Colour Coded Wiring

Notes on Wiring

Dealing with wiring can often appear more complex than it really is. The wiring diagram of a modern vehicle is usually broken down into seperate units that cover multiple pages, but most vehicles built before the mid 1980's often have quite simple electrics, which can be represented by a wiring diagram that fits on one page. The electrical accessories and wiring in older cars, including Morgan, are wired following the British Standards BS-AU7 wiring code, which utilises a colour code system to identify wires as they emerge from the loom. This makes life a lot easier when restoring an old Morgan and with a few simple tools and access to the suppliers of colour coded auto wire, you should find little difficulty in servicing the wiring loom. Holden Vintage and Classic have a good selection of auto cable which can purchase by the metre. The BS-AU7 code uses a base colour and thin trace line to denote the source and destination of wires emerging from the loom. The wiring diagram in the "Morgan Owners Handbook" labels the wires with initials that indicate their colour. At first sight the colours indicated are not always obvious and a key assigning initials to a colour is not always present. For example, the colours blue, brown and black all begin with the letter "b" so blue is assigned the letter "U" and brown the letter "N". Therefore UW refers to a blue wire with a white trace, which denotes a main beam headlamp wire connected to the dip switch. Figure 1 above shows the the colour codes of cables in my 1972 car and should be relevant to the wiring diagrams for any Morgan built between 1970 and 1985.

The size of automobile wire is described by two numbers which are important in calculating the correct power rating. This is calculated as shown in Figure 1. Most wires are protected by a fuse, but this does not include the main battery feed and the brown wire with white trace connected to the alternator. A short circuit involving either of these wires will not only burn out the wires, but could lead to a car fire. This is why the battery should always be disconnected when working on the car electrics. In Morgans built during the 1970's, the main battery cable lies alongside the plastic petrol supply from the fuel tank!

If you add an electrical accessory it must be run from the correct terminal on the Fuse box, because one has to consider whether the ignition auxiliary , constant current auxiliary, or lamp circuit is required. A reversing light for example would be wired through the ignition auxiliary circuit, which reqires the ignition to be switched on. Spotlamps (if fitted) are wired to the lamp circuit fuse, because they do not require the ignition to be switched on. The headlamps are not fused. They are wired to the battery via the ammeter. The reason for this is to prevent loss of night vision due to fuse failure. Two fuses connected in parallel give extra protection to the tail and side lights. If spotlamps are fitted, they must also be connected, via a switch, to the dual lamp fuses.

---

Figure 2

Wiring Loom Components

Take a look at the wiring loom after removing it from the car and you will notice a large portion of it is wrapped in what appears to be black insulating tape, but closer inspection reveals it to lack the adhesive backing. It is loom tape and is used to bind groups of wire together forming the main part of the loom, which is located behind the dashboard. If you are renovating the wiring loom during a rebuild do not be tempted to substitute loom tape with insulating tape, because if you need to replace it at a later date, you will find it leaves a sticky mess over the wiring. Even though loom tape is not adhesive it tends to stick to itself quite nicely as it spirals around groups of wires. Where the tape terminates, heat-shrink is used to stop it from unravelling. Heat-shrink is supplied in different sizes and is cheap and readily available from many suppliers on the 'Internet'. The Loom gives strength to the wires it contains, but their current ratings will be less than that of the same wire running alone in air. Current ratings of long wires will also suffer particularly, when wire is near its maximum capacity, where it is wise to go up a size. Wiring should not be uprated unnecessarily, because the use of heavier cables reduces loom flexibility. Be careful if you uprate headlamp bulbs or add powerful spotlamps. Two standard 60w headlamp bulbs will consume 5 amps each, therefore the wires back to the fuse via the dip and lighting switch will have to handle 10 amps. The size of these wires will have to be 28/0.3, but 14/0.3 wires will be adequate for most of the loom.

Fuel tank sender   green with black trace   -   black   -   bullet snap connectors

*NOTE   4 way connectors are used to loop on to another component requiring the same wire. For example, the left and right stop lamps are both actuated by the same brake switch, therefore the green wire withe purple trace is looped on from the right hand stop lamp to the left hand one.

