Innovate Motorsports ST-12 - Manual (Page 21)

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Most modern EFI systems have trigger wheels that create reference pulses through a hall effect or
variable reluctance sensor. These trigger wheels look like toothed gears with one or two teeth missing
(some instead have extra teeth).
Some systems also have only a single magnetic trigger reference from the flywheel or balancer and use
the starter ring gear to provide extra pulses. The extra pulses are needed by the ECU to determine the
exact crank angle when to fire the sparks for the different cylinders.
The ST-12 is not concerned with firing multiple cylinders, but only with the timing of one cylinder (typically
cylinder 1). Therefore it does not need the additional pulses, but can identify the reference trigger from
each.
Another concern is the “phase” of the pulses. The timing can be measured either from the rising or falling
edges of the spark pulse to the rising or falling edges of the reference pulse. Which pulse edge for each
has to be known to allow accurate measurement. Very often this can be only determined by trial and
error. This means you have to go through all four possible combinations until you measure the correct
advance, verified with a timing light.
For this reason the use of the inductive clamp as RPM source is NOT recommended for spark advance
measurement, because it’s phase is indetermined can changes depending on which way around you use
the clamp.
For example if the source for the spark pulse is the negative side of the ignition coil (inductive ignition),
the negative side of the coil goes to ground (negative edge of pulse) to charge the coil. When the coil
discharges (spark happens) the voltage rises to several hundred volts and then returns to 12V. In this
case the spark pulse would use the rising edge.
The same is true for the trigger pulse from the trigger wheel. Depending on the sensor used, the output
pulses can be negative or positive. This can either be found out with an oscilloscope or by programming
the ST-12 for one way or the other and finding out which is the correct one.
The tryout should be done at different RPMs, because under some circumstances you could get a correct
reading at idle, but a shift at a different RPM.
NOTE:
Variable reluctance sensors cannot be read by the ST-12. VR sensors do not output a clean pulse,
but a short wave, whose 0 Volt crossover is measured. The wave amplitude is dependent on RPM
and sensor. For these applications a special VR amplifier like the LM1815 needs to be used.
Example voltage trace of a VR sensor output:
Example output of a VR sensor amplifier: