The SM’s electrical system is quite standard for a car of its era. As a luxury model items such as electric windows were fitted as standard. Complete and part wiring looms are available if sections need replacing.
Most owners choose to fit an electronic ignition system to their cars as the the original manual system was not entirely satisfactory.
Being a Citroën you would almost be disappointed if there weren’t some quirks or unusual features. Probably the most notable electrically is the rain sensitive wipers. Although these are now common place on cars the SM does not use some sophisticated optical sensor glued to the windscreen. It simply works on resistance and a bi-metallic strip. As the windscreen dries the resistance builds up and the heat in the strip breaks the contact and the wipers stop. Ingenious. Of course this is not an off the shelf part either!
Reproduced below is a basic wiring diagram for the SB SB carburettor car, although there are three wiring diagrams, one for the SB SC Efi cars and a separate one for the American specification cars, but all of these are in the relavent workshop manuals with an index to the wiring diagrams.
SM Wiring