How to test automotive electrical issues with a test light

Use a test light to diagnose electrical issues

There are two types of automotive test lights; powered and unpowered. The unpowered test light versions are the most common and least costly. I’ll show you how to use an unpowered light to check fuses, connectors and ground connections.

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 connected it to the positive and negative terminals of your battery.

How to test fuses using your test light

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