If you haven’t guessed, the Haynes manual’s wiring diagram is VERY wrong especially for US 1500’s. Tom O’Malley has spent many hours redesigning wiring schematics for maximum readability. They are the best diagrams I have ever seen saving me many hours troubleshooting. They are so good I wish I had thought of them. Tom, if you are out there, email us!
Modified Tom O’Malley diagrams:
GT6 Mk III (non emissions), download this link.
Non-Tom O’Malley diagrams:
Spitfire MkIV wiring diagram from the factory manual.
North American 948 Heralds, Spit Mk2 and Triumph 10(outside link), posted by Andrew Mace
Here is a SET of wiring diagrams for the Overdrive wiring in PDF format. Since many manuals show the diagrams differently or poorly I have shown all options from all manuals I could find.