Place a wire from the coil about 1/4" from a grounded part of the
vehicle.
Insert the exposed lead from an ordinary lead pencil in the middle of the
gap.
(don't hold pencil in bare hand)
cause the coil to spark--A brighter reddish flash should emanate from one
side of the pencil lead---This should be the coil to sparkplug wire. If
not, reverse primary leads.

Orrin Iseminger wrote:

> How do I identify which ignition coil terminal is positive and which
> is negative.
>
> I'm installing new wiring in a Model A Ford and the instructions
> specify that certain wires should be connected to the + terminal and
> other wires connected to the - terminal. However, this particular
> coil has no markings on the terminals.
>
> If I hook it up wrong, what will the symptoms be or how can I tell if
> it is hooked up right or wrong?
>
> This particular coil carries the "SP" logo. I assume it's the same
> outfit that made Model A distributors.
>
> TIA
>
> Orrin