Test switch is used with an amp meter to isolate parasitic amperage draws on all GM vehicles or any vehicle with a side- or top-post battery. This tool attaches between the negative battery lead and the battery, then the vehicle road is tested. The next step is to connect an amp meter to each side of the switch, thereby allowing all available current to pass through the meter. Fuses are then removed one at a time until the circuit source of the parasitic draw is located. Similar to J-38758.