RM18 to 12v - wires

In case this is not answered directly.

Get hold of 2 wiring diagrams, one for 6V RM19 and one for 6V RM18 and compare the alternator wire colours referenced on the diagrams to the connections to the lighting switch and the alternator coils. You should be able to xref from the RM19 to the RM18. Other way is to check the ohms readings between all combinations of 2 wires to identify the 2 coil vs 4 coil connections.

According to this site

https://www.jrcengineering.com/technical-support/lucas-alternator-tips/

Light Green is Green White

Green Yellow is Green Yellow

Dark Green is Green Black

This is half way down the page

As you have 2 conflicting sources then you are back to my other 2 suggestions. Comparing diagrams and testing which combination of 2 wires are 2 or 4 coils in circuit.

The JRC info is direct from a Lucas manual but only for RM13/14/15 so you need same for RM18 which should be somewhere in a Lucas PDF on the internet.

Looking good.

The 'do I add the 6V to 12V upgrade' is no longer a simple yes. With the advent of 6V leds which are as bright as the 12V leds (they are all 5V DC internally at the led, a voltage regulator is always inside) but if you have coil ignition then 12V allows the use of EI and keeping the two 6V coils wired in series for the sparks so less outlay.

The current headlamp H4 LED bulb I use is labeled 6 to 24V and works in neg and post earth circuits as it has as part of its voltage regulation circuit a bridge rectifier. It draws 24W and is brighter than the H4 halogen 55/60W bulb it replaced. 

Indeed such is here. I can see where it comes from (these old colours are on my web site) I will explain later (with a very clear head). But some little points.

LED are not 5V they are ABOUT 2.5V. But to be used in any Automotive situation they need regulation. The power produced at your rear wheel is NOT affected by the power from the alternator. (In fact being pedantic you MIGHT get a smidgen more at the rear wheel if you allowed your alternator to be open circuit).

12V allows the use of EI and keeping the two 6V coils wired in series for the sparks so less outlay.

Again NOT quite so, the Dominator would have had a single 6V coil in the beginning, so buying it 2X6V coils does not save anything. But more on Alternator colour confusion after I have though about it. PS you can't damage the alternator by misconnection, you just get a lower output-nuisance.

Again NOT quite so, the Dominator would have had a single 12V coil in the beginning

? 1958 motorcycle with 12V coil.

No, Al has edited his error.

Dominators from the factory were 6V!

Yes there was colour of cable error on my web site, it had been there best part of 20 yrs!! But it is/was not significant as it transposed the High and Low outputs of the very early alternators. When converting to 12V these get connected together any way! And as John says I delved into the world of 1950s alternators as best I could and corrected my web site.

By the way for your delectation, another one of my hobbies is repairing old and not so old radios. I have been trying to repair a 1980s Sony radio, the printed circuit board around the control pad is quite corroded. It is positive earth!!

I am resurecting a Technics  Hi fi stack for my grandson  who has (out of the blue)  developed a passion for old  Beatles records  after a week in Dublin drinking Guinness  and getting off with an Irish girl.