Tiger2, post 19:
The diagram in post 13 is nicely drawn. It is easily understood by experienced Model A persons. It is lacking a few details that will be needed by the beginner Model A person that is wiring his car for the first time.
The diagram shows a 6 volt battery with the + pos post grounded to the frame. It is important that the newbie know what battery and which cable is grounded to the frame. 12V or 6V, and Pos + or Neg - grounding is being used. A 12V, negative ground system requires a reversal of the wires at the ammeter and the coil from those of a 6V + positive ground one.
Professional wiring diagrams (for those with formal training) are generally drawn with no relationship of the components to their actual position in the vehicle. Components are identified within a symbol. Actual component positions in the car might be commented on the diagram in a convenient location. IMHO, this drawing is a duke's mixture as it depicts renditions of the components and their somewhat relative positions in the vehicle, which actually helps the beginner. However, it lacks the markings of + plus and - minus on the key components. The perspective of the drawing seems to be from the front of the car looking toward the firewall.
Assuming it is the backside of the ammeter that is depicted, the two wires attached there should be drawn with one crossing over the other. The ammeter will read backwards if these connections are incorrect side to side. The wires that run from the terminal box studs to the ammeter on a stock Model A are not yellow and yellow. They are yellow and yellow / black stripped. Refer to the front face of the ammeter for proper connections.
At the ignition coil on a stock Model A, the short black wire that is sourced from the terminal box powers the primary coil winding. It must attach at the - neg post of the coil and the opposite post, the + pos post must power the ignition points via the switch, else the polarity of the spark at the spark plug will be reversed causing a weak spark. The car will run, but it may start and run poorly if there are poor connections or weak components. There are methods to test the polarity of the ignition.
There is a service bulletin which moves the source of the short black wire, which powers the coil primary from the yellow wire / terminal box stud to the yellow / black stripe terminal box stud. This bulletin can be found in the November 1929 chapter. This change causes the opening and closing of the ignition points to be depicted on the ammeter when starting the vehicle and at lower rpms. This is useful when diagnosing ignition problems. The newbie should make sure he is wired accordingly.
Snyder's Model A Parts sells a very good 11 x 17 wiring diagram, which is produced on a rugged plastic sheet for a small amount of money. It depicts the last of the wiring changes for the Model A 28-31.
https://www.snydersantiqueauto.com/p...6763&cat=41801
I hope someone finds this post helpful.