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Secrets of British Wiring.
Coping with that rat's nest of faded
and tattered wires under the dash and in the engine compartment of your prized Rover, MG,
Jaguar, Hillman, Triumph or other examples of fine British automobilery, or meandering
along the frame of your favorite British two-wheeler, can be irritating at best.
Some of you are fortunate that the original colors are still visible and so are easily
traced.
The rest of you may not be so lucky.
Do not despair!
Even if you do not own a wiring diagram for your particular car, the following information
may be enough so that you can properly connect, repair, or even make a new loom (or harness) to the
manufacturer's exact coding. At least, you will be able to identify unknown loose wires.
Also included is a list of places where you may he able to get your harness repaired or replaced
with an exact duplicate.
What's in a Loom?
Early wiring looms were covered with a black cloth wrapping interwoven with various yellow or
other light colored tracers. Later ones were covered with black or other colored plastic tape.
In the construction of all looms, friction tape is used along the various legs, and at branch
junctures to hold individual wires in place while the harness is being wrapped by a special machine.
There are usually three or four different sized wires to carry the current loads of various
circuits. The most common sizes are: battery main feed (44 strands x 0.012" (appr. 5-gauge U.S.)
- 22 amperes); generator main feed (28 x 0.012" (appr. 12-gauge) - 14 amperes); and other
circuits (14 x 0.010" (appr. 16-gauge) - 5 to 6 amperes).
Other sizes include 37 x 0.036" (20-gauge) 0.68 amperes and 61 x 0.036" or 61 x 0.048" (18-gauge) -
1 ampere.
Note that the smaller the number in the American wire gauge system, the heavier the wire. Only
these wire sizes were used on vehicles prior to 1967. After that, a number of additional sizes were
needed to handle newfangled and sometimes more powerful appliances.
Color me What?
There were only seven basic colors used (solid, or with various contrasting stripes or tracers)
during the early years (post-war to 1967). They were:
(1) Brown - Battery circuit. From the battery or starter switch to the ammeter or
voltage regulator (control box), and feeding light and ignition switches, and radio from control
box terminal Al.
In addition, brown was the basic color used for leads from the starter switch to the electric
clock, inspection lamp sockets and battery auxiliaries fuse (from which are fed the horns,
cigarette lighter, interior lights, etc.).
(2) Yellow Generator circuit. From the generator armature terminal to the D terminal
on the control box, and to the ignition warning light.
(3) White - Ignition circuit. This color is used for all (usually unfused) items that
are powered up when the ignition switch is turned on - such as fuel pump, starter solenoid switch,
overdrive, etc.
(4) Green - Auxiliary circuits. All circuits fed through the ignition switch and
protected with fuses or circuit breakers such as stop lights, turn signals, fuel gauge, wipers,
etc.
(5) Blue - Headlamp circuits. Fed from terminal S2 (or H) on the headlamp switch.
(6) Red Side (park), dash and rear lamp circuits (some manufacturers used purple in
their dash-light circuits). Fed from terminal S1 (or T) on the lighting switch. Included in these
circuits are fog lamps and other lamps supplied with power only when the park lamps are in use.
(7) Black - Earth (ground) circuits. Used for all devices that are not grounded
internally.
Components of each circuit consist of three wires: the feed; the switch wire and the return.
The return wire is not needed, of course, if the component is grounded directly to the chassis or
through the body of the car, or frame of the bike.
Some variations in fusing, switching circuits, and so on were employed, but the three-wire principal
is carried out through all vehicles. I must add, though, that since the introduction of solid-state
technology, this last statement may no longer be true for all circuits and/or components.
Feed or supply wires are always of a solid color. Switch wires have the main color of the feed wire,
but with a tracer. Ground wires are always black.
Be especially careful when fitting wires to any component. Don't mix them up because not all
circuits (especially lighting) are fused. One careless mistake could cause your loom to fry all the
way from the component to the source of power when you switch on that errant circuit.
In fact, it is recommended that you install an in-line fuse holder (and appropriate fuse) in all
circuits that do not contain a protection device. A convenient and out-of-sight mounting place can
usually be found somewhere under the dash, or under the gas tank on a motorcycle. You will usually
connect one end of the fuse holder to the switch and join the other end to the wire(s) that usually
go to the switch. Fuses for headlamps should be 50-amperes, and for park, tail or license lamps,
35's.
When components are controlled with switches in the ground circuit (like most windshield wipers),
the fact is noted by a wire that has a black tracer. You will want to keep a sharp eye out when
playing with other wires in these circuits because they are "hot" ones, and some of them are not
ever "dead" unless the battery has been disconnected.
What's in a Diagram?
Early British wiring diagrams are coded with numbers representing the color of a wire in any
given circuit. Later schematics use only letters. If you run across a diagram that is missing the
translation, there is a list of the codes for both systems near the end of this article.
Going it Alone
When you are trying to salvage or repair an existing harness, and you can't identify the color
of a particular wire because of fading or deterioration, you can usually cautiously cut and peel
hack a bit of the cloth wrapping to a point where it reveals a wire's true flavor. It you are
careful, you probably won't ruin too much of the cloth wrap. It is a good idea to keep a needle
and thread handy to prevent further raveling by stitching up the area you peeled back after you
identity wires or make repairs.
If it becomes necessary to completely strip the harness of its outer covering, don't fret; there
are several companies who will re-wrap it, just as it was. Mind you, don't fail to save a sample
of the original pattern.
The English Connection
Most early British vehicles use an archaic method of connecting wires to their terminal points -
they let you fuss with bare wires that have to be held in place while you run down a brass set
screw to secure them. This operation can be made a whole heck of a lot easier even when you have
as many as four or five wires to cram into the tiny space provided. The secret to beating this
inconven-ience is in the proper preparation of the ends of the wires.
If you strip each wire the same amount i normally 4" to 3'8"), you can almost never succeed,
except when you are dealing with only a single wire. When you get it almost tight, one or more of
the wires will invariably squirt out from under the screw.
By stripping just a fraction more from each succeeding wire, no insulation can get under the
screw as you tighten it. You can make a tidy "bed" of the copper, and so allow the set screw to
clamp down on only the copper strands, as the insulated portions will automatically move to one
side or the other.
Start your securing operation by making sure the loom is supported so that it has enough slack to
allow all the wires you are working with to remain in position. If they seem too strained or too
short, adjust the loom for a little slack in the area in which you are working, and tie it off
with a piece of string so the wires can't move away from where they are to be connected.
If you wish, you may solder the batch of ends all together, inserting a heavy lead in the end of
the pack to use as the sole wire to be trapped under the screw. All ends should be sparklingly
clean before you solder.
In fact, for the best possible contact at any connection, you should make sure all wire ends
are bright and shiny before you secure them.
After twisting each of the individual ends, remove the set screw and insert the wire that has
the least copper exposed. Make sure that when the wire is fully inserted, the insulation clears
the opening.
Then, carefully take the next two wires and slip them over the top of the first, letting the
insulated portions find their own spots.
Follow with the balance of the wires in the same fashion and, while gently squeezing them from
top one to bottom and pressing the ends of all of them toward the "hack" of the hole, insert and
tighten the set screw. If you still have difficulty, you might try taping the wires together
to help hold them in the right pattern while you tighten the screw.
Wiring is really easy if you pay attention to putting the right wire in the right place and
remember, BLACK is always GROUND!
With these few hints and accompanying tables, you should be able to figure out where everything
goes and how to deal with some of the mysteries of wiring.
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© RWP dec. 2004