Fluctuating neg. to ground on ignition coil

For rough comparison two coils were used in '73 Dodge cars.
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Mattax said
The voltage the coil is getting while running is going to fluctuate with current through the resistor.
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Look up how a points distributor and coil work. Then this will make more sense. All the ECU is doing is converting the signal to electronicly do the same as points.