Cylinder firing indicator

In a cylinder compression test apparatus for a spark ignition engine:

• improved circuit means for generating an electrical pulse representing the firing cycle in a selected cylinder, said pulse-generating means comprising a signal attenuator having a hardwire connection to the high tension electrode in the spark plug for the selected cylinder, said attenuator having an output signal voltage whose magnitude is approximately one-tenth of 1 percent of the high tension voltage;
• a low-frequency-pass filter receiving the output of the attenuator, said filter having a capability for rejecting the attenuated highest voltage spike generated in the high tension electrode, while passing the attenuated high tension steady state signal occuring during the cylinder fire period;
• the filter output being essentially a pulse having a width that is approximately the same as the duration of the cylinder fire signal, whereby said pulse, after additional filtering and conditioning is suited to act as a trigger means for a digital-logic element, e.g. a flip-flop.