Ignition Coil Primary
1972 Dodge Challenger · 400ci V8
Sensor Type
inductive
Waveform Type
pulse-width
Location
Coil mounted on driver side firewall, primary connects to ballast resistor and points distributor
Pins
2-pin connector
Idle Frequency
33.3 Hz
Timebase
10ms/div
Voltage Scale
50V/div
Trigger
3V, rising edge
Wiring
Blue wire from ignition switch through ballast resistor (positive), Dark Green/Black wire to distributor points (negative/ground side)
Expected Voltage Range
At Idle
0V to 12V
At Higher RPM
0V to 350V
Expected Pattern
Square wave dwell period followed by sharp voltage spike to 250-350V when points open, then oscillating decay rings
Known-Good Waveform
Primary should show clean 6-9V dwell during points closed, followed by sharp spike to 250-350V when points open. Spike should have 3-5 oscillations (ringing) that decay smoothly. Dwell angle should be 28-32 degrees on this 8-cylinder points system with standard coil.
Common Failure Patterns
Low voltage spike under 200V with weak or no oscillations
Cause: Weak ignition coil or poor primary winding, often due to internal breakdown or overheating
Irregular dwell period or missing firing events
Cause: Worn distributor points, incorrect point gap (should be 0.017 inch), or weak points spring tension
Excessive voltage spike over 400V with prolonged ringing
Cause: Open or high resistance secondary circuit, bad spark plug wire, or fouled spark plug causing kickback
Diagnostic Tips
Check ballast resistor voltage drop (should reduce 12V to 6-9V). Measure dwell angle with meter - should be 28-32 degrees. Points gap directly affects dwell time and coil saturation.
Related Diagnostic Trouble Codes
P0351P0352P0353P0354
Always confirm waveform data with your vehicle's OEM service manual before diagnosing.
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