GearWrench High-Low Voltage Tester The GearWrench High-Low Voltage Tester test’s for voltage in 6 and 12V systems for voltage and spark plugs for spark by grounding the clip and touching the probe to the point being tested. The Tester Bulb will light if power is present. To use the GearWrench High-Low Voltage Tester as a Spark Tester, screw the probe to the opposite of the handle.