How to Use a Multimeter on a Car

How to Use a Multimeter on a Car

When it comes to troubleshooting problems with your car, electrical issues are often the trickiest to solve. When a mechanical aspect of your car isn’t working properly, there is usually a direct and obvious reason why. To identify an electrical issue, you’ll need to use a multimeter to see whether or not voltage and continuity are present in the wiring of your vehicle. We’ll explain exactly how to use a multimeter on a car in the rest of this article!

How to Use a Multimeter for Dummies

Auto mechanic testing an electrical circuit using a multimeter

If you’ve never heard of a multimeter before, it is a special tool that can measure voltage, resistance, and current. They are often digital, rectangular-shaped handheld devices that work by connecting probes to wires to you want to test. When an electrical component of your car isn’t working, it generally means there is a defective wire causing an open or short circuit. 

Identifying Electric Problems

How to find a short in a car with a multimeter?

Multimeters can help you distinguish exactly which wire isn’t working, as you’ll notice a lack of voltage from the defective wire. Finding a short is quite a tedious process, and will require you to measure continuity to identify where a short circuit is occurring.

Depending upon what unit you’re looking to measure, you’ll want to set the range and parameters for your multimeter. If you’re going to test for amperage, make sure to always use the fused port for your probes. Otherwise, you can use the non-fused ports. When looking at your probes (or leads), the black one is negative and the red one is positive. No matter what you plan on testing, make sure to apply the parking brake before you begin!

You might also like: How to Read Car Wiring Diagrams for Beginners

How do you check the voltage with a multimeter?

If you’re checking the voltage on your car, you’re going to want to use DC voltage. 20V is the right setting if you want to test your battery. Simply adjust the dial to the correct range/setting, and connect your probes to whatever component you wish to get a reading for.

How do you check a headlight with a multimeter?

In order to test the wiring of your headlight, you’ll actually need to find the ground wire. From the connector that attaches to your headlight, there are two-four wires. With two or three wires, only one of them is a ground wire, while four-wired connectors will have two ground wires. Whichever wire is connected to the chassis is the ground wire. To test it, you’ll want to set your multimeter for resistance. You can place one of your probes on the ground wire, and the other on your car battery’s negative terminal. If you do not read continuity, then you have a problem with the ground wire, meaning it needs replacement.

How to test a car ground wire with a multimeter?

To test ground wires, you’ll first start by measuring resistance. Test along the wiring to verify your reading is five ohms or less. If it exceeds five ohms, you’ll need to test the wire further. Switch your multimeter to DC voltage and turn on whatever electrical component you’re having issues with. Test the wiring once again to ensure the reading doesn’t exceed 0.05V. If it does, you’ll need to use a different place to ground the accessory or use a bonding strap.

How do you check if a fuse is blown with a multimeter?

Troubleshooting a fuse is extremely simple with a multimeter. Simply set your multimeter to the lowest ohms setting and place your probes on either side of the fuse caps. Fuses don’t have polarity, so the probes you use don’t matter. If you have a very low ohms reading, the fuse is working great. If the resistance value didn’t change after connecting your probes, the fuse is blown.

You can also test a fuse using a test light. Turn the key ON and touch both sides of the fuse with the tip of the test light. If it lights up, the fuse is good. If it doesn’t, the fuse needs to be replaced.

How to Use a Multimeter on a Car Battery

Auto mechanic testing an electrical circuit using a multimeter

If you’re having difficulty getting your car to start, one of the most common reasons is a weak battery. This is something you can easily test with a multimeter, which will also give you an accurate idea of how much charge the battery actually has left. Testing your car battery first is a great way of ruling out a likely-culprit for many electrical issues.

When testing your car battery, you’re going to test for voltage, setting your multimeter to 20V DC. You can connect your probes to both battery terminals, matching up each color to the same-colored terminal. If you have the ability to have someone turn the headlights on, do so now so you can get a better reading of what the car can actually handle. You’ll do this with the car turned off.

In most conditions, you’ll have a reading somewhere between 11.8V and 12.6V. The higher the voltage reading you see, the more charge is left in the battery. A 12.5V reading would indicate a like-new battery with an almost full charge, while an 11.9V reading would indicate one that the battery will need to be replaced soon.

How to Use a Multimeter on a Car Alternator

Auto mechanic testing a dead battery using a multimeter

Another reason why your car won’t start is a failing alternator. Unlike your car battery, this isn’t a part that you can directly connect your probes to. Instead, you’ll actually test the car battery once again. An alternator is responsible for recharging your battery while you drive using the natural mechanical movements of your vehicle. So by checking how your battery responds under various electrical loads, you can see if your alternator is actually the problem.  

To test your alternator, you’ll want to set your multimeter to 20V DC again. Make sure the vehicle is off before popping the hood. Match up the corresponding probes to their correct terminals on the battery (negative to negative, positive to positive). Check the reading to see where you’re starting at, hopefully, you’re starting with at least a 12.4 reading.

Now, you’ll want to start the car. Check the battery reading once again. If you see a reading of fewer than 14 volts, then it is likely your alternator is, in fact, the issue. A healthy reading will be above 14 volts. If you’re seeing a healthy reading, go and turn on anything you can think of that requires electricity. 

Things like your stereo, all lights, heater, and seat warmers are all good options here. You’ll want to read the multimeter once again, seeing how much the voltage dropped by. If you have a reading above 13 volts, then the alternator isn’t the problem. However, a reading less than 13 volts means that the alternator is actually the issue.

Using a Multimeter on Your Own Car

Two auto mechanics looking for an electrical problem using a multimeter

There’s no secret that identifying and solving electrical problems can be an absolute nightmare. Some vehicles have more than a mile of wiring (if you unraveled it all and stretched it out), meaning there’s a lot of potential wiring that could go haywire. Finding exactly which portion isn’t working will require a lot of effort, but it is well worth it once you finally fix the problem

If you want to test the wiring of your vehicle, you’ll need to acquire and learn how to use a multimeter, as well as an OEM repair manual for your vehicle. To make your life easier, you can find how to use a multimeter on a car pdf to walk you through the process. With this, you can follow step-by-step instructions that will illustrate what unit you should be measuring and where to place your probes! Backed with a multimeter and a pdf instruction guide, you can tackle any electrical issues in your own garage!

About Ren R

Loves cars and helping others learn how to take care of their own vehicle. Drives a Honda Accord and enjoys every second of it!

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