Paul wrote the quotation below, and he is exactly
right. My wording was imprecise. Here is what I should have said:
At higher engine speeds the coil has less time to
saturate. When operating as engineered there is
adequate time to produce a hot spark, even at red
line. However, when the ignition performance is
degraded, such as with a resistive primary
circuit, the current available at high engine
speeds may be insufficient to provide a reliable
spark. (This was exactly the problem I faced on
a BMW 2002tii, except that the voltage drop was
over 2 volts (static)). So the translation is
not that the primary voltage drop is worse at
high engine speeds but rather, the secondary
voltage may be worse because of the reduced
primary current ... even if similar to the idle
values.
Did that translate into English this time? And
thanks for your sage advice Paul.
Rick
> Whilst this voltage drop does occur, and is often the cause of a dim
> ignition warning lamp glow when the turn signals or brake lights are used,
I
> have to question the statement that the volt-drop is worse with the engine
> running than when stopped. When the engine is stopped and assuming the
> points are closed (which mine usually seem to be) there is a constant
> current flow through the coil, which is the biggest single load on that
> circuit - twin electric cooling fans or heated rear window possibly
> excepted. With the engine running the points are opening and closing and
> switching the current through the coil on and off, in the ratio of about
> 2/3rds on and 1/3rd off. This will definitely result in an analogue meter
> showing a lower average volt drop than with the engine stopped. Digital
> instruments will vary, some never settling and showing a steady voltage.
> That was the theory, and when I went and checked my 73 roadster I measured
> the voltage between the brown and the white at the fusebox (i.e.
effectively
> the volt-drop in the ignition switch) and got 0.05v with the ignition on
and
> the coil disconnected, 0.4v with the coil connected and points closed but
> the engine stationary, and 0.3v with the engine running. This difference
> between stationary and running is pretty-well what I expected.
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