This month's Texas MG Registry's newsletter had some excellent information about our car's wiring color scheme which I'm sharing below. In addition, For MGB owners (as well as the lesser Triumph marque), a friend of mine, Steve Carrick, who owns Advanced AutoWire (which offers the absolute best aftermarket & upgraded wiriing harnesses for our cars), has the most complete set of wiring diagrams for Triumphs & MGB's that I know of. They can be found at http://advanceautowire.com/ in the "stock schematics" section. It is worth downloading & printing the schematic for your year and even laminating it to keep in the car.
This information below is what was in the Texas MG Registry's newsletter and is an excerpt from 'Rewiring Your MG' by Dave Braun.
Wiring Colors
Lucas wiring codes are simple and although there are occasional exceptions to the color codes, memorizing the codes is a worthwhile practice for someone rewiring or trouble-shooting a British car. The common color code letters are included here for reference. Please note that each of these codes have sub-codes that are denoted by colored 'tracers' but still follow the main code traits.
BROWN (N) is a direct feed from the battery. It is always hot (un-switched) and un-fused. The only way to isolate a brown circuit is to disconnect it from the battery. Brown feeds go to the ignition switch, headlight switch, some hazard flashers, the starter relay, the alternator and the fuse box.
PURPLE (P) is what become of a brown wire after it goes through the fuse box. Purple is always hot (unswitched) but fused. Purple circuits go to the horns; some hazard flashers; courtesy and door lamps and the flash to pass switch (yes...two 12-volt sources in that little dipping switch!). You can isolate the purple circuit by removing the fuse.
WHITE (W) is what becomes of a brown wire after it goes through the ignition switch. White is only hot when switched and is un-fused. All the components that come on with the need to run the engine are on the white circuit, including the fuel pump, the ignition coil, on some cars the overdrive, any feeds to seatbelt circuits, the radio and the fuse box to provide a switched source of power.
GREEN (G) is from the white wire after it goes through the fuse box. Green is switched and fused. Green circuits provide power to most of the gauge functions (including the infamous 'voltage stabilizer'), the turn signals, heater fan, wipers and brake lights. You can isolate the green circuits by removing the fuse.
BLUE (U) is from the brown wire after it goes through the headlight switch. Blue is only hot when switched and un- fused. Blue goes to the headlight dipper where it becomes two important sub-codes - blue with a red tracer (UR) to the dipped beam lights and blue with a white tracer (UW) to the main beam lights.
RED with GREEN TRACER (RG) is also from the brown wire after it goes to the headlight switch. I think this is where someone at Lucas went to lunch, had a few pints, came back and mixed up the codes. This is the code that goes to the fuse box with a tracer and comes out of the fuse box as a single color. So RED WITH GREEN is only hot when switched by the headlamp switch and it is un-fused. RED WITH GREEN goes to the fuse box as mentioned and also the gauge lamp switch or dimmer. A special subset of the RED WITH GREEN tracer is the RED WITH WHITE (RW) tracer wire that comes out of the gauge lamp switch (dimmer). The RED WITH WHITE tracer is the instrument lighting circuit.
RED (R) denotes the RED WITH GREEN tracer wire after it goes through the fuse box. As such, red is switched and fused. Red circuits are the front parking lights, the tail-lights and the license plate lights.
BLACK (B) wire is a ground wire. Any other color wire with a black tracer is likely the ground side switching wire for that component.