The method of connecting wiring to electrical components in the loom varies. For example, H4 headlamps may be connected by a ' pigtail' which ensures the correct connection to the three terminals on the rear of the lamp. The the three pigtail leads are connected to the main loom by '2 way bullet snap connectors', as shown in Figure 2. This type of connection is used under the front wheel arches in a hostile environment, where the use of bullet snap connecters are more suited. It is essential to ensure the correct connections to the main loom. This is where the colour coded wiring is important. The white wire from the ignition switch feeds into three different parts of the circuit via a '6 way bullet connector'. The 'Rear end wiring' is connected to the various lamps by bullet connectors and is connected to the main loom by a 9-way multi connector. Multi connectors are a convenient way to connect accessories like wiper motors which perform multifunctions. The 'alternator plug has a retaining clip in place to prevent accidental disconnection. If the plug comes out with the engine running, the alternator will be damaged. If you wish to break into an existing feed wire for an additional accessory, a 'Scotchlok self stripping connector can be used . You must make sure this will not overload the wire you splice into. 'Eyelet connectors' that can be bolted to the chassis are often used to terminate earth leads. Electrical components attached to the ash frame or dashboard cannot rely on their metal casing for an earth contact and often use this type of connection. Lucar connectors are commonly used for connecting to most electrical components. A typical example is the Fuse box wiring. 'Crimp terminals' are used to join the ends of a group of wires and you will find quite a few in the Morgan loom.

Universal Crimp Tool

If you are restoring the old loom, it is worth replacing old oxidized connectors with new ones. The 'universal crimp tool' illustrated in Figure 2 can be purchased from 'Halfords' or the 'internet' at a reasonably low price and is capable of crimping, cutting and stripping. Connectors will often have a colour coded sleeve which denotes the size of cable they fit. For example, a yellow sleeve would fit a 12 or 10 'S.W.G' size wire. The crimp tool indicates the colour codes and cable sizes alongside the appropriate part of the tool to be used. The wire strippers also show 'S.W.G' wire sizes for which they can strip the outer plastic sleeve of cables without damaging the internal copper core. The mysterious five holes around the swivel pin of the tool are used to cut small bolts and the yellow numbers indicate the 'UNC' size of bolt to be cut. The 'ignition terminals' hole is used to crimp ignition lead terminals. There are more better, more expensive tools on the market than the one illustrated in Figure 2, but if you do not deal with electrical work often, the one shown is good value for money.

Expandable braided sleeving

Expandable braided sleeving'. can be used to tidy and protect the wiring under the bonnet. When it is slightly compressed along its length it expands in diameter allowing it to slide over groups of wire. It can be sealed at the ends with 'heat-shrink' to prevent fraying. Holes may be cut in the sides of the braid to allow groups of wires to branch to other destinations along the loom. These junction points should also be protected with'heat-shrink'. Under the wheel arches of my Morgan I used convoluted plastic sleeving to protect wiring from stones and dirt thrown up from the wheels. It is easy to fit these types of sleeving when restoring the loom, but if the wiring has not been removed from the car, it is not so easy. The solution is to bundle wire in situ by using 'spiral binding'. Companies like 'AES' will supply all these types of sleeving and more.

---

Schematic of The Wiring Loom

Figure 3

Layout of the Wiring Loom

The schematic of the wiring loom shown in Figure 3 divides into four main areas which relate to submenu options under the 'Electrical System' Menu. Each area has been assigned a different colour, although the interior and engine sections overlap, because the footwell is forward of the dashboard. This gives the illusion that the main part of the loom and dashboard wiring is situated in the engine compartment, which is not the case. Most of the main loom and wiring to instruments and controls are behind the dashboard inside the car.To avoid confusion loom wires in the engine compartment are labelled with blue text, whilst interior loom wires are labelled with red text. The front end wiring is labelled with green text and the rear end is labelled with grey text.

If the loom has been carefully renovated, it should be able to be laid out in a similar fashion as shown in Figure 3 and reach both ends of the car. All terminals should be clean and all wiring should be intact and properly sleeved in loom tape or braided sleeving. If the circuit diagram in the '4/4 Morgan Owners Handbook' has been adhered to, the correct coloured wires should emerge from the various branches of the loom as follows: