I know at first glance, a digital multimeter might look pretty confusing and maybe even a bit intimidating. They’re filled with strange symbols and multiple test lead ports for plugging in just two test leads. It’s enough to leave anyone stuck scratching their head.
In the right hands, a digital multimeter is a powerful tool that can be used in countless applications. In fact, it’s one of my most commonly used tools when I’m working on an installation. And by the time you’re done with this post, your hands will be plenty capable of putting a multimeter to use in your next project.
In this post, I’ll walk you through step by step on how to perform a variety of tests with a multimeter. And for those of you who are more visual learners (like me!), I’ve included some videos from my YouTube channel where I walk you through everything from setting up the meter to showing you specific examples of how you can put it to use.
So let’s dive in! And what better place to start than why you’ve made a smart choice in learning how to use a digital multimeter over other alternatives.
Why Multimeters are Better than Test Lights
To test for a live circuit, many will reach for the familiar test light. Besides having no idea what the actual voltage is, let me explain why these are not only a bad choice, but could actually cause damage to your vehicle.
A test light works by allowing current to flow through it to light up the light. The current is low enough that it’s not really a problem in many of the circuits in your vehicle but it can be a big problem for others.
Modern vehicles are being packed with more and more computers, and these computers are filled with circuits that are designed to carry extremely low current. If you happen to come in contact with a wire leading to one of these circuits with a test light, the current that is drawn by the test light can quickly damage the circuit board.
A digital multimeter, however, typically has an input impedance of somewhere around 10 million ohms! The meter is practically invisible to the circuit because it draws virtually zero current.
Depending on the type of meter you have, they can be capable of measuring everything from voltage, current, and resistance, to light intensity, sound level, temperature, capacitance and more. For this post, we’ll be focusing on the most commonly used functions for installations: voltage, current, resistance, and continuity.
Auto Ranging Vs. Manual Ranging Multimeters
Auto ranging meters like the one pictured above are by far the most user friendly (and newby friendly) meters to use. With an auto ranging meter, you don’t have to know the expected range of voltage, resistance, current, or whatever you’re measuring before you take a measurement. All you have to do is set the meter to the type of measurement you want to make and the display will automatically adjust itself to give you the most precise reading.
When we talk about range in the context of digital multimeters, it is typically in reference to the different settings available on a manual ranging meter like the one shown above.
Unlike an auto ranging meter, with a manual ranging meter you need to either have an idea of the expected range of what you’re measuring or you’ll need to step through the ranges to find the one you need. For example, if you’re measuring a circuit on your vehicle’s 12V electrical system, you wouldn’t set the meter to the 0-2V range. Instead, you would set it to the 2-20V range because 12V falls within that range.
If this sounds really confusing, don’t give up yet! In the videos I posted below, I show examples using both an auto ranging meter and a manual ranging meter.
How to Measure DC Voltage with a Digital Multimeter
The video below gives detailed instructions on how to set up your meter to measure DC voltage, how to connect it to the circuit to get a reading, and examples of how you can put this skill to use. Read on, past the video for a summary of what’s in the video and further explanations.
Before you can take a voltage measurement, first you need to set your meter up to measure DC voltage.
Set the dial to the setting marked with a V (for voltage) with a straight line by it like the one in the picture. This is the symbol for DC voltage. Then plug the black test lead into the port labeled COM or common and the red test lead into the port with the same DC voltage symbol.
When you’re measuring voltage, you are measuring the difference in potential, or voltage, across a particular portion of the circuit. In order to do this, you’re meter needs to be connected to the circuit in parallel.
Now, I know this might sound confusing, but it basically just means that you connect the red (positive) test lead to the positive side of the circuit and the black (negative) test lead to the negative side.
Uses for Measuring DC Voltage
Measuring DC voltage is easily the most common use for a digital multimeter when installing mobile electronics.
It can be used for troubleshooting voltage drop issues, identifying a poor ground connection, or troubleshooting a device that isn’t powering on just to name a few.
In many instances, it’s the only tool that can help you get the job done right. Installing a remote start and security system, for example, where you need to be able to identify all of the ignition switch circuits, door lock circuits, brake wire, parking lights, etc. Or if you find yourself needing to hardwire a stereo like in the scenario I describe in the post How to Rewire a Car Stereo From Scratch.
The list goes on and on but I think you get the picture. So let’s move on to the next type of measurement.
How to Measure DC Current with a Digital Multimeter
Have you ever had a car battery that drains overnight for no apparent reason? Your problem could be parasitic current draw on your vehicle’s battery. In the video below, I walk you through setting up your meter to measure current, how to connect your meter to a circuit in order to measure current, and I show you an example of how to test to see if you have parasitic current draw and how to identify where it’s coming from.
The auto ranging meter I use in the video has three different settings for measuring current depending on how much current you are measuring; micro-Amps (µA), mili-Amps (mA), and Amps (A).
Unless you know the range of current you expect to be measuring, when you’re determining which setting to use on the meter, it’s always a good idea to start with the highest range first.
When you plug in your test leads, make sure you pay attention to which test lead port corresponds to which setting. The black test lead will always go in the COM port, but the red test lead will go in the port marked with the same symbol as the setting the dial is set to.
To measure DC current, your multimeter will need to be connected to the circuit in series so that the meter is physically a part of the circuit. In order to do this, first you will need to disconnect the circuit.
You’ll get the same result by measuring from either the positive or negative side of the circuit, but as a best practice, I always disconnect the negative side. To reconnect the circuit with the meter, attach the black test lead to the source side of the circuit and the red test lead to the load side.
I know this can be difficult to visualize without actually seeing it in front of you so definitely check out the video to see how this is done. I promise, it’s easier than it sounds!
Uses for Measuring DC Current
There are a lot of uses for measuring current with a multimeter, but in the mobile electronics installation world, it’s not something that comes up all too often. But there is one situation that might come up from time to time where knowing how to use your meter to measure current can save you a lot of time and money, and that’s a car battery that drains overnight.
If you run into this problem on an otherwise healthy battery, the cause is usually a parasitic drain on your electrical system. This happens when a circuit in your vehicle stays on or draws current in some way when it should normally be off when the vehicle is off.
How to Measure Resistance with a Digital Multimeter
In the video below, I show you how to set up your meter to measure resistance and test for continuity, I explain what resistance and continuity are, and give specific examples of how you can use each of them.
Resistance is measured in ohms and the symbol for ohms is the Omega symbol like the one in the picture. When you want to measure resistance with your multimeter, you’ll need to set the dial to the setting marked with this symbol. Then plug the black test lead into the COM port and the red test lead into the port marked with the Omega symbol.
When you measure resistance, it’s best to disconnect the component you’re measuring from any circuits. That way you know you’re only measuring the resistance of that component. Then, just touch one test lead to the positive side of the component and the other test lead to the negative side. It doesn’t matter which test lead goes to which side, the reading will be the same.
Uses for Measuring Resistance
One of the most common applications for measuring resistance when you’re installing mobile electronics is to measure the resistance of a speaker’s voice coil or multiple voice coils connected in series or parallel.
I go into great detail on this topic in my Subwoofer Wiring Guide post. Check it out if you’d like to learn more!
How to Test for Continuity with a Digital Multimeter
When you test for continuity, you’re testing if there is a continuous path for current to flow between the two points that you place your test leads. When there is continuity, the meter will beep to let you know.
To set up your meter to test for continuity, set the dial to the setting marked with the symbol shown in the picture. It should look like sound waves or a sideways wifi symbol. The test leads will typically go in the same ports you use to measure resistance.
Uses for Testing for Continuity
In the video, I show you an example of using continuity to quickly find a blown fuse in a fusebox. Another common use is to determine if a speaker wire is grounded out causing an amplifier or stereo to go into protect mode.
Instead of tracing each speaker wire along its entire path, just connect one test lead to ground and touch the other test lead to each speaker wire. If the meter gives you a beep on one of the wires, then you’ve quickly identified your problem.
I hope that after reading this post and watching the videos, your multimeter is much less intimidating. If you give some of these tips a try, I guarantee you’ll be wondering how you ever did installs without a digital multimeter. If you learned something here, I’d love to hear about it. Let me know how you put your meter to use in the comments below.