Ignition Coil Secondary
1972 Dodge Challenger · 225ci I6
Sensor Type
inductive
Waveform Type
ac-sine
Location
High tension output from coil tower to distributor cap center terminal
Pins
1-pin connector
Idle Frequency
12.5 Hz
Timebase
5ms/div
Voltage Scale
5kV/div
Trigger
2kV, rising edge
Wiring
Black high-tension wire from coil tower to distributor cap
Expected Voltage Range
At Idle
-15000V to 15000V
At Higher RPM
-20000V to 20000V
Expected Pattern
Sharp voltage spike (firing line) to 8-12kV, horizontal burn line at 1.5-3kV for 1-1.5ms, followed by coil oscillations
Known-Good Waveform
Secondary pattern should show sharp firing lines between 8-12kV with relatively consistent height across all six cylinders. Burn time should be 1.2-1.8ms with burn voltage around 2-3kV. Spark lines should be evenly spaced at 60-degree intervals in parade pattern, with slight increase in firing voltage on compression stroke.
Common Failure Patterns
Firing line exceeding 15kV or one cylinder significantly higher than others
Cause: Fouled or worn spark plug, excessive plug gap, high resistance in plug wire, lean mixture on that cylinder
Very short burn time (under 0.8ms) or absence of spark line
Cause: Shorted spark plug, carbon tracking in distributor cap, cracked rotor
Erratic firing lines with voltage spikes or dropouts in parade pattern
Cause: Worn distributor cap terminals, corroded rotor contact, moisture in distributor
Diagnostic Tips
Use a capacitive pickup on #1 plug wire to avoid damaging the high-tension system. The slant-six distributor is prone to worn cap terminals and moisture intrusion - look for consistent firing voltage across all six cylinders within 2kV variation.
Related Diagnostic Trouble Codes
P0300P0301P0302P0303
Always confirm waveform data with your vehicle's OEM service manual before diagnosing.
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