Power is supplied to a vehicle's engine ignition, electric door locks, alarm systems and other accessories by the 12-volt circuitry that runs throughout the vehicle. When a circuit fails, you will find that a 12-volt tester is an invaluable tool for locating the source of the problem. If you are a do-it-yourself auto mechanic who understands basic electrical circuits, you can verify the presence or the absence of electrical power in your vehicle quickly and confidently by using a 12-volt circuit tester.
Turn on the vehicle's electrical power. The accessory position on the ignition switch powers up everything except the engine ignition circuit. To check the ignition's 12-volt wiring, turn the switch to the "On" position.
Connect the ground clip of the 12-volt tester to a well-grounded source. The battery's negative terminal is the best ground source, but any grounded metal source will do.
Place the pointed end of the tester on or in the circuit terminal or wire to be tested. The point may be used to penetrate the insulation of a wire to make a voltage test if you can't access a terminal. The tester handle lights up if the circuit being tested is in good working condition.
Check the grounding integrity of a circuit by reversing the connections and attaching the ground wire of the 12-volt tester to the positive side of the circuit being tested.
Avoid short-circuits by keeping the metal shank of the 12-volt tester clear of other metallic surfaces when performing voltage checks. It is possible to set off airbags in an automobile with a 12-volt tester if you probe the wrong wire. These wires are conspicuously marked with yellow tape and should only be tested by a qualified technician.