Quickly Checks Spark at Plug Wire. A handy tool to check ignition spark without removing the spark plug. The tool is specially calibrated to measure ignition voltage. Works on both standard and electronic ignition systems. Easy to use. Simply adjust calibration screw to proper gap size. Attach spark plug wire to end of tester. Clamp tester on engine to ground. Crank engine and check for spark. *On the Black body of the tool, there are markings for the setting of the gap to the approximate voltage (KV is 1000 volts, so each number x 1000) of the ignition system.