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An automotive test light or circuit tester can check the presence or absence of electricity between two locations in a vehicle. This tool is used to test load carrying electrical circuits. A 12 volt test light is a handy visual indicator that voltage is available, but can’t measure current flow higher than the test light’s own current consumption.

Test lights for cars are composed of a handle similar to a screwdriver except that the handle is hollow with a flexible wire passing through a removable rear cap that provides a contact on the inside of the cap. Typically, there will be a spring between the bulb and the inside end of the probe, which extends like a screwdriver bit out of the handle except that it has a sharp tip.

car test lights image
There are high impedance test lights with LEDs in them that pull almost no current, and there are low impedance test lights (like the one shown) that pull about 0.25 amps.| Image Source: Richard McCuistian

Using the tool involves clipping the wire onto a metal component like your vehicle’s chassis or your battery’s negative terminal and then using the probe to check if any electricity passes through the probe into the wire or vice versa. If there’s electricity, then a light on the probe will illuminate.

In this article, we’ll discuss how to use a test light on a car.

Checking for Voltage

Just clip the wire to a ground source, which can be your battery’s negative terminal or your vehicle’s chassis, touch the probe to the positive battery terminal to make sure your clip connection is good and you have source voltage at the battery,  and then use the probe to check for voltage at the test point.

diagram of a test light
Diagram showing how to use a test light | Image Source: Richard McCuistian

If the circuit is switched on and you don’t have voltage at the test point, you need to find out why by checking the same circuit at a point closer to the source of the voltage. When you find voltage, the open circuit is between the place that illuminated the test light and the place nearest to it that didn’t. 

In the video below, we demonstrated how all the voltage that was supposed to be leaving the battery wasn’t even making it into the battery terminal, even though the connection looked just fine. We found the problem with a test light.

If you’re checking a switched load, you can check the fuse that feeds the switch first, then check the terminal that feeds power to the switch. Then turn the switch on and see if the power passes through the switch. If it doesn’t, the switch is faulty.

But there is a caveat here. If there is resistance in the feed to the switch, you may read voltage on the “hot” side of the switch until the switch is closed, and then the voltage may go away. At this point, you need to find the point of high resistance.

More Reminders About Using a Test Light

Connect the clip to the B+ battery post when checking for ground. But remember, even a low impedance test light only pulls ¼ amp (0.25). If the load pulls 5 amps, your test light might light up as if you had a good ground, yet as soon as the load is energized the ground may not carry the load. For more on this, watch this short video:

Even a low impedance test light only pulls ¼ amp (0.25). If the load pulls 5 amps, your test light might light up as if you had a good ground, yet as soon as the load is energized the ground may not carry the load.

Richard McCuistian, ASE Certified Master Automobile Technician

In the video, a test light indicated there was power and ground at the fuel pump, but when the fuel pump was energized, the power and ground that would operate the test light bulb weren’t enough to operate the pump, which pulls at least 20 times as much current as the test light. 

That’s the point of the short video: there was resistance in the fuel pump ground (dark) side of the circuit. It takes both good ground and good power (B+) for a circuit to work right.

You might notice in the video that we found the problem by using a headlamp bulb, which consumes about the same amount of current as the vehicle’s fuel pump when it’s pumping fuel (5-7 amps). When the tank is empty, the pump will pull only about 1.5 amps because the pump is spinning free.

Checking For Blown Fuses

You can also use a test light to look for blown fuses in the vehicle’s fuse box. When certain electrical components are malfunctioning, finding the blown fuse can be challenging. Finding blown fuses using a test light is fairly easy. You can simply clip the test light onto your chassis, which provides a negative charge. Then, you can probe both metal contacts of each fuse to check if the current can pass through. If the test light only illuminates on one side of the fuse, then the fuse is likely blown.

image showing a blow fuse check using test lights
Illustration showing how to check for blown fuses using a test light | Image Source: Richard McCuistia

Checking for Faulty Relays or Control Units

Aside from blown fuses, you can also check for faulty relays or control units for certain components. The process involves clamping the wire to a ground source like your vehicle’s chassis and then using the probe to check the positive and negative terminals of the suspect component. If the test light doesn’t illuminate when you probe the part’s negative terminal, then the component isn’t conducting electricity and it’s likely faulty. If the test light doesn’t illuminate when you probe the component’s positive terminal, then the part’s power source could be to blame.

, How to Use an Automotive Test Light

Pro Tips are nuggets of information direct from ASE-certified automobile technicians working with CarParts.com, which may include unique, personal insights based on their years of experience working in the automotive industry. These can help you make more informed decisions about your car.

Pro Tip: You need a schematic in most cases to do a comprehensive check of the circuit or relay and you need to know how a relay works and which pin is which in the relay socket.

Checking the Ground Path

All vehicle electronics have a ground wire that leads back to the vehicle’s battery or the vehicle’s chassis. The purpose of the ground wire is to give a circuit a way to discharge excess electrical current and protect the equipment from excess voltage and short circuits. Vehicle electronics need a reliable ground path, or else they won’t function. You can use the test light to check if an electric device has a good ground connection. 

After confirming that a device has a reliable positive power source, connect the test light’s clamp to the positive terminal. Then, use the probe to check the ground component in question. If the tester illuminates, then it means there’s a solid ground connection. However, if there’s no illumination, you might need to clean contact points and inspect the ground path. Fortunately, re-establishing a ground path is a fairly easy task.

Keep in mind that you can potentially damage vehicle electronics if you don’t use the test light properly. If you connect the test light’s clip to a high-voltage power source and connect the probe to a sensitive electronic part, you can cause short circuits. Also, don’t poke your test light randomly. You should know what you’re inspecting so that you don’t damage sensitive components. 

Some vehicle test lights can have a red and green light that can indicate whether you’re probing a ground circuit or a positive circuit. These types of test lights can be useful so that you can check the type of circuit that you’ve connected to the clip before you start probing.

diagram showing a test light used to determine ac clutch continuity
The Illustration shows how you can use a test light to determine the continuity of the A/C clutch as well as the integrity of the clutch ground. The light should light when connected as shown with the A/C clutch fuse removed. First, find the hot terminal by connecting the test light clip to ground. Then move the test light clip to B+ 12v and test the opposite side of the fuse. It should show a ground coming back through the load. You can also check fuel pumps and other electrical loads this way if they aren’t working when they’re engaged. | Image Source: Richard McCuistian
About The Authors
Written By Automotive and Tech Writers

The CarParts.com Research Team is composed of experienced automotive and tech writers working with (ASE)-certified automobile technicians and automotive journalists to bring up-to-date, helpful information to car owners in the US. Guided by CarParts.com's thorough editorial process, our team strives to produce guides and resources DIYers and casual car owners can trust.

Reviewed By Technical Reviewer at CarParts.com

Richard McCuistian has worked for nearly 50 years in the automotive field as a professional technician, an instructor, and a freelance automotive writer for Motor Age, ACtion magazine, Power Stroke Registry, and others. Richard is ASE certified for more than 30 years in 10 categories, including L1 Advanced Engine Performance and Light Vehicle Diesel.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.

File Under : Electrical System , DIY
Garage Essentials
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