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5. MEASURING MODES
There are four different ignition system parameters that can be selected for measurement,
depending on the engine’s ignition system:
• Engine RPM.
• Spark burn (firing) time.
• Spark plug voltage.
• Dwell angles (for ignition systems equipped with high voltage spark plug wires).
• Primary ignition coil current ramp time (for coil on plug and coil near plug ignition systems).
5.1 Engine RPM
Measures engine RPM (revolutions per minute) in 2 and 4 stroke engines, and may be
used in combination with other measurements to evaluate ignition system
performance at different engine speeds.
5.2 Spark burn (firing) time
The spark burn time, also called “firing time”, is the measurement of the period from
the moment a spark is initiated up to the point when it is extinguished, and is the most
indicative measurement of performance of the ignition system. Abnormally long or short
burn times may indicate some problem in the ignition module, spark plug, fuel mixture,
cylinder compression, etc.
5.3 Spark plug voltage
Spark plug voltage values are useful for comparing the performance of the ignition system
between cylinders. This may be used to diagnose common problems like misfires, broken
spark plug wires, etc. Spark plug peak voltages may vary widely, are less consistent and
not always indicative of a properly working ignition system. On the other hand, observation
of the spark plug voltage waveform or trend may prove useful in diagnosing ignition and
mechanical problems.
5.4 Dwell angle (for ignition systems using spark plug wires only)
Dwell angle is a measurement of the angle of rotation of the crankshaft, between the moment
in which the primary of the ignition coil is energized (e.g. point contacts closed) and the
spark is generated (e.g. point contacts opened). Its main use is in determining if there is
enough angle (or time) to energize the primary of the ignition, and not so much as to cause
the ignition coil to overheat and fail. Dwell angles are specified for a particular engine and
RPM, and usually adjusted in the mechanical distributor/point assembly.
5.5 Current ramp time (for coil on plug and coil near plug only)
Ramp time is measured from when the primary coil current first begins to increase to
when it stops increasing. The current may stop increasing because the coil has reached
its saturation point, or it is interrupted to generate a spark.
Ramp time measurements are useful for diagnosing problems in the primary circuit of
the ignition module. Abnormally long ramp times may indicate excessive primary circuit
resistance, low input voltage, that the wrong module is installed, etc. Shorter than normal
ramp times may indicate a shorted primary coil, that the wrong module is installed, etc.
