the analog noise threshold signal on line 115 providing a noise threshold for the ignition input signal to noise threshold comparator 20 based on characteristics of the ignition input signal itself fig3 a and 3b illustrate an exemplary embodiment of probe 12 which includes metal contacts 120 insulating material 122 coaxial cable 123 having inner conductor 126 and outer conductor 128 handle 124 spring member 130 pin 131 and screw 132 probe 12 is preferably a spring clamp similar to a clamp on a battery jumper cable with metal contacts 120 forming opposing jaws of the probe insulating material 122 and handle 124 coaxial cable 123 is coupled to probe 12 to carry electrical signals to the tachometer circuitry of fig1 and 2 insulating material 122 electrically isolates metal contacts 120 from handle 124 of the probe metal contacts 120 of probe 12 are electrically connected to center conductor 126 of thin coaxial cable 123 to supply induced ignition pulse signals to the tachometer circuit handle 124 is electrically connected to outer conductor 128 of coaxial cable 123 to electrically ground handle 124 to circuit common this arrangement of handle 124 is preferred to prevent 60 hertz noise from the hand of a user from interfering in the operation of.