How To Use A Test Light Like A Pro

The automotive test light has to be one of the simplest diagnostic tools on the planet. Consisting of nothing more than a few wires and a light bulb, a test light can help you locate the most difficult to diagnose electrical problems. With such a simple tool you can imagine learning how to use a automotive test light should be easy.

Well…… it is. 

It is with a little bit of education and practice. 

What is an automotive test light?

An automotive test light is an automotive diagnostic test tool that is capable of checking positive and ground circuits quickly and easily. As mentioned above, the test light is very simple, but in the right hands can be very capable of testing the most difficult electrical circuits. 

While there are many different types of test lights, the construction is generally the same. There is a housing that contains a probe and a light bulb. On the other end of the light there is a wire and a clamp that is used to “complete the circuit” of the bulb.

When using a test light you are completing a circuit that allows a light bulb to illuminate. A simple circuit requires a power source (automotive battery), conductors(wires that connect the circuit), and a load (the test light bulb). 

Using our years of professional diagnostic equipment we have compiled a list of the best automotive test lights that will fit your needs best. Take a look to see which test light would fit your needs best. 

Test the test light

Before you first use the test light the first thing you should do is to make sure the test light works correctly. This can be done very easily.

Install the ground clamp to the negative post of the battery or a known good ground connection (eg. body ground, alternator ground).

After this, simply probe the positive side of the battery post to make sure that the light in the test light illuminates correctly.

If it lights up okay, the test light passes the test and is good to use for testing. 

Where can a test light be used

An automotive test light is very useful for testing many different automotive electrical systems. The most common types of systems that you can test are fuses in a fusebox or power distribution center (PDC). Testing power supply to fuel pump relays and other components are also  common tests that you can perform. 

We have outlined some of the more common tests that you can perform below. 

How to use a test light to check automotive wiring

Now that you have made sure the test light is working correctly, let’s jump in and learn more about how to use a test light for electrical diagnosis. 

Testing a positive circuit with a test light

The most common use of a test light in electrical diagnosis is to check for available power on a wire. This can be used in many different types of situations. These situations can range from installing a stereo, testing for power at a light bulb, and even making sure the battery is attached correctly.

incandescent test light probing an alternator for power

To test a power feed circuit with a test light you will first attach the clamp of the test light to a good ground, try to use the battery as your source of ground. This is the easiest way to make sure that you have a good ground. 

Use the probe to make contact with a conductive surface that you are testing. This would be a wire terminal in a connector or a bare wire that has the insulation removed. 

If the test light illuminates it indicates that there is power present at the test point. If the light does not illuminate it indicates that there is no power present at the test point. 

Testing a ground circuit with a test light

Most people are familiar with testing for available power with a test light, it can also be used for testing a ground circuit as well. This takes some electrical diagnosis experience, but just remember one thing.

When using a test light you need to “complete the circuit”.

Knowing this we need to change the way that we attach the automotive test light when testing. 

First you will need to attach the test light clamp, for testing ground circuits you will need to clamp the test light to the positive terminal of the battery.

We then need to complete the circuit by probing a ground circuit with the test light and see if the light illuminates. 

As before if the light illuminates it indicates the ground circuit is okay, if the light does not illuminate it indicates that the ground circuit is faulty. 

How to test a fuse with a test light

The easiest way to test a fuse is by using a test light when the fuse is still installed in the fuse panel. This test is very simple and can be done very quickly.

automotive fuse panel with different amperage fuses and relays

First consult the correct wiring diagram by using the best automotive service manual. The key will need to be placed in the on position prior to doing any testing. 

Attach the ground clamp to a known good ground (battery ground or chassis ground). Once the fuse panel cover is removed locate the suspected faulty fuse. Using the probe, you will make contact with both sides of the exposed fuse test points. 

If the lamp illuminates when testing both sides of the fuse, the fuse is okay and passes the test. If the test lamp illuminates on one side of the fuse only it means the fuse has popped and needs to be replaced. 

automotive test light probing a 15 amp fuse in a fuse panel

What does a dim test light mean

While the test light is simple to use, it does have some downfalls when compared to using an automotive multimeter. A test light can tell us if the circuit is complete and has continuity but it cannot tell us if there is power present and how many volts are present when testing. 

The only other indication the test light gives us when testing, is the brightness of the light bulb. If for example the test light bulb illuminates brightly it means there is full voltage present, or 12 volts in an automotive setting. 

If the bulb is dim it indicates that the voltage is lower. This is caused by excessive voltage drop before the test point in the circuit. So if the bulb is dim there is reduced voltage present at the test point, possibly 6 volts for example. 

This method of testing is very important when you are checking ground circuits. For example when testing a ground circuit with an automotive test light it lights dim. This means that there is a bad ground wire, or bad ground connection present in the circuit. 

Current draw is important

Using a test light it is important to know how much current is being drawn when testing. It can be possible when using a test light to damage advanced electrical components such as a powertrain control module(PCM) for example. 

LED test light probing a fuse in a fuse panel

This is one of the main differences between an incandescent test light and an LED test light. An incandescent test light may have a current draw of close to 0.5 amps or 500 milliamps when using. 

Using an LED test light on the other hand can have a current draw between 20-50 milliamps or 1/10th of the incandescent test light. This makes it important to consider when you choose between these two different types of test lights. 

Using a test light is only one tool you can use

A test light is one of those amazing additions that can be added to your toolbox or travel tool bag. It is very versatile and can be of great use when you are out in the field testing a problem. Sometimes initial testing with a test light will lead to you pulling out your best automotive test light to continue doing more testing. 

John Morris
John Morris
John Morris is the technical editor for His years of experience in automotive repair as well as an automotive professor have prepared him to ensure that even the most technical information is accurate and concise at all times.

Latest Articles