In a message dated 97-12-29 17:43:17 EST, cb1500@erols.com writes:
> After dealing with quite a few Spits, I've never seen a stock ballast
> resistor mounted on the fire wall next to the coil. Depending on the
> year, there was a drive resistor that was part of the electronic
> ignition. It was ususlly mounted to one of the alternator or air pump
> bolts.
Joe, Charlie, and others:
This weekend, whilst visiting the B&N bookstore in Marietta, GA, I picked up a
copy of "Triumph Spitfire and GT6 - A Guide To Originality," by John Thompson.
I own neither a Spitfire nor a GT6, but I think they are pretty neat cars, so
I bought the book just for general interest.
FWIW, on page 67 there is a very clear photo of a ceramic ballast resistor
mounted on the bulkhead next to the ignition coil. The photo is clear enough
that I can trace the wiring and can confirm that it is indeed a ballast
resistor, and not part of an electronic ignition set-up. Just below that photo
is a photo of another Spitfire, showing the heater control valve. The ballast
resistor can also be seen in this photo. The caption under the first photo
reads:
"An ignition ballast resistor was introduced on the MKIV for improved cold
starting. Comprising a coil of resistive wire protected by a white ceramic
insulator, it was secured using one of the ignition coil fixing screws. On
1973 model cars this separate ballast resistor was replaced by a resistive
wire built into the loom."
Which brings me to another point: several people have refered to ignition
coils as having either "internal" or "external" ballast resistors. For the
cars we are interested in at least, this is not quite correct. If a coil has a
ballast resistor, it is always external. A coil may or may not have an
internal resistor, and it may have one with or without an external resistor,
but it is not a ballast resistor. The two resistances perform totally separate
functions. The internal resistors are meant to limit current, particularly at
low speeds or when the engine is stalled with the ignition on. External, or
ballast resistors, on the other hand, are meant to reduce the voltage applied
to the coil, for the sole purpose of being bypassed to apply full voltage to
the coil for ease of starting.
Yes, it is true if you reduce the voltage you automatically reduce the
current, and vice versa, but the electrical characteristics of the two
resistors are quite different because of the differing functions. The internal
resistor is made of a coil of soft iron wire, which has the characteristic of
increasing resistance when hot. When the engine is running, the dwell angle is
such that the points are closed, and the coil is drawing current, about 60% of
the time. This means the average current through the coil is about 60% of the
maximum current. When stalled, however, if the points are closed the coil
draws 100% of the maximum current, and could overheat without the resistor.
The resistance of the iron wire increases from the heating, and limits the
current to prevent overheating.
When the points close, current through the coil does not reach its maximum
value immediately, but builds over a period of time. At high speeds, the
current hasn't enough time to reach maximum before the points open again, so
overheating is not a problem. At low speeds, however, the coil can reach full
current, and maintain the maximum current for a significant length of time.
Because of this, if the coil circuit is designed to produce a hot spark at
high rpm, it can overheat at low rpm, thus the need for an internal current
limiting resistor.
The resistance of the ballast resistor stays pretty much the same, whether hot
or cold. It is intended to be in the circuit to reduce the voltage, and the
current, to the coil when the engine is running, so the coil in a ballasted
system must be designed to produce about the same output spark energy with the
reduced input voltage as a non-ballasted coil would with full voltage. For
starting, the ballast resister is bypassed, and the full voltage is applied to
the coil, producing a much hotter spark than is produced while running. As
soon as you turn the key from the start position, the ballast is no longer
bypassed, and the coil gets the lower voltage. The benefits of the ballast
resistor is realized when it is NOT being used.
All of the above is, of course, only general, and various car/ignition system
manufacturers use different ignition schemes, depending on their own ideas,
and on the intended usage of the automobile, but that is my understanding of
how they work. There are also other ways to limit current rather than using
internal resistors.
How to determine if a particular automobile has a ballast resistor or not?
Very simple, just measure the voltage at the positive terminal of the coil and
at the positive post of the battery with the key on. If the two voltages are
equal, you do not have a ballast resistor. If the coil voltage is
significantly lower than the battery voltage, you do. (If the battery voltage
is lower than the coil voltage, you have a miracle!). By comparing the two
voltages, you eliminate any misreadings from a bad battery or charging system,
and you don't have to know what the actual voltage should be. Some coils use 6
volts, others 8, and others 9. I'm sure there are other values used as well.
BTW you two, keep up the good work! I look forward to reading your posts, as
they are always informative, and usually entertaining as well.
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://www.sky.net/~boballen/mg/Masters/
'74 MGBGT---3000mile/year driver, original condition
'68 MGBGT---organ donor for the '74
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