Ignition Coil Primary
1972 Dodge Challenger · 318ci V8
Sensor Type
inductive
Waveform Type
pulse-width
Location
Primary negative terminal of single oil-filled canister coil mounted on firewall or intake manifold, distributor ignition system
Pins
2-pin connector
Idle Frequency
21.3 Hz
Timebase
10ms/div
Voltage Scale
100V/div
Trigger
50V, rising edge
Wiring
Blue or dark blue wire to negative terminal (breaker points), brown or red wire to positive terminal (battery feed through ballast resistor)
Expected Voltage Range
At Idle
0V to 350V
At Higher RPM
0V to 400V
Expected Pattern
Sharp voltage spike to 300-400V on points opening, followed by damped oscillation (ringing) settling to battery voltage during dwell period
Known-Good Waveform
Primary waveform should show clean dwell period at battery voltage (12-14V) followed by sharp spike to 300-400V when points open. Spike should have 3-5 oscillations (ringing) before settling. Dwell angle should be approximately 30-34 degrees for 8-cylinder engine with points-type ignition.
Common Failure Patterns
Low voltage spike under 200V or excessive ringing beyond 8 cycles
Cause: Weak ignition coil with shorted windings or degraded capacitor in distributor
Erratic dwell period or irregular firing pattern
Cause: Worn distributor shaft bushings, pitted or misadjusted breaker points, or worn distributor cam lobe
Diagnostic Tips
This points-ignition system has no electronic control. Check dwell angle mechanically (30-34° spec). Ballast resistor bypass during cranking is normal - primary voltage will be higher during start.
Related Diagnostic Trouble Codes
P0351P0352P0353P0354
Always confirm waveform data with your vehicle's OEM service manual before diagnosing.
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