Usage: Test spark plug performance and ignition strength Application Range: 12V Gasoline vehicles car spark plugs Widely Application: 100V-240V input voltage. JF109 Spark Plug Test principle: In automobile ignition system, the high-voltage ignition coil generates a high-voltage (10000 volts or more), and the spark plug diverts it form high-tension ignition coil into the cylinder engine, spark between the spark plug electrodes the mixture. The JF109 could tell the good spark plugs from the bad by the strength of the spark. Spark Plug Tester Using Note: 1. Please Turn Off Power before you place spark plug 2. The Spark Plug Tester is a high-voltage detector, so do not touch spark plug with hand at work to avoid danger. JF109 Spark Plug Tester User Manual: 1. Place the Spark Plug into tester hole 1 or 2; If you want to compare the new spark plug and old one, place new spark plug into hole 1 and the old on into hole 2 2. Connect to power supply 3. Turn the key switch 4 clockwise to turn on