Michael Lundsford wrote:
"Would someone out there enlighten us on the coil voltage issue. It was my
understanding that unless your coil had an internal ballast you needed an
external ballast, either through the old fashoned ballast resistor or through
the resistor wire built into later models. I have the pertronix ignition in
mine with a ballast resistor and it began to stall at stop lights when the
RPMs
got down around 800. After a lot of head scratching I discovered that the
new coil I recently installed had an internal ballast and, since I had a
ballast resistor also I was reducing the voltage twice. I just bypassed the
external ballast and the car now runs fine. Is this line of reasoning correct
or
is this just another example of Lucas electric being FM."
and Vance Navarrette wrote:
"While it is possible to have the ballast resistor in the coil
itself, that would mean that the coil would have three terminals on it,
so that the ballast can be bypassed during starting. I have never heard
of an internally ballasted coil, but that doesn't mean they don't exist."
We need to stop thinking about voltage and start thinking about current. The
maximum current a set of points can switch over a long period of time is
about 4 amps. On a 12 volt system, this means the coil (and ballast) resistance
should be about 3 ohms. A set of points will burn out in about 1000 miles if
the current exceeds 4 amps by very much.
To hold the current to 4 amps or less, you can either use a ballast resistor
with a low resistance coil, or use a high resistance coil alone. A low
resistance coil would have about 1 1/2 ohms, so the ballast resister would need
to
be about the same. A high resistance coil would be about 3 ohms.
In the earlier days, before all the pollution controls were added, our cars
didn't have a ballast resister; they used a 3 ohm coil. When the cars got hard
to start along around '73/'74, they went to a coil/ballast setup for the
sole purpose of being able to bypass the ballast on startup. By removing
(bypassing) the ballast resister while the engine was cranking, the current
through
the coil exceeds the 4 amp max briefly, giving a hotter spark for starting.
On our cars, the ballast resister is in the form of an iron wire, rather
than a discrete resister. If you look at the pink/white ballast wire, you'll
see
that it is not copper like the other wires. Iron wire has the property of
having more resistance as it heats up. On initial startup, even after the
ballast is switched back in the circuit, the coil will draw more current than
it
will after the ballast wire heats up from the current through it.
On an internal ballasted coil, part of the coil winding is also made of
iron, providing less resistance on startup than on running. It is still a two
terminal device.
At just about every TR6 car show I go to, I see at least 2 - 3 cars with a
double ballast - the factory ballast wire and an added-on discrete ballast
resister. If you have a late model TR6, you should check your coil/ballast
setup
to make sure you aren't double ballasting!
There's a lot more to it than that, but I hope this helps a bit.
Dan Masters
Check out the new British Cars Forum:
http://www.team.net/the-local/tiki-view_forum.php?forumId=8
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