How to use a circuit tester on a car

Updated July 19, 2017

A circuit tester is a simple mechanism in the form of a pen with a light bulb and a ground lead with an alligator clip. The circuit tester is used for locating live (battery positive) wires and ground wires. It could also be used for locating and identifying short circuits and battery draws. The volt/ohmmeter is a step up from the circuit tester—besides acting as a circuit tester, it displays the amount of voltage and the resistance in the line.

Attach the circuit tester's negative lead to a good ground. Probe the cavities in the fuse box with the tip of the tester. When you find a live terminal, the tester lights up, indicating power. If you are looking for switched power, check the fuse box with the key on first. Once you identify a cavity, turn the ignition key off and recheck the cavity. If it is still lit, repeat the process until you find a cavity that has power only with the key on.

Check for the positive live terminal within a harness connector by pulling the connector apart. Make sure to use the connector opposite the appliance. Connect the black negative lead to a good ground and probe all the terminals with the key on, then with the key off, looking for power. If you want to check for power to a failed fuel pump, separate the fuel pump harness connector behind the gas tank. Connect the black negative lead to a good ground and probe the harness side of the connector. The computer activates the fuel pump for only two seconds at a time unless it senses the engine starting. Have a helper cycle the switch on and off at five-second intervals while you probe the terminals.

Check for a bad ground connection by pulling the harness connector apart and locating the live terminal with the circuit tester. Record the exact terminal. Turn the terminal so you can see the corresponding colour of the wire the terminal is connected to. Place a paper clip in the other half or appliance side of the connector in the terminal with the same colour wire. Attach the black lead alligator clip to the paper clip and probe the opposite connector at the same terminal. If the light does not light up, the ground is gone or the mechanism is not functioning.

Check the battery for a draw by disconnecting the negative terminal and the negative terminal connector. If the circuit tester light is brightly lit, there is power being drained from the battery. If this test shows a draw, you can locate the circuit in which the short is located. If there is no draw, the battery has a bad cell. A very dim light is normal because a small amount of power is needed to keep the circuits alive. This is not enough to drain the battery. If the light is bright, pull one fuse at a time in both fuse boxes until the light goes out to find the circuit containing the short.


When connecting a piece of equipment such as a radio, amplifier, GPS, etc., you are looking for a wire with battery power. The best way to find the live wire is to search for an open cavity in the fuse box under the dash.

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About the Author

Don Bowman has been writing for various websites and several online magazines since 2008. He has owned an auto service facility since 1982 and has over 45 years of technical experience as a master ASE tech. Bowman has a business degree from Pennsylvania State University and was an officer in the U.S. Army (aircraft maintenance officer, pilot, six Air Medal awards, two tours Vietnam).