Ignition Coil Secondary
1972 Dodge Challenger · 400ci V8
Sensor Type
inductive
Waveform Type
pulse-width
Location
Secondary tower of coil on driver side firewall, connects to distributor cap center tower via coil wire
Pins
1-pin connector
Idle Frequency
33.3 Hz
Timebase
5ms/div
Voltage Scale
5kV/div
Trigger
2kV, rising edge
Wiring
Single high-tension wire (black insulation) from coil tower to distributor cap center
Expected Voltage Range
At Idle
0V to 25000V
At Higher RPM
0V to 35000V
Expected Pattern
Sharp firing spike to 8-15kV, then burn line at 1.5-3kV for spark duration, followed by coil oscillations at 300-500V
Known-Good Waveform
Secondary pattern should show sharp firing line reaching 8-15kV depending on cylinder load and compression. Spark line should be steady at 1.5-3kV for 1-2ms duration. All eight cylinders should fire within 3kV of each other in parade pattern. Coil oscillations after spark should show 4-6 clean peaks.
Common Failure Patterns
One cylinder shows firing spike 5kV or more higher than others
Cause: Lean mixture, vacuum leak, high resistance spark plug wire, or worn spark plug on that cylinder
One cylinder shows firing spike much lower with short or absent spark line
Cause: Fouled spark plug, shorted plug wire, carbon tracked distributor cap or rotor on that cylinder
All cylinders show firing spikes over 20kV with erratic spark lines
Cause: Excessive secondary resistance from worn distributor cap, rotor, plug wires, or overly wide spark plug gaps
Diagnostic Tips
Use parade pattern to compare all 8 cylinders - firing order is 1-8-4-3-6-5-7-2. Check rotor and cap for carbon tracking. Original plug gap is 0.035 inch; wider gaps increase firing voltage demand.
Related Diagnostic Trouble Codes
P0300P0301P0302P0303
Always confirm waveform data with your vehicle's OEM service manual before diagnosing.
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