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How to Use a 12-Volt Circuit Tester

by Max Stout

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.

Step 1

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.

Step 2

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.

Step 3

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.

Warnings

  • 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.

Items you will need

  • 12-volt circuit tester
  • Maintenance manual

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