So you just installed an aftermarket part like an electric fan, head unit or security system but now you’re stuck on the wiring part. Or maybe you’re tired of waking up to a dead battery that’s suffering from a parasitic drain of some kind. You frantically search the forums and read a bunch of how-to articles and they all tell you to simply use a multimeter to troubleshoot your electrical woes away.
The problem is that no one ever talks about how to actually use one of these things. If you’re modifying your car or even own one for long enough, sooner or later you’re going to have to develop a good relationship with the electrical system. A multimeter is a very handy tool that can help you pinpoint electrical problems, save money on repairs, and also help you install aftermarket parts and accessories the right way so you don’t end up overloading your system or having to deal with headaches down the road.
There are two main types of multimeters – digital and analog. I prefer a digital multimeter (DMM) because the display screen is much easier to read and they’re more resistant to damage. You don’t have to break into your piggy bank either. You can get one for right around $20 but it’s a tool you’ll wind up using a lot so it’s also worth the few extra bucks for a better model. The best way to get familiar with your multimeter is to put it to use with some easy exercises so take a moment to read the manual so you get familiar with the parts and settings of your specific unit and let’s get started.
1. How to Measure Voltage
This is probably the function you’ll use the most as a DIY auto enthusiast and comes in handy for troubleshooting a variety of electrical issues and identifying 12V power sources. In this exercise you can either choose to measure the voltage of your car battery or a household AA. Plug the black probe into the COM (common) jack and the red probe into the mAVΩ jack (this jack may be labeled differently on your unit). If you choose to go with your car battery, turn the dial on your multimeter so it’s set to 20V in the DC range (DC is the V with the straight line next to it, AC is the V with the wavy line). If you’re measuring a AA, then set your multimeter to 2V.
Now why do we have to use different voltage scales for the car battery versus the AA? Most multimeters are not autoranging so you have to set it to the range you expect to measure. A setting of 2V will measure voltages up to 2 volts while a setting of 20V will measure voltages up to 20 volts. If you’re unsure of the voltage, the best thing to do is start at the highest scale to find the approximate value of the voltage. Then select the optimum range according to that number. This ensures your unit doesn’t receive a higher voltage than expected and safeguards it from getting damaged. You’ll know when you’re out of range (need a higher setting) if you get a reading of “1” that appears to the very left of your screen. You want to set the range as close to the voltage you’re measuring as possible for the most accurate reading. So if you’re measuring a 12V battery, a setting of 20V will be more accurate than a setting of 200V.
With the multimeter set correctly, touch the black probe to the negative terminal (-) and the red to your positive one (+). For a car battery on a warm day, a reading of 12.6 means it’s fully charged. If it reads less than 12.4 volts then it’s low and needs to be recharged. Your household AA battery should read about 1.5 volts if it’s a fresh one.
If you’re out by your car for this exercise let’s take it a step further. Turn on the car to test the battery’s charging voltage. With the car at idle, you should see a reading of 13.5 – 14.5 volts. If it’s less than 13.5 volts then the alternator isn’t putting out enough current to keep the battery charged.
When testing other circuitry in your car the black probe or clamp should always contact a ground connection and the red one at the connection or point in the circuit where you want to read the voltage. Knowing how to use the voltmeter function of your multimeter is useful for a variety of tasks like identifying blown fuses, broken connections or finding 12V sources for your aftermarket parts.
If you get your blacks and reds mixed up, it’s no big deal. You’ll still get the same numeric reading but it will have a negative sign in front of it.
2. How to Measure Resistance
As the name suggests, resistance measures how much a circuit resists the flow of electric current in ohms. Knowing how to test for resistance and continuity (whether or not there’s an electrical connection between two points) is important when working with fuses, switches, relays, sensors and other components of your car or truck like the ignition coil, lighting, stereo and more.
Anytime you measure for resistance or continuity it’s important that you disconnect the circuit or component from power. The easiest way to do this is to remove the red cable (+) from the battery or remove the object (like an ignition coil, light bulb or subwoofer). This protects your multimeter from damage since it has an internal battery that’s used to create a current through the circuit or device being tested.
