Wiring Mysteries Unravelled

Car wiring often makes experienced mechanics weep, possibly because electricity can neither be seen nor heard in the wires and only short glimpses of the car's loom are seen before it disappears through a hole or hidden corner of the boot. Indeed the routes taken by the wiring are often as mysterious as their function. An exploratory glance into the gloomy recesses under the dashboard can be very off-putting. This cats-cradle of spaghetti is not as difficult as it may first appear. Unlike spaghetti, the wires are colour coded. MGs use the Lucas code of seven main colours and a secondary tracer colour. This code combined with a wiring diagram in your workshop manual will enable you to identify most wires without trouble. The colours are usually abbreviated in these diagrams by their initial letter. Some colours start with the same letter so another letter is chosen, B is for Black so N is for brown, U is for blue. Each colour gives a clue to the wire's function:-

BROWN (N) A direct, unswitched, unfused supply for the battery. These wires usually lead to the fuse box, voltage regulator (cars with dynamos) or the ignition switch. Sometimes a brown wire may feed the lights switch. It is generally prudent to disconnect the earth strap from the battery before playing around with brown wires as there is no fuse and you could burn the insulation and in extreme cases cause an electrical fire.

YELLOW (V) Used as an alternative to brown on some older cars. More recently used on overdrive switches and solenoids.

PURPLE (P) A direct, unswitched but fused supply from the battery. Often leads to the lights switch or the horn. On older cars before purple was introduced as a colour, brown with a secondary tracer colour was used.

WHITE (W) Unfused supply from the ignition switch. Usually leads to the ignition auxiliary fuse or to components which are switched via the ignition switch but are not fused.

GREEN (G) Fused supply switched via the ignition switch. Usually leads to components such as the windscreen wiper switch, stop lamp switch, flasher unit, heater fans and the bi-metal instrument voltage stabiliser.

LIGHT GREEN (LG) This colour is usually associated with direction indicator lights.

BLUE (U) The main colour for supplies from the light switch to headlamps, fog and spot lamps.

RED (R) The main colour for side, rear, number plate lamps and, occasionally, spotlamps.

BLACK (B) Main colour for leads running from components to earth.

Components can be switched either from the supply side (battery-switch-component-earth) or the earth side (battery-component-switchearth). In the latter case, the wire coming from the component usually has a black tracer colour. The colour code is your friend. Without it, trying to trace a fault would be more time consuming. If ever you replace a wire or connect up some new component or accessory, respect the colour code - it may help you if it stops working. To buy a complete set of different colour wire from an electrical supplier can be difficult and/or expensive. A trip to your local scrap yard can be very rewarding, giving you a full range of colours for a few pence. Incidentally, if you are removing an electrical component from a car in a scrap yard, do not pull the connectors off-cut the supply wires leaving an inch or so connected. This will save making notes at the time of removal and head scratching when connecting it up later as sometimes components have more terminals than connected wires. The same tip applies if you are replacing the loom on your car as there is rarely room to have both the old and new looms in the car at the same time to enable an easy swap.

Check a new loom with the wiring diagram and become familiar with it before taking it anywhere near the car. Identify and label the ends. Masking tape folded over the wire and stuck to itself can be labelled using a bait-point pen. If masking tape is left on for too long the glue may remain behind when the label is removed ... chrome cleaner will dean this up. Do not use self-adhesive clear tape for sticking labels as the glue will dry out.

Another factor to consider when wiring a car is the thickness of the cable you are using. The number of strands dictates the electrical current the wire can carry. 14 strand wire is fine for most parts of the car, 28 strand should be used for headlamps, spotlamps and foglamps, 65 strand for dynamo and control box feeds and also ammeters, 97 strand wire must be used for the main feed from the battery to the control box. Supply and earth connections to a component can be more trouble than the component itself.

Wires are usually connected to components or other wires by three types of connector, the screw terminal, the bullet or the Lucas-type. Invest in a handful of the correct types of connector for your car.

Tools I consider essential for working on car electrics are a decent wire stripper (penknives or cheap strippers tend to damage the wire strands as well as remove the plastic insulation), a pair of fine nosed pliers (available from small radio shops), a bulb with a couple of wires attached to find volts (avoid 'sparking wires to discover it they are live!), a small soldering iron (try your radio shop -but do not expect it to fix radiators!) A small multimeter is very useful, a cheap one may not be especially accurate but will give years of useful service if treated and stored with care.

I prefer soldering connectors to wires rather than crimping them on. The results are more original and are less susceptible to vibration. Cleanliness is the key to successful soldering - wash you hands between doing an oil change and reaching for the soldering iron! Use flux-cored solder for wiring as these will not corrode the connector and break the circuit; The task can be made much easier by tinning (covering with a thin layer of solder) the iron, the wire and the connector. Tinning the iron is the first and easiest operation - melt a small blob of solder over the iron tip and wipe on a clean, damp sponge or rag. The wire is then tinned by holding the iron against the wire and placing the solder against the wire, when the wire warms the solder will flow between and around the strands of wire. This is where the fine nosed pliers are useful - if they grip the wire next to the insulation, they will act as a heat sink preventing the insulation melting and also saving burnt fingers. The connector can be tinned in the same way. Any plastic covers or grommets should now be slid over the wire. The components can be joined and fixed by a touch from the iron; any additional solder required should now flow to the correct places when heated. There is generally a shortage of hands when soldering, an elastic band wrapped around the handles of a pair of pliers enable wires and connectors to be held securely but gently - one of my little vices.

Lucas type Connectors should also be crimped onto the insulation of the wire using a small pair of pliers. After joining the wire, give it a little tug, if it is mechanically sound it is likely to be electrically sound. As an aside I have found that tinning the ends of the multi-strand throttle and choke cables has saved lots of frustration when re-assembling carburettors as there is usually one strand that points out sideways and refuses to line up when threading the cable through the linkage.

Grommets are essential where wires pass through holes in panels. Vibration will cause chaffing and short circuits to earth in difficult to find places. Sometimes a grommet will have perished or vanished. Disconnecting the loom passing through the panel, sliding on a new grommet, reconnecting may be a major job. A solution is to cut through one side of a grommet, fit it around the cables and re-join the cut faces with a dab of super glue.

Tidying the loom is best done using electrical binding tape. This tape is similar to insulating tape but does not stick to itself with the same ferocity making it easier to wrap around wires. This feature also enables the ends to unwrap unless secured with a couple of turns of insulating tape or masking tape (painted black?). The older type of woven loom casing cannot be reproduced using DIY equipment, to be original you will have to buy a ready bound full or part loom.

I hope these tips on how to break the Lucas colour code and some of the dodges involved in vehicle wiring may help your MG look as though it has been wired by The MG Car Company rather than by a pack of Boy Scouts practicing their knots!

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