[Mgs] Excessive Voltage?

[Paul Hunt]

The first question is why?  But if you *have* added a ballast resistance then
you must also use a matching coil with it, not the original 12v that a 72 or a
68 had, MGBs didn't get a ballast and 6v coil until rubber bumpers.  And when
they did they also got a special solenoid with a bypass contact to give a
boost voltage to the coil during cranking to improve starting.  Without that
you have gained nothing by changing a 12v coil for a 6v with ballast, except
something else to go wrong.

12v coils measure about 3 ohms across the primary (spades) or about 2.4 ohms
for a 'sport' coil.  6v coils measure about half that i.e. 1.5 ohms for
standard or 1.2 ohms for sport.  The factory ballast resistance for these
similarly measures about 1.5 ohms.  There are very many lower resistance coils
that should not be used except with a specialist electronic ignition system
designed for them.

On the face of it if you have 12v on the ignition side and 6v on the coil side
with the points closed then you have equal resistance coil and ballast which
is correct, but until you measure the resistance of each (wiring disconnected)
then you won't know if they are both wrong or both right.  Two 3 ohm items
will give the same voltages, but only half the current and so weak sparking.

Running voltages between coil and ballast are affected both by the car system
voltage and the dwell angle of the points or trigger.  The correct running
voltage of the cars systems is about 14.5v at 3000 rpm, not 12.6v.  That is
determined by the voltage regulator or control box, but will reduce towards
12.8v as the load on the cars electrics increases i.e. recharging the battery,
headlights on etc.  In your case 13v is good enough if that was idle or only
just above it.  It depends on what meter you are using, and what dwell angle
in the distributor, as to what voltage you will see on the coil side of the
ballast.  I'd have said 11v was a bit high, ordinarily with an input voltage
of 13, equal ballast and coil (as you seem to have), points distributor and an
analogue meter I'd expect to see about 9v.  This can be very different with
electronic ignition of most types and I couldn't say what you might read with
a digital instrument.  11v with points and an analogue instrument says to me
you have low dwell.

PaulH.
  ----- Original Message -----


  I've installed a generic ballast resister in my ignition circuit and
  voltage is correct while the car is not running: 12+ volts & 6  volts.  When
the
  car is running, I measure 13+ volts on the ignition  side of the resister
  and 11+ on the coil side and at the coil.  Is this  correct?

  Also, does the voltage regulator affect the operating voltage of the car
  generally, or just the charging system?
  I thought voltage throughout the car should not exceed battery  voltage,
  i.e. 12.6V.