Starting with something simple, let’s say your headlight or tail light goes out. Some of the headlight bulbs on today’s cars can cost $50 so let’s be sure the bulb is indeed dead before getting the credit card involved.
Remove the bulb in question from the socket and housing. Never touch the bulb with your bare hands because the natural oils from your skin will cause the glass to overheat and reduce the life of your bulb. If you don’t have one handy, we can do the same test on a regular household light bulb. Plug the black probe into the COM jack and the red one into the mAVΩ one. Just like you did when measuring voltage, flip your selection dial to the ohms (Ω) side and select the range in which you expect your measurement to be. For a car bulb the resistance is generally low so pick one of the lower scales.
If you’re not sure and you’re testing a component like an ignition coil or subwoofer, sometimes it will be printed on a sticker and other times you’ll have to look it up in a manual or check with the manufacturer. If you’re testing fuses, wiring or troubleshooting other circuitry you can also find the correct range by starting high and dialing down until you get an accurate reading.
Tail lights typically have 2 contact points at the bottom – a tail light filament and brake light filament. A headlight will often have 3 prongs or contacts – ground, low beam and high beam.
For a tail light, touch your black probe to the metal sleeve of the bulb on the side. Then touch your red probe to each of the contact points. You should get a reading of 1-3 ohms. For a headlight bulb, touch your black probe to the ground contact while testing each of the other two contact points with your red probe.
You can test your household light bulb the same way. Touch your black probe to the very bottom of the bulb’s sleeve (button) and touch your red probe to the side of the metal sleeve.
If a filament is burned, you’ll get an infinite reading (open circuit) and depending on your multimeter you’ll commonly see “OL”, “OFL” or another indication (consult your manual).
Most digital meters are also capable of reading continuity, which is symbolized by speaker or sound cone icon. Sometimes it’s part of the resistance settings and with other meters it’s a separate function. This is convenient for testing fuses, troubleshooting broken wires or connections, and any part that needs to pass current. When there’s continuity through the part, wire or connection, the meter will beep. You use continuity just like you do resistance but it speeds up the process of testing many wires or fuses because you don’t have to keep looking for a reading. If you hear a beep, then it’s good.
3. How to Measure Current
When you’re dealing with aftermarket parts, knowing how much current is being drawn by a particular component is a valuable skill for troubleshooting or preventing electrical overloads. It’s also useful for identifying parasitic battery drains and measuring power output of your alternator among other things.
Let’s test our battery for a potential drain. Connect your black probe into the COM jack. This time your red probe most likely has to go into another jack labeled 10A (or 20A depending on your unit). This allows it to read higher amps than in the milliamps range. A parasitic drain on most late model vehicles should be less than 50 milliamps but if your battery goes dead often then you may read something much higher. You never want to “surprise” your meter with a higher load than it’s scaled for or you can burn the fuse or damage it.
Remove the negative cable from the car battery. You’re going to connect your multimeter in series with the circuit, so touch one of the probes to the negative battery terminal and the other probe to the disconnected negative cable lying nearby. This completes the circuit and the current passes through the multimeter which reads it and then displays the amp draw on the screen. If you know you don’t have a drain problem you may have to scale down to a lower setting to get a reading.
- Multimeters can be easily damaged or blow a fuse when testing a voltage or current that’s higher than expected so if you’re unsure always start with the highest scale and work down from there. If you get a reading of “1” then you know you’re out of range and need a higher scale.
- If you want to change ranges during a measurement, make sure that at least one of the test leads is disconnected before changing the range selector or the multimeter could damaged by spark, etc.
- Remember to disconnect your circuit or component from power if measuring resistance or continuity because your meter will supply its own power from an internal battery. Leaving power on while testing will damage the meter.
- Watch your fingers. Touching the resistor or the probe with your fingers can cause inaccurate readings due to your body absorbing current from the circuit.
- If your multimeter stops working, check the fuse. You may have blown it.
- Temperatures affect your readings. For instance, voltage readings will drop with temperature roughly .01 volts for every 10°F.
- Disconnecting the battery in certain models of cars will reset the PCM requiring modules to relearn and erase your radio and clock settings. It’s best to plug in a battery backup into the cigarette lighter to protect the KAM while disconnecting the battery cable.