looms, it comes with a number of connection systems and terminations.

Tapes come in a number of differing types:

Insulation tapes should be avoided as their problems have been previously highlighted.

PVC or harness tape is the original tape used to wrap wiring looms, it is stretched as it is would round the wiring loom, it is non adhesive but adheres to itself as it is stretched around the wiring loom, it is advised to put heat shrink at each end, or termination.

Rayon cloth tape is another recent innovation, it is similar to adhesive insulation tape, but rayon ensures it sticks much better than insulating tape, it is therefore a useful tape for wiring loom wrapping.

PIB (polyisobutylene) tape is a self amalgamating tape, it is non sticky but adheres to itself to form one solid mass as it is wound around a wiring loom, it is totally waterproof and has a multitude of other uses. PIB tape can be used to seal hoses in cooling systems as a permanent repair so is a nice toolbox accessory.

More than one type of harness handling system can be employed, where a vehicle is subject to considerable immersion in water or mud it is very suitable, and terminations can be covered with heat shrink tubing, or PIB tape.

Now we have the capability to design and connect our wiring loom, we now need to know the fundamentals of mounting our wiring loom into our vehicle correctly, there are many systems so it is down to particular or individual choice. Wiring looms always appear to sag, there is a good reason for this, this is a vehicle body always moves, particularly on a 4X4, so a degree of wiring loom flexibility is necessary to counter this movement.
If a wiring loom is left tight in its mountings, with no slack or sag between the mounting points it will simply stretch our wiring loom and pull it apart over time; this is not something we want or need, so always remember this, and practice it when installing wiring looms.

Many mounting systems use a variation of the tie wrap, these are:

Anchor ties; these are a traditional tie wrap with a plastic barbed end, a suitable size hole is drilled into a panel, the barbed end pushed into the hole and the wiring loom is bundled up and tied with the tie wrap.

Eyelet ties are similar, their end has an additional plastic piece designed to attach to the panel with a countersunk screw or bolt.

Tie mounts are a separate or individual plastic base, these may be self adhesive or incorporate a hole to accept countersunk screws or bolts, and some may be self adhesive and incorporate a countersunk hole. Tie wraps are pushed through slots and around the wiring loom to secure them.

Mounting clips, cable clamps, and harness clips are similar, they attach to a panel with either a barbed hook into a pre-determined hole or attach with self adhesive pads or screws, they have a hinged top and the wiring loom sits inside them. The top is closed to secure the wiring loom in place, most can be opened to release the wiring loom, and some can be connected to other clamps to allow hoses to be mounted.

“P” clips are made from plastic and metals, they are in the shape of a P and a mounting hole in the top allows them to be mounted, metal versions usually have a rubber coating inside them to prevent wiring looms from chafing.

Now we know how to select our cable sizes, make up a wiring loom and secure it; we now need to route it around the vehicle, this usually means passing our wiring through steel or other panels, and onto accessories such as fuse boxes. To do this correctly we use a grommet, this is basically a rubber washer with a groove around its circumference, and the groove locates into a predetermined hole where the groove locates it firmly. The centre of the grommet has a hole of a specific size through which passes the wiring loom, this prevents the wiring loom chafing on the sharp metal.
One variation this is the sleeved grommet which is like an ordinary grommet, but has a hollow sleeve or tail, these are used mainly for wiring entering items such as fuse boxes or waterproof housings as the sleeve can be filled with a waterproofing compound.
Blanking grommets are simply a grommet without the centre hole and are used to fill holes which are no longer required.

Fitting a grommet is easy, select the centre hole size you require to pass the wiring loom through, then refer to the grommet chart and select the suitable size, this will tell you what size hole to drill to fit the grommet.

If a suitable sized grommet is not available PVC beading may be used, this is simply a PVC strip with a groove down one side, it is inserted into the hole and slid over the panel, once installed it is cut to length.

Fitting grommets often causes corrosion to occur where they are mounted, this is usually down to poor preparation, once a mounting hole is drilled simply clean off any burrs and paint the now exposed metal to prevent rust forming. One other tip is to use a rubber based adhesive and fill the grommets circumference mounting groove, install the grommet and it will adhere to the steel; the rubber adhesive still allows some flexibility.

Proceed to Section 2