Date: Mon, 1 Dec 2003 21:51:30 -0700
Reply-To: Nancy Eilers-Hughes <keithahughes@QWEST.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Nancy Eilers-Hughes <keithahughes@QWEST.NET>
Subject: Re: Cold Starting
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Sorry, but this just doesn't wash.
> I'll be glad to, but I'll tell you that most of your
> eyes will be glazing over soon.
> Put a DC current thru any coil, and you get a magnetic
> field.
Right so far...
> Remove the power the field collapses, causing
> a reverse voltage into the source. If this source was
> removed by a switch
i.e. points or ICU
> the voltage will jump the gap
> at the switch and crate an arc.
Hence the capacitor (condensor) which provides a current sink when
the polarity reverses. Remember, the primary has relatively few
windings, so relatively little back-emf is generated in the primary
windings (even if the secondary did not exist).
> This is a basics of the
> ignition system. The VW coil is set for a given gap,
> and several other standard things like wire resistance,
> etc.
No, the ratio of secondary to primary windings is determined based
on the primary supply voltage, the desired secondary voltage, and
the expected range of dwell angle (i.e. charge, rest, & discharge
times). The physical construction of the coil, heat sinking/heat
dissipation parameters, are designed around these variables.
Virtually no current is required so changes in secondary resistance
have a miniscule affect. Since the voltage is increased over a
thousand fold (i.e. 12vdc to >17Kvdc), the current decreases
proportionally. Since the voltage drop across the secondary is =
IR, with I so tiny, huge increases in R have little impact. Now,
compare the DC resistance of any amount of wiring or metal to the
DC resistance of 0.028" of air gap.
> If for any reason this should change (with Bosch Platinum
> plugs) you run the risk of damaging the coil by over heating.
OK, reality check here. The DC resistance of the secondary ignition
loop CANNOT increase outside the coil, no matter what plug you put
on it - the plug is an open circuit! You can increase the
resistance of the secondary coil windings which will heat the coil
*and* reduce secondary voltage. And, you *can* change the plug gap
sufficiently such that you don't have enough potential to arc
across the gap. The result is a dead miss. Either the EMF is high
enough to arc, in which case the secondary is sinked to ground and
the field energy dissipates as current (very low current BTW)
instead of heat, or it's too low, and you get a miss (and the field
energy ends up as heat in the coil).
Granted, too wide a gap and the inter-electrode ionization is
weaker, and the spark is weaker. But it's the gap that causes the
issue, not the electrode material. The major causes of coil
overheating are excessive and/or variable dwell angle which results
in insufficient time between charge/discharge cycles (i.e.
excessive duty cycle). Take a look at a Motronic coil. Why does it
have all the extra heat sinking capability? Because the ECU
continually adjusts timing on a per cylinder basis, thereby
creating a highly variable coil duty cycle.
> This is exactly what I have found, as the coils still work,
> but several internal shorts would not let them charge
> enough to jump the plug gap any more.
Internal shorts are heat sources themselves. Those shorted windings
will still collect field energy, most of which is dissipated as
heat in the coils fill oil. You may have seen correlation between
Platinum plugs and dead coils, but I think it's correlation only.
The coils were already defunct.
I can tell you from recent experience, cold starting problems and
cold running problems *can* be coil related. If your coil has
windings that are shorting, when the coil heats and expands, these
shorts may disappear. You should also see more problems at higher
RPMS vs. lower RPMs as the secondary voltage of a compromised coil
will be significantly reduced as the duty cycle increases.
Keith Hughes
