Hello JonE. I am the author of the wiring diagram from the Dorset A7 club website. It was an attempt to combine disparate information from several sources, and to draw things in a more understandable "flow" fashion rather than the mechanical "birds eye view" style favoured in the 1930's.

The PLC ignition switch on my car has an internal link between the A terminal and an unmarked one which is one of the inner pair for the ignition switch part. This gives you two places to attach a wire to, which is easier than stuffing two wires into just one terminal. Other examples of this switch may differ of course.

Current flows from the battery via the ammeter and the white/purple wire into the unmarked terminal, through the ignition switch and out again via the IG terminal to the ignition. It also flows via the internal link to the lighting switch part (shown in the SIDE position), and to the cutout via the A terminal and another white/purple wire. Current flow direction depends on whether the battery is charging or discharging of course.

For the starter wiring, I recommend the earth return wire is taken direct from a lug under any of the starter securing bolts straight back to the battery. You also need a lighter gauge earth return link back from the body for the lights etc. This can either be direct to the battery or via the engine block and starter return as the diagram shows. If you don't have this body link then current may flow instead via the choke cable outer, the oil pressure gauge pipe or some other less than satisfactory path.

There is no harm in adding supplementary earth links, but remember that these cars did all work when they left the factory. Assuming no fibreglass bits, I wouldn't bother unless you are getting a significant resistance that you cannot resolve in some other way, such as cleaning under the heads of the bolts that attach the wings.

Location: New Forest

John.. And others.. Thanks.
John.. I tried to email you unsucessfully as I've linked to that diagram on my blog (it was recommended to me by Ruaridh and i too found it very useful.) hope ok to use.. Ive added the Dorset website onto the image.

Interesting the other point about keeping a wiring diagam in the car. I always though one of the 'spaghetti' diagrams would make a good T shirt... But sadly i dont think the images are out of copyright. Perhaps the earliest are...?

Current email is as per my post above, feel free to PM me. The one on the diagram is now out of date as it was from my place of work and I retired 6 months ago when I got to 60 !

Location: New Forest

I notice in John's excellent diagram that the headlamps are shown as 24 watt cf the 18 watt of the handbooks. A point perhaps to watch for those attempting to match dyanamo output to load.

Any introductory guide for new owners should include the Cornwall Club "Charging Refresher" article (with correction!), John's diagram, and a table of original lamp sizes in watts (And original plug gaps, all three ignition timing settings with distributor id, and much more...)

Bob Culver

Location: Auckland