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Features Archive:
Joseph Lucas: The Prince of Darkness.
(1890s translucent overalls whilst undeniably stylish proved sadly impractical..)
So there you are, grubby Haynes Manual in one hand, circuit tester in the other trying to work
out why the lights/horn/wipers/cigarette lighter/clock etc. won't work, (apart from the fact that it
has 'Lucas' stamped on it that is.) Tracing the circuit usually works eventually but often you
find yourself with several loose wires and no clues as to where they go, in which case you
need a clue or two.
There is a British Standard for vehicle wiring number BS-AU7a which defines the colours and
allows you to trace where the wires should go from first principles rather than a diagram. It
also allows accessories such as electric fans to be added with the correct coloured wires
which assists later diagnosis enormously.
None of this helps the fact that the things on the ends of the wires are usually made by Lucas
and are therefore of er.. variable quality! I'm particularly fond of a description of an E-type
Jaguar's headlamp switch as having" three positions: DIM, FLICKER and OFF.
Anyway, that's enough potentially libellous comments (I'm sure all Lucas products are simply
fab nowadays and the repro stuff is only crap because it is so authentic :) This info should not
only allow you to trace original wires but also to use the correct colours when adding
additional equipment like spotlights, ejector seats or whatever.
Firstly the basics- What The Colours Mean:
BROWN is for direct, unswitched, unfused supply from the battery. Seen on wires to
alternators, dynamos, control boxes and ignition switches etc. Always live and often carrying
heavy current.
YELLOW sometimes used as an alternative to brown on older cars. Also used on solenoids
and overdrive switches.
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PURPLE Direct, unswitched but fused supply from the battery. On older cars with no purple
wires, brown wires with a secondary tracer colour are used.
WHITE Unfused supply from the ignition switch.
GREEN Fused supply switched via the ignition switch. Used for things like the wiper motor,
indicators and brake lights which only work when the ignition is on.
BLUE The main colour for front lights- headlamps and spotlamps.
RED The main colour for rear and side lights.
BLACK Usual colour for wires from components to earth points on the body.
These refer to the main colour of the wire. The second tracer colour which is on many wires is
the thinner line and is used to identify the wire. There is a certain amount of logic to them too
as you will see.
Below is the colour code table for BS-AU7a which was amended in the 'eighties so there will
be a few variations on our '50s to '70s cars..
Main colour is given first , tracer second
MAINLY BLACK WIRES
Black
Black/ Brown
Black/ Blue
Black/ Red
Black/ Purple
Black/ Green
Black/ Light Green
Black/ White
Black/ Yellow
Black/ Orange
All earth connections
Tachometer generator to tachometer
Tachometer generator to tachometer
Electric or electronic speedometer to sensor
Temperature switch to warning light
Relay to radiator fan motor
Vacuum brake switch or brake differential pressure valve to warning light
and/or buzzer
Brake fluid level warning light to switch and handbrake switch, or radio to
speakers
Electric speedometer
Radiator fan motor to thermal switch
MAINLY BLUE WIRES
Blue
Blue/ Brown
Blue/ Red
Blue/ Light green
Blue/ White
Blue/ Yellow
Blue/ Black
Blue/ Pink
Blue/ Slate
Blue/ Orange
Lighting switch (head) to dip switch
Headlamp relay to headlamp fuse
Dip switch to headlamp dip beam fuse. Fuse to right-hand dip headlamp
Headlamp wiper motor to headlamp wash pump motor
a) Dip switch to headlamp main beam fuse
b) Headlamp flasher to main beam fuse
c) Dip switch main beam warning light
d) Dip switch to long-range driving light switch
Long-range driving light switch to lamp
Fuse to right-hand main headlamp
Fuse to left-hand dip headlamp
Headlamp main beam fuse to left-hand headlamp or inboard headlamps when
independently fused
Fuse to right-hand dip headlamp
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MAINLY BROWN WIRES
Brown
Brown/ Blue
Brown/ Red
Brown/ Purple
Brown/ Green
Brown/ White
Brown/ Yellow
Brown/ Black
Brown/ Slate
Brown/ Orange
Main battery lead
Control box (compensated voltage control only) to ignition switch and lighting
switch (feed)
Compression ignition starting aid to switch. Main battery feed to double pole
ignition switch
Alternator regulator feed
Dynamo 'F' to control box 'F' Alternator field 'F' to control box 'F'
Ammeter to control box. Ammeter to main alternator terminal
Alternator to 'no charge' warning light
Alternator battery sensing lead
Starter relay contact to starter solenoid
Fuel shut-off (diesel stop)
MAINLY GREEN WIRES
Green
Green/Brown
Green/ Blue
Green/ Red
Green/ Purple
Green/ Light green
Green/ White
Green/ Yellow
Green/ Black
Green/ Pink
Green/ Slate
Green/ Orange
Light green
Light green/ Brown
Light green/ Blue
Light green/ Red
Light green/ Purple
Light green/ Green
Light green/ White
Light green/ Yellow
Light green /Black
Light green /Slate
Light green/ Orange
Accessories fused via ignition switch
Switch to reverse lamp
Water temperature gauge to temperature unit
Direction indicator switch to left-hand flasher lamps
Stop lamp switch to stop lamps, or stop lamp switch to lamp failure unit
