Ignition Coil Secondary
1972 Dodge Challenger · 318ci V8
Sensor Type
inductive
Waveform Type
pulse-width
Location
Center tower of distributor cap, fed from coil secondary high-tension terminal through coil wire
Pins
1-pin connector
Idle Frequency
21.3 Hz
Timebase
5ms/div
Voltage Scale
5kV/div
Trigger
2kV, rising edge
Wiring
High-tension ignition wire (typically black or dark-colored spark plug wire)
Expected Voltage Range
At Idle
0V to 25000V
At Higher RPM
0V to 30000V
Expected Pattern
Firing spike to 8-15kV followed by spark line at 2-4kV for 1-2ms, then coil oscillations and dwell section
Known-Good Waveform
Secondary pattern shows initial firing spike of 8-15kV (lean mixtures higher, rich lower) followed by horizontal spark line at 2-4kV during combustion. After spark extinguishes, coil oscillations appear as 4-6 decaying waves, followed by zero voltage during dwell. All eight cylinders should show consistent firing voltages within 3kV of each other.
Common Failure Patterns
Firing spike exceeds 18kV with extended spark line duration
Cause: Excessive secondary resistance from worn spark plugs, bad plug wires, or wide plug gap (over .035 inch)
One cylinder shows firing spike 5kV+ higher than others or intermittent absence of firing line
Cause: Defective distributor cap terminal, carbon tracking in cap or rotor, or cracked distributor cap
Shortened or absent spark line with low firing voltage under 5kV
Cause: Shorted spark plug, carbon-fouled plug, or grounded secondary wire insulation
Diagnostic Tips
Use capacitive pickup on #1 cylinder wire for parade pattern viewing of all 8 cylinders. Mopar distributors of this era are prone to cap and rotor carbon tracking - inspect carefully for cracks.
Related Diagnostic Trouble Codes
P0300P0301P0302P0303
Always confirm waveform data with your vehicle's OEM service manual before diagnosing.
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