Unpowered Test Lamp

This tool consists of a 12-volt lamp with leads. The ends of the leads usually have alligator clips, but various kinds of probes, terminal spades and special connectors also are used.

The 12-volt test lamp continuity tester uses the vehicle's battery to provide voltage to the circuit being tested. 12-volt testers are manufactured with a variety of tips to permit touching them to connectors, bare wires, insulated wires, or even wires within wiring harnesses. To check the tester before use, briefly touch the clip to one side of the battery and the probe to the other. 12-volt testers are NOT sensitive to polarity in a circuit and can be connected either way.

The 12-volt test lamp generally has a sharp probe tip so it can be inserted into connector terminals or through the wire insulation for testing. It is important to keep the probe tip sharp to minimize damage to wire insulation. When the test is complete at a particular point, be sure to tape any holes made in wire insulation.

The unpowered test lamp is used on an open circuit. One lead of the test lamp is grounded and the other lead is moved around the circuit to find the open. Depending on the physical layout of the circuit, sometimes it is easier to start at the voltage supply, and other times it is easier to start at the circuit load or ground circuit.

Once one becomes familiar with the test lamp and the brilliance of the bulb in a normal circuit, high-resistance circuits can be recognized by the effect they have on the bulb. As the current drops in a high-resistance circuit, the bulb in the test lamp glows less brightly. Although the 12-volt test lamp cannot be used as a foolproof test for high resistance, a less than normal brilliance of the lamp is an indication of circuit high resistance. Further testing will verify the condition and locate the cause.

NOTE: Test lamps are to be used only on circuits that do not contain solid-state devices. If a test lamp is used in a circuit containing a solid-state device, the current that the test lamp would draw would be above the current that the solid-state device would be able to handle. Using a test lamp on a solid-state device may damage the device.