Ignition Coil Connections

The ignition coil does not change the direction of current flow hence it does not change polarity.  However, there are more coils in the secondary winding inside of the coil, thus there is more resistance to incoming current flow at the secondary winding terminal, so the coil’s high voltage output to the spark plugs will be less if the battery is inadvertently connected to the secondary winding terminal.  By the way, the higher resistance wastes primary current in the form of heat and the coil will run hotter if the battery (current-in) is connected to the secondary terminal instead of the primary terminal.  Heat reduces coil life!


So now that we know the primary terminal on the ignition coil is where we want to connect the battery wire, two questions arise to get the connection right in the Model A: (1) Which is the primary terminal on the coil? (2) Which is the correct wire from the battery that connects to the primary terminal?

    (1) Using an ohmmeter with one lead in the high voltage output socket, measure the DC resistance at each wire terminal.  Connect the battery wire to the terminal with the least resistance to get the highest secondary voltage output to the spark plugs.

    (2) Assuming that the Model A has a 6-volt system wired in accordance with the factory diagram, the BLACK wire from the driver’s side of the terminal box is the correct wire. If your Model A has non-factory wiring, then find the wire coming from the CHARGE side of the ammeter and trace it down to the terminal box stud.  Connect this stud to the primary terminal of the coil.


Do not rely on the markings that may be cast into the coil top at the terminals.  Before 1955, coils were marked  (-) or BAT at the primary winding terminal, and (+) at the secondary winding terminal because the ignitions of the time were 6-volt (+) ground.  By 1956, ignitions went to 12-volts (-) ground, so the primary terminal was marked (+) or BAT.  So you can see the coil markings may cause you to make the wrong connections for a pre-1956 Ford.


If you find the above confusing, here is a much simpler solution.  Buy from your local Model A parts supplier, a gizmo known as a “Ignition Spark & Coil Tester.”  It costs in the ballpark of $25, and it has lights to indicate if the wiring is right or wrong.  This gizmo will earn its price back in gasoline mileage and performance.


Most all coils, 6 and 12-volt alike, for breaker point ignitions are (the same) designed for 6 to 8-volt operation.  Note the resistance wire in the figure.  In a 12-volt ignition system, the ignition switch has 2 run positions.  The START position allows 12 volts into the primary coil to get a hot shot high-voltage engine start.  When you release the key to the RUN position, the resistance wire is cut into the primary circuit to drop the coil voltage to 6 to 8 volts.  The heat from a constant 12-volt input will shorten the life of a breaker point ignition coil.


When converting from 6 to 12-volt operation, you can run a reproduction “Ford” script 6-volt coil with an external resistor so the coil operates at 6 to 8 volts.  You can also run a 12-volt coil with an external resistor.  However, some 12-volt coils have an internal resistor and you need to know this to avoid having two resistors on the primary side of the coil.


Running an antique ignition coil is fool hardy because eventually coils breakdown and cease to function without warning.  Do not run an original antique coil in a 12-volt conversion because these old coils will not take the stress of 8 to 12 volts.  Also make sure that the brass terminal ends are soldered to the conductor of the high-voltage wire between the coil and the distributor.  Keep the primary wire connections bright & tight and the plastic parts of the coil and distributor clean.

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