How to diagnose electrical issues with an automotive test light

Use an automotive test light to diagnose electrical issues

An automotive test light is a valuable tool for any DIY mechanic or professional technician. By understanding how to use it properly, you can quickly diagnose a wide range of electrical issues in vehicles.

An automotive test light is a simple tool used to check for the presence of voltage or ground in a circuit. It consists of a probe, a ground clip, and an indicator (usually a light bulb or LED).

Types of Test Lights

There are two types of automotive test lights; powered and unpowered. Both types can test for power and ground, but the powered units can check for ground more easily. The unpowered automotive test light is the most common and least costly. I’ll show you how to use an unpowered light to check fuses, connectors, and ground connections.

In addition to powered and unpowered versions there are also test lights that use an incandescent or LED and test lights that draw minimal current, making them computer safe.

How to use your light to check for a good  ground connection

Before using a test light, you should test it using your car battery. That confirms the bulb is working and you’ve got a good ground connection.

Next, connect the alligator clip to the positive terminal of your battery. Touch the probe to the ground point you want to test. If the ground is good, it’ll illuminate as brightly as when you connect it to the positive and negative terminals of your battery.

How to test for power using a fuse test as an example

Some auto fuses have power even with the key out of the ignition switch, but most don’t. Unless you know whether the fuse is “Hot at all times”, you should turn the key to the RUN position so each fuse is powered.

Then, connect the alligator clip to a good ground. Then, touch the probe to each side of the fuse.

There are two test points on the tops of these fuses. One side is power into the fuse, the other is power out. If the fuse is blown, your light will only show power on one side

©, 2023 Rick Muscoplat