Is a new ignition coil easy to fit (in comparison to fitting my alternator,
which was simple)? Is there any way to test wether the current one is shot?
cheers
John
----- Original Message -----
From: "Richard B Gosling" <Gosling_Richard_B@perkins.com>
To: "goalie_john" <goalie_john@yahoo.co.uk>; "spitfires"
<spitfires@autox.team.net>
Sent: Tuesday, November 28, 2000 3:38 PM
Subject: Re: warning light and alternator
> 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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