A prominent up side to Lucas wiring is that the colours for all of the basic circuits are the same regardless of year, make, or model. This goes for all Lucas wired cars from the late ‘40’s through to the late ‘80’s, (it may continue later but I don’t want to be responsible for someone’s cooked car). Once you have figured out your Mini, you can graduate to the neighbours MGB and look like the all knowing hero.

The solid colour wires are always between the power source (battery or fuse box) and the switch. The wires with a tracer are always between the switch and the load.

The colours, while not conventional, are very consistent. By un-keyed I’m referring to the ignition switch.

Brown – un-keyed, un-fused power - aka. Battery. e.g. – starter solenoid to ignition switch and fuse box.

Purple – un-keyed, fused power – e.g. – fuse box to interior light, fuse box to horn on two wire horns
White – keyed, un-fused power – e.g. – ignition switch to fuel pump and ignition coil

Green – keyed, fused power – e.g. – to all keyed accessories, fuse box to heater switch, flasher unit, . . wiper switch, etc.

Blue – headlamp circuit – solid colour from the fuse box to switch, with tracer from to dipswitch (blue/yellow) and dip-switch to lamps (blue/red – low beam, blue/white – high beam)

Red – park lamp circuit – solid red to lamps and to dash light resistor, red/white from resistor to instruments.

Black – ground – this means a good ground, not some painted/rusted scrap. The larger the load, the more important this becomes.

Black as the tracer? – this usually indicates a switch to ground – e.g. white/black is coil-ve to the points, purple/black is horn to horn switch (as the switch goes to ground).

So….. with this knowledge you now know that when you are looking at two wires going to a switch the solid wire should have power, and with the switch in the on position the wire with a tracer should also have power. And if the wire is green and you have no power on the solid wire, the next place to look would be the fuse box. Simple!

Some other tips for preventing the curse of Lucas:

• When restoring your car, or when having problems, replace the fuse box. I can’t stress this enough, look at where the old one has been for 20+ years, they corrode on the contacts and will cause no end of grief.
• Use new female bullet connectors – the metal connector covered in rubber – they are available new and are usually toast after being removed. The cost of new ones is far outweighed by the hassle saved of chasing problems later.
• When installing your new harness and new bullets, put a little dab of Silicone Di-electric Grease in the connector (Q-tips work great for this) to prevent future corrosion.
• Be sure to push the bullet fully home into the connector. A push on the bullet with little screwdriver until it clicks in is needed. I’ve seen far to many cars with this not done which can lead to shorts from the still exposed end, or the wire falling out leading to shorts and/or much cursing when the lights die at night (trust me on this).

As with any general rules, there are always exceptions so go probing wires at your own risk.