How do I check for power with an unpowered test light?

Connect one test lead to a good ground or the NEGATIVE battery terminal, then use the other lead to check wires, connectors, or components. When the light comes on, there is power at that location.

How do I investigate a blown fuse condition?

Turn off all equipment powered through the fuse, disconnect unswitched components (unplug motor connectors, remove light bulbs), turn the ignition to RUN if needed, then connect a test lead to the hot end of the blown fuse and the other to ground. If the light glows, power reaches the fuse; move the grounded lead to the other end of the fuse — OFF indicates a short in a component, ON indicates a short to ground in the wiring.

What precaution must I take before using a self-powered test light?

CAUTION: Disconnect the NEGATIVE battery cable from the battery before using the self-powered test light.

How do I make my own jumper wire?

Attach alligator clips to each end of a length of heavy wire, such as 16 gauge automotive primary wire, so extra resistance isn't placed into the circuit. For momentary jumping, insert a small nail into one of the clips as a probe.

Why shouldn't a jumper wire be used to bypass a lamp or motor?

Bypassing a high-resistance load connected between the hot circuit and ground creates a direct short circuit and may cause personal injury, fire, or other damage to the vehicle.

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1979 Ford Bronco/F100-F350 Electrical Troubleshooting Tools - Test Light Guide

Tools for electrical troubleshooting: unpowered test light, self-powered test light, jumper wire

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Ford troubleshooting manual page explaining electrical test tools - unpowered test light, self-powered test light, and jumper wire - with how-to steps for checking power and continuity.

AI caption August 2014 · Photo 3 of 48

This page, titled "Tools for Electrical System Trouble-Shooting," is from a multi-page reference set filed for a 1979 Ford Bronco (covering Bronco and F100 through F350 trucks). It explains three basic diagnostic tools and how to use them. First, the unpowered 12-volt test light: how to check for power at wires and connectors, a step-by-step procedure (steps a–d) for investigating a blown fuse condition, and how to locate a wiring short to ground by disconnecting connectors (C-1, C-2, C-3) one at a time, illustrated with a simple circuit diagram showing a fuse holder, battery, and test light. Second, the self-powered test light: a caution to disconnect the negative battery cable first, then instructions for checking circuit continuity, switch continuity, and grounds. Third, the jumper wire: how to make one from 16-gauge automotive primary wire with alligator clips, how to use it to check a switch circuit, and a caution never to bypass a high-resistance load such as a lamp or motor, which would create a direct short circuit and could cause injury, fire, or vehicle damage. The page includes photos of each tool in use with captions. A reader learns fundamental DC electrical troubleshooting technique applicable to the vehicle's fuse, ground, and switch circuits.

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Frequently asked questions

How do I check for power with an unpowered test light?: Connect one test lead to a good ground or the NEGATIVE battery terminal, then use the other lead to check wires, connectors, or components. When the light comes on, there is power at that location.
How do I investigate a blown fuse condition?: Turn off all equipment powered through the fuse, disconnect unswitched components (unplug motor connectors, remove light bulbs), turn the ignition to RUN if needed, then connect a test lead to the hot end of the blown fuse and the other to ground. If the light glows, power reaches the fuse; move the grounded lead to the other end of the fuse — OFF indicates a short in a component, ON indicates a short to ground in the wiring.
What precaution must I take before using a self-powered test light?: CAUTION: Disconnect the NEGATIVE battery cable from the battery before using the self-powered test light.
How do I make my own jumper wire?: Attach alligator clips to each end of a length of heavy wire, such as 16 gauge automotive primary wire, so extra resistance isn't placed into the circuit. For momentary jumping, insert a small nail into one of the clips as a probe.
Why shouldn't a jumper wire be used to bypass a lamp or motor?: Bypassing a high-resistance load connected between the hot circuit and ground creates a direct short circuit and may cause personal injury, fire, or other damage to the vehicle.