John,
I would agree that 16.9V sounds too much. The cause of this is almost
certainly the alternator - this is all the more likely because you took a
working, used one, rather than a reconditioned (i.e. good-as-new) item. If I
understand it right, the alternator generates voltage proportional to the
speed of the engine; it is then corrected to the desired output by some
electronics in the back of it, which also converts the electicity generated
from AC to DC. If you are generating that high a voltage, I would suspect the
electronics in the back of your alternator are dodgy. It will work, but it
will damage components due to overheating over time. I'm also not sure what
the effect will be on your battery of having that high a voltage across it for
a sustained period - it might overheat the battery. Have a feel of the
battery casing after a long run.
As for the coil, this is a cylindrical thing that is bolted onto the bulkhead
near the battery, with a connection at each side of the end, and one from the
middle of the end. This generates the very high voltage needed to make a
spark in your spark plugs. There are actually two coils inside it. The first
is connected to the live supply from the battery. It is then earthed, via a
switch in the distributor. The coil has electricity passing through it, which
builds a large magnetic field within the coil. At the crucial moment, the
distributor opens the switch to earth, so no more current can pass; the
magnetic field immediately collapses. This sudden change in magnetic field
strength creates a current in the second winding, with a very much higher
voltage. The current is very low, but that's OK. The current flows along the
HT (high tension, which actually means high voltage) lead to the top of the
distributor, which sends it to one of the spark plugs, and this current leaps
across the spark plug gap to cause the spark. The distributor then
re-connects the LT (low tension, i.e. low voltage) side of the coil, to build
the magnetic field once more.
This can cause poor running at high revs because, if the first winding is
dodgy, it may not have time to build much of a magnetic field at high revs
(the faster the engine, the less time there is between sparks). There will
therefore not be much of a current generated in the HT circuit, so the spark
at the plug will be weak. However, this is but one of a number of causes of
poor high speed running - timing out, centrifugal advance on the distributor
(which makes the spark earlier at high speeds, to give the petrol more time to
burn) not working, partial fuel blockage, weak fuel pump, mixture incorrectly
set on carbs for starters. My instinct is that electical drain is not the
most likely cause of your high speed running problem, but is competely
separate, or is simply paranoia as you said!
Well, that may be more than you wanted to know about coils, but you did ask!
Richard and Daffy
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