How to use a Test Light (and how to find the one Right for you)
Your car's electrical system can be wrecked from small issues.
A simple test light can be the answer to finding the problem.
The use of a test light can help diagnose your car’s electrical components and can be the easiest way for a DIY mechanic to check for a blown fuse.
When working on your car's electrical system, always use safety first and don't probe any circuits you don't know. The last thing you want is for a useful tool to become a nightmare for your wiring harness.
What is a test light?
A test light, or test lamp, is a simple piece of electronic equipment used to test the presence of electricity in a circuit. Typically, it is used to check the voltage on a circuit, like on a wire or on a fuse, but it can also be used to check grounds.
A test light mainly consists of a light bulb, usually an incandescent lamp (bulb) or LED bulb, a probing end, and a clamp.
When testing for voltage on a circuit, the clamp is connected to a known good ground, like the negative side of the battery. When the probe side of the test light touches voltage, like the positive side of the battery, the bulb will light.
Different types of test lights can be found in a mechanic's toolbox.
There are many different test light types. Each test light can vary with the type of bulb and the amount of electrical current that flows through them.
For example, a test light with an LED bulb will flow less current than a test light with an incandescent bulb.
In most applications, professional mechanics prefer a test light with an incandescent bulb.
Fuse Box
What does a test light do?
A test light is an electrical test instrument used to test for electricity in a circuit. When electricity is found, the test light will illuminate.
To test for voltage on a circuit, you would first have to connect the test light's clamp to a known ground source, this is usually the negative battery terminal.
After that, you will use the probe end of the test light to test the contacts of fuses in the car's fuse box panel or pierce a wire.
When the test light illuminates, this means there is voltage on that circuit.
The opposite can be done to test for a good ground. The clamp of the test light will be connected to the positive battery terminal and the probe end will be used to find a ground.
Connect Test Light to battery ground
How do I find a short with a test light?
A "short" in an electrical circuit can mean the circuit has been compromised and has either a direct path to ground or positive.
It is important to understand the circuit before testing for a short circuit.
To test for a short circuit to ground:
Connect the clamp side of the test light to the battery positive (positive power source).
Connect the other end to the wire (or circuit) in question.
When the test light illuminates, it means there is a short to ground.
Be sure it is the correct circuit you are testing. The circuit may be grounded for a reason.
Wiggle the wire or wiring harness until the test light does not illuminate.
Inspect the area that you wiggled. There, you will find the short.
A short to positive can be tested the opposite way.
Instead of clamping the test light to the battery positive, it will be connected to a ground connection (battery negative).
Testing a fuse with a test light
What is the best automotive test light?
A test light preference depends on the user. Typically, the best automotive test lights have a long, sharp probe to pierce wires or have a proper connection to a fuse in the fuse panel, a long cord length, and a replaceable bulb.
Typically, any test light will work for a beginner mechanic. Even for a small price, a test light can a great investment.
For advanced mechanics, it is important to keep in mind the current draw of the test light when doing electrical tests. For example, a test light can be placed in a circuit in place of the load component. The current draw of the test light can eliminate bad grounds or powers.
What are other types of electrical equipment or circuit tester(s)?
Some other electrical equipment and circuit testers include:
Digital Multimeter
A digital multimeter is a tool used to test multiple electrical values. A basic multimeter can display voltage, amperage, and ohms (resistance).
Power Probe
A Power Probe is a tool with a similar shape as a test light but with extra functions. It has two clamps that connect directly to the battery and can send power and ground to a circuit when needed. It can also display the voltage on the integrated screen along with indicating that a circuit has a battery positive or negative.
Oscilloscope
An oscilloscope is a tool that can graph voltage changes on a screen. The oscilloscope has a high sample rate and can graph irregularities in voltage waveforms. This tool is used by advanced automotive technicians on the hard-to-find problems.
It is important for a professional mechanic to be up-to-date to be able to perform proper tests on modern cars.
Check out this case study of a suburban with an electrical problem with the cooling fan.
How do you use a test light?
Below is how you can use your test light to check for a blown fuse.
The first step is to connect your test light to the negative battery terminal or a good ground.
Next, test your test light by touching it to the positive battery terminal. If it lights up, then it is in good working order, if not then there may be a bad connection or a blown bulb.
Find your fuse panel.
Use the probe end of the test light to touch the test contacts of each fuse.
The test light should illuminate on both sides of the fuse. If it does not, then there is your blown fuse.
Where can I find the right wiring diagram for my car?
Finding a wiring diagram can take hours of searching through various websites and forums.
Even when you have found a wiring diagram, how do you know it's the one you need or for your car?
An easy way to find the correct wiring diagram for your car is to use AlldataDIY.
AlldataDIY has all the wiring diagrams for your car and step-by-step repair procedures to fix your car the right way.
What is an LED test light and how does it work?
An LED test light is just like an incandescent test light, except with an LED bulb. Most mechanics will not use an LED test light because of the low current that it takes to illuminate.
What does red mean on a test light?
A red color on a test light will usually mean the circuit has been completed. This may mean that the test light has touched power (when the clamp is connected to ground) or ground (when the test light is connected to positive).
Always check your test light's manual if there is one.
How do I check for a battery with a test light?
Checking that the battery is in good working order in not possible with a test light. There are some test lights that have a voltage meter on them, but it only displays the voltage of the battery and not the condition.
A battery tester is needed to check the battery and the car's electrical system.
Try your local auto parts store for a cheap test light.
It may be able to get you out of a jam with a blown fuse or broken wire.
If you have questions about test lights and their capabilities they may or may not have, ask them in the comments below or find a trusted mechanic!
Disclaimer and Disclosure:
Due to factors beyond the control of DiagnosticMechanic.com, it cannot guarantee against unauthorized modifications of this information, or improper use of this information. DiagnosticMechanic.com assumes no liability for property damage or injury incurred as a result of any of the information contained in this website. DiagnosticMechanic.com recommends safe practices when working with power tools, automotive lifts, lifting tools, jack stands, electrical equipment, blunt instruments, chemicals, lubricants, or any other tools or equipment seen or implied in this website. Due to factors beyond the control of DiagnosticMechanic.com, no information contained in this website shall create any express or implied warranty or guarantee of any particular result. Any injury, damage or loss that may result from improper use of these tools, equipment, or the information contained in this website is the sole responsibility of the user and not DiagnosticMechanic.com.
DiagnosticMechanic.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com. In many of our case studies, articles, and tool reviews, we may earn a small commission when readers purchase products through our links.