SDC Forum #71720 (May 2013) and addendum
Either 6 or 12 volt coils should show roughly .4 to 2.0 OHM resistance in the primary
windings, which are the (-) to (+) poles on the coil perimeter.
Secondary Winding (Hi Voltage to either perimeter poles) should be 6 - 15,000 OHM .
Newer coils with segmented core construction typically read 6,000 to 8,000 ohms,
while others can may read as high as 15,000 ohms.
A 12 volt coil with an internal resistor will show 3.0-3.5?. (internally wound
to create higher resistance.
Points Ignition:
A 12 volt coil without internal resistance will show 1.5 OHM. The working
ohmage of a 12 volt coil on a V8 should be ~3.0 ohms to prevent running hot
and burning points. The higher Ohmage is supplied by an External resistor.
Measure your coils primary resistance (-) to (+) on the coil perimeter.
If less than 1.5, then add the correct ballast resistor to bring the
resistance up.
Most 6 volt coils have a primary resistance somewhere between 1 and 1.2 ohms
and 12 volt coils have a nominal resistance of 1.5 ohms when used with a ballast
resistor and around 3 ohms with an internal resistor.
(From SDC#71720)
I use a vacuum tube
voltmeter to check coil resistance because they can be set to show a zero
reading when the leads are touched together. Many of the late 30's and mid
40's Fords used an external resistor with their 6 volt coils. Bud
From (http://www.yesterdaystractors.com/cgi-bin/viewit.cgi?bd=ac&th=138584)
THERE ARE HARDLY ANY COILS, 6 OR 12 VOLTS, THAT CONTAIN ANY INTERNAL STAND ALONE DISCRETE
"RESISTORS".
YES there were a few very old, mostly automotive, coils that contained hidden inside
the can (often a ringed seperate compartment) a stand alone discrete "resistor".
HOWEVER what many lay people refer to as "internally ballasted" or a coil with an
internal ballast/RESISTOR IS NOT USUALLY THE CASE.
Whats commonly used on a coil thats designed for nominal 12 volt operation is EITHER
additional windings OR higher resistance wire, such that the total primary winding
resistance is in the 3 ohm range so theres around a 4 amp draw which the points have to
carry and switch on and off so they dont burn up prematurely.
Coils labeled "12 volts" OR "12 volts NO external ballast required" are designed for 12
volts and some call them "internally ballasted".
HOWEVER a coil labeled "12 volts, for use with external ballast resistor" is in essence
a 6 volt coil and the voltage dropping (12 to 6) external ballast is indeed needed or
the coil will overheat........
So call them coils with an internal resistor or internally ballasted if you like BUT
THERES NO "RESISTOR" INSIDE THEM!
However, a 12 volt coil has enough wire/windings or wire with sufficient X ohms per
unit length resistance such that the total primary resistance is in the 2.5 to 4
ohms range (3 typical) so that NO EXTERNAL BALLAST IS REQUIRED I will grant you that.
A 6 volt coil usually CONTAINS NEITHER an internal "resistor" nor internal wire/coil
windings containing a total resistance much over 2 ohms (1.5 typical) and a good parts
man wouldnt ask the question if you asked for a 6 volt 50 or 60's vintage coil.
Although, if you asked for a 12 volt coil, its a legimitate question to ask if it uses
an external ballast or not, and if so it will be labeled "12 volts for use with external
ballast". If NOT, its labeled "12 volts" or "12 volts NOT for use with ballast"