A monitoring resistor in series with the spark coil primary winding and the electronic interruptor switch provides a signal to an integrator for shifting the control thresholds of a threshold switch that controls the interruptor away from their quiescent values that are nearer the zero crossover of the timing voltage wave provided by an engine driven timing signal generator. The integrator output voltage remains constant during the time the interruptor switch is open. While current flows through the interruptor circuit, the integrator increases the control voltage until the primary winding current reaches a predetermined level and then decreases the control voltage until the timing wave recloses the interruptor. While the control voltage is being decreased, the coil current is allowed to rise to a limiting value and is then held constant by another circuit controlled by the monitoring resistor that slightly reduces the conductivity of the interruptor switch in its closed condition, but this occurs only while the engine is accelerating to the operating speed range. During this time the switch-on threshold for controlling the interruptor is raised towards the peak of the timing wave, but the switch-off threshold is clamped to its initial value. With further increase in engine speed, the net effect of the integrator operation changes sign and the switch-on threshold is lowered. As soon as the switch-on threshold goes below its initial value, the switch-off threshold is unclamped and is depressed along with the switch-on threshold towards the negative peak of the timing wave by the integrator action. The result is to keep the amount of energy stored in the spark coil at the time of primary circuit interruption constant over a wide range of speed.