C15 Alternator Wiring

One question I have been asked many times, is which wires are which on the C15 alternator. So hopefully this will be useful to many people.

Background

The RM18 & RM19 Lucas alternators used by BSA have six output coils. In the 6v configuration the output is divided so that when the machine is running on ignition alone one third (two coils) of the output is used and when the lights are switched on the remaining four coils are brought into use. This entails some complicated switching and wiring. You have only to look at the wiring diagram to see that the ignition switch has about ten wires and numerous link wires in it. If the switch develops a fault, it can result in lack of current when the lights are on, which will result in a flat battery. Due to the complication of these switches, they are expensive if and when you can find them.

Lucas RM18 or RM19 Alternator

The biggest problem comes when you have disconnected the alternator without looking at the colour of the wires and have not made a note of the connections. Because of their age it is possible that someone may have soldered on new wires of none standard colours. When you come to put it back together you are suddenly faced with a problem of which wire goes where? The most common choice of new wires are Red, Black & White for some reason. Probably due to the fact that those colours are available to none automotive workshops.

The Wires

Below is a section of the alternator which has six coils connected to three wires.

So given this information you should be able to resolve your problem and get the bike back on the road. You may retain the original 6 volt system or convert to 12 volt electrics, which present better lighting and are easier to set-up.

Should you change the wires back to standard? Well that is up to you. A lot depends on the condition and your ability. If you do not, then at least make a note some where, as no doubt the next owner will be asking the same question.