(THIS MEMO COMES FROM BOB BIDONDE)
Ignition Coil Hookup 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
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, and here's why. 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 2 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. And "Oh by the way," 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.
Bob Bidonde
