Scope and Multimeter Operations Ignition Scope
142
Start of Ignition
Figure 9-74 (1) - High voltage is induced, by mutual induction, into the secondary
winding because of the primary to secondary turns ratio. The secondary voltage
overcomes the resistance in the secondary circuit up to the spark plug gap. The
spark plug gap is ionized and current arcs across the electrodes to produce the
spark that initiates combustion.
Spark Line
Figure 9-74 (2) - Voltage is reduced to about 1/100th of the secondary voltage,
because of the 100:1 step-down ratio. This is the actual discharge across the air
gap between the spark plug electrodes.
End of Ignition
Figure 9-74 (3) - The coil energy is no longer able to sustain the spark across the
electrodes at this point.
Coil Oscillations
Figure 9-74 (4) - An oscillating voltage results (in step with the secondary voltage)
because of the step-down ratio as current dissipates in the primary circuit.
No Current in the Primary Coil
Figure 9-74 (5) - The trace flattens out when there is no longer current in the
primary ignition circuit. Battery or charging system voltage available at this point is
about 12 to 15 volts.
Primary Current Turns On
Figure 9-74 (6) - The primary circuit switching device switches on the primary
current. This starts the dwell period.
Start of Primary Current
Figure 9-74 (7) - Current in the primary circuit establishes a magnetic field in the
ignition coil windings. The strength of the field builds during the dwell period.
Current Limiting
Figure 9-74 (8) - This represents the dwell period or “on-time” of the ignition coil
primary current:
• The rise in voltage (primary waveform) indicates that current limiting is
occurring.
• On ignition systems that use current limiting to control the coil current, a current
hump or voltage ripple appears on the “on-time” portion of the primary
waveform during the dwell section.
Primary Current Turns Off
Figure 9-74 (9) - The primary switching device stops the primary current flow
suddenly. The magnetic field that had built up collapses. This induces a high
voltage in the primary winding by self-induction.
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