Pertronix Ignitor Inside Info.

[jlandry@domain.elided (John A. Landry)]

Reapers,

For those of you who have (or plan to get) a Pertronix Ignitor, I have some
interesting inside information I got from one of their engineers today
which I'll pass along.

I called them to find out why an old hand held analog Craftsman tachometer
/ dwell meter I have won't correctly display the true engine rpm since
installing the Ignitor.  The engineer was nice enough to spend plenty of
time explaining how the Ignitor worked and how it differs from breaker
points.

He explained that the reason we have so many problems trying to make
tachometers work with various ignition systems is because tachometers work
in several different ways in order to figure out the engine rpm.  In a
conventional breaker-point ignition system, present at the negative coil
terminal is a rising and falling sine wave of voltage potential from pretty
much zero to maybe 200 volts as the coil is "grounded."  Some tachometers
count these waves or "pulses" to determine the engine rpm by looking for
the zero voltage portions between some higher voltage.  Some look for
simple rises in voltage or even just the drops in voltage, not caring about
hitting a zero or any particular peak voltage.

The problem occurs because modern electronic ignition systems don't
necessarily have a wave form that matches the conventional breaker-points.
With a breaker point system, the wave form present at the negative coil
terminal rises rather rapidly as the points close, reaching a sort of
rounded peak as the points remain closed for a period and then have a
tapered tail as the points open and the current is broken (combined with
the effect of the condenser).   In the case of the Pertronix Ignitor, the
wave form is controlled by the electronics in concert with a capacitor.
The wave form has a characteristic which includes a very sharp rise, a
level plateau of 10 to 15 milliseconds (which duplicates the closed points)
and a exponentially tapering tail.  With the ignitor, the voltage never is
allowed to completely drop to zero.  Instead it hits a low of around 1.6
volts and rises to a high or around 400 volts.  This waveform combined with
these voltages causes the coil primary windings to saturate quickly and
fully... much more so than breaker-points.  This is why the Ignitor is
capable of assisting the coil in delivering much higher than normal voltage
output (if needed).

The dwell in the Ignitor is electronically preset and is not affected by
the air gap between the magnets and the module (so long as the magnets are
close enough to the module to trigger the internal Hall effect generator).
The dwell angle is also not affected by rpm.  If properly measured, the
dwell would be found to be much higher than with breaker-points.  A larger
(longer) dwell is better to insure complete current saturation in the
coil's primary windings at higher rpms.  Fast switching electronic ignition
systems are capable of delivering this without the mechanical restrictions
and shortcomings of conventional breaker-points.  Mechanical breaker-points
resorted to dual point systems to extend the dwell.

So an older tachometer that was designed around the predictable wave forms
associated with breaker-points may not work at all with a modern electronic
ignition system.  In my case, without a wiring schematic, it's impossible
to know what the old Craftsman hand held tachometer is looking for to gauge
the engine rpm.  I and the engineer at Pertronix suspect it's probably
expecting the voltage to drop closer to zero between pulses, so it may be
counting and averaging several ignition pulses as one.  Of course,
tachometers that are designed around newer electronic ignition systems or
tachometers which use an inductive pickup work fine.

On an aside, one note of interest to current or potential Pertronix
customers... the engineer told me it's their unwritten policy to always
replace an Ignitor that fails no matter how old it is or whether it's still
covered under warrantee.  The only exception might be a clear case of abuse
(crushed with a hammer for example).  As a matter of fact, he said they are
talking about making the policy official by offering a written *lifetime*
warrantee up front.  Of course I heartily encouraged him to further promote
the idea with the Pertronix management.  <grin>

Hope someone found this information as interesting as I did.

Regards,

John
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