Hazard flasher unit to hazard pilot lamp or lamp failure unit to stop lamp bulbs
Direction indicator switch to right hand flasher lamps
Heater motor to switch single speed (or to 'slow' on two- or three-speed
motor)
Fuel gauge to fuel tank unit or changeover switch or voltage stabilizer to tank
units
Fuse to flasher unit
a) Heater motor to switch ('fast' on two- or three-speed motor)
b) Coolant level unit to warning light
Low fuel level switch to warning light
Instrument voltage stabilizer to instruments
Flasher switch to flasher unit
a) Flasher switch to left-hand flasher warning light
b) Coolant level sensor to control unit
c) Test switch to coolant level control unit
Fuel tank changeover switch to right-hand tank unit or entry and exit door
closed switch to door actuator
Flasher unit to flasher warning light
Start inhibitor relay to change speed switch; or switch to heater blower motor
second speed on three-speed unit
Low air pressure switch to buzzer and warning light Flasher switch to righthand warning light; or
differential lock switch to differential lock warning light
Front screen jet switch to screen jet motor
Fuel tank changeover switch to left-hand tank unit; or entry and exit door open
switch to door actuator
Rear window wash switch to wash pump; or cab lock-down switch to warning
light
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MAINLY ORANGE WIRES
Orange
Orange/ Blue
Orange/ Green
Orange/ Black
Orange/ Purple
Orange/ White
Orange/ Yellow
Orange/Light green
Orange/ Pink
Orange/ Slate
Wiper circuits fused via ignition switch
Switch to front screen wiper motor first speed timer or intermittent unit
Switch to front screen wiper motor second speed
Switch to front screen wiper motor parking circuit, timer or intermittent unit
Timer or intermittent unit to motor parking circuit
Timer or intermittent unit to motor parking circuit
Switch to headlamp or rear window wiper motor feed, timer or relay coil
Switch to headlamp or rear window wiper motor parking circuit timer or relay
coil
Timer or relay to headlamp or rear window wiper motor feed
Timer or relay to headlamp or rear window wiper motor parking circuit
MAINLY PURPLE WIRES
Purple
Purple/ Brown
Purple/ Blue
Purple/ Red
Purple/ Green
Purple/ Light green
Purple/ White
Purple/ Yellow
Purple/ Black
Purple/ Pink
Purple/ Slate
Purple/ Orange
Accessories fed direct from battery via fuse
Horn fuse to horn relay when horn is fused separately
Fuse to heated rear window relay or switch and warning light
Switches to map light, under bonnet light, glove box light and boot lamp when
fed direct from battery fuse
Fuse to hazard flasher
Fuse to relay for screen demist
Interior lights to switch (subsidiary circuit door safety lights to switch)
Horn to horn relay
Horn to horn relay to horn push
Rear heated window to switch or relay
Aerial lift motor to switch up
Aerial lift motor to switch down
MAINLY RED WIRES
Red
Red/ Brown
Red/ Blue
Red/ Purple
Red/ Green
Red/ White
Red / Yellow
Red/ Black
Red/ Pink
Red/ Slate
Red/ Orange
Red/ Slate
Main feed to all circuits mastered by sidelamp switch
Rear fog guard switch to lamps
Front fog lamp fuse to fog lamp switch
Switches to map light, under bonnet light, glove box light and boot lamp when
sidelamp circuit fed
Bulb failure unit to right-hand-side and rear lamps
a) Sidelamp fuse to right-hand side and rear lamps
b) Sidelamp fuse to panel light rheostat
c) Fuse to panel light switch or rheostat
d) Fuse to fibre optic source
Fog lamp switch to fog lamp or front fog fuse to fog lamps
Left-hand, sidelamp fuse to side and tail lamps and number plate illumination
Sidelamp fuse to lighting relay
Lamp failure unit to left-hand side and tail lamps
Fusebox to rear fog guard switch
Window lift main lead
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MAINLY WHITE WIRES
White
White/ Brown
White/ Blue
White/ Red
White/ Purple
White/ Green
White/ Light green
White/ Yellow
White/ Black
White/ Pink
White/ Slate
White/ Orange
White/Yellow
(or just plain yellow)
Ignition switch or starter solenoid to ballast resistor
Oil pressure switch to warning light or gauge, or starter relay to oil pressure
switch. Choke switch to choke solenoid (unfused) and/or choke to
switch to warning light, or electronic ignition distributor to drive resistor.
Starter switch to starter solenoid or inhibitor switch
or starter relay or ignition (start position) to bulb failure unit
Fuel pump no 1 or right-hand to changeover switch
Fuel pump no 2 or left-hand to changeover switch. Start switch to starter
interlock or oil pressure switch
to fuel pump or start inhibitor switch to starter relay or solenoid
Ballast resistor to coil or starter solenoid to coil. Ignition coil contact breaker to
distributor contact
breaker, or distributor side of coil to voltage impulse tachometer
Ignition switch to radio fuse
Current tachometer to ignition coil
Hazard warning lead to switch
a) Overdrive
b) Petrol injection
c) Door locks
d) Gear selector switch to start
YELLOW WIRES
Yellow
(or yellow/white)
a) Overdrive
b) Petrol injection
c) Door locks
d) Gear selector switch to start
PINK WIRE(!)
Pink/ white
Ballast terminal to ignition distributor
That's all folks. The usual disclaimer applies., ie that This is correct to the
best of my knowledge but you use this information at your own risk. So don't
sue me if you blow your alternator or fry all your relays...