How to Read and Interpret a Voltmeter
Why Own A Voltmeter?
When it comes to working on your car's electrical system, a voltmeter is a "must have" tool. Some people call them a VOM, Volt-Ohmmeter or Voltmeter. No matter what you call it, they all do the same thing. A decent-quality voltmeter can tell the condition of your battery, if a wire is broken internally, or if a relay isn't working, a fuse is blown (even if it looks good), a distributor shaft is worn, alternator or generator malfunctioning, light socket not grounded or what's not functioning in that radio circuit. If that's not enough, the voltmeter can tell you what's wrong with your vacuum cleaner, toaster or microwave oven! Now are your interested? All you need to do is learn how to use one...
Which Voltmeter is best for me?
Here's the good news: a perfectly useable voltmeter can be purchased for less than $20. Of course, you can spend hundreds for meters that are self-protected and have features that you'll never use, but you don't have to do this to get a meter capable of doing all the necessary things.
These meters, from left to right, are a $6 throwaway meter found at a hardware store, a venerable 0ld 1960's Simpson VOM that weighs 3 pounds and a top-of-the-line Fluke digital meter that costs about $100.
There's only one decision you have to make: digital or analog? Frankly, it makes no difference which one you choose, so if you like numbers flashing in front of your eyes go digital. Otherwise, the sweep of a needle on an analog meter is much easier for many people to see, but you have to know which scale you're on to properly interpret readings. Look at the photo and decide for yourself. Three meters, two analog and one digital, are shown connected to a 12-volt motorcycle battery. The two analog meters' scales are set on 10 volts, so their needles are "pegged." The digital meter is precisely measuring the battery's voltage at 11.77 volts.
All meters read Voltage in AC or DC mode, Resistance in several scales and Milliamps, or thousandths of an amp. Some have overcurrent protection, a nice feature for those who forget which scale they left the meter set at, and then plug the probes into a 110 Volt AC line (unprotected meters will get fryed.)
Ignore features such as "battery check," which is nothing more than a 1.5 volt DC scale, and "fuse check," which is nothing more than a resistance reading. If the meter comes with these at no extra cost it's no big deal, but don't pay for them.
Voltmeters take batteries. The reason for this is that the resistance scale requires the meter itself to put a little current into a wire to measure its resistance. Otherwise, the batteries aren't necessary to read voltage or milliamps. Always leave the meter set on a voltage scale to prevent draining the battery.
Okay, I got my own Voltmeter, Now What?
We need to do a little "refresher" on basic electricity (see our six-part series on Automotive Electrical Sysems for a more in-depth discussion of basic electricity.) so here goes...
You need to know about Amps, Volts and Resistance, or Ohms, in order to troubleshoot electrical systems. The Amp is the basic unit of current in electricity. Current, or flow of electrons, is what's doing the work in a circuit. It's the current that heats up the filament in a light bulb, flows through the circuitry in a radio, creates electromagnetism in a solenoid (relay) and does everything we're accustomed to enjoy from electrical devices.
Voltage is the "pressure" of electricity. That is, current can't flow without a voltage potential forcing it to do so (think of it this way: if current is the stream of water flowing out a hose nozzle, voltage would be the water pressure in the pipes that is pushing it through the hose.) In cars, of course, the electrical system runs on either 6 or 12 Volts DC, or direct current.
Resistance (Ohms) is anything in a circuit that resists the flow of current. All circuits have resistance, because that's how electricity does work for us. Take, for example, a headlight. Attaching a 12-volt source to the terminals on a headlight causes it to light up. The reason it does so is that the filament in the bulb has a designed-in resistance. That resistance requires a high current flow (relative to the wires attached to the terminals) to pass through, heating up the filament until it becomes incandescent. A 100-Watt headlight will consume a little over 8 Amps of current. Wait a minute! Where did Watts come from?
Glad you asked. The Watt is the product of Amps times Volts (W = A x V). It is the unit of electrical power, and we use the formula to calculate the current. Adding Resistance to the mix, we come up with the formula: Volts = Amps x Resistance. Since we know the current in the example above we can plug the values into the formula to find the resistance of the headlight's filament: 12 = 8 x R, or R = 1.5 Ohms.
Why did we do that? Because most meters can't read Amps. They can read milliamps - thousandths of amps - but not the real McCoy. Doing that takes rugged, heavy-duty meters called Ammeters, but you don't need one if you know the basics.
One More Thing...
Electricity doesn't do any good without having a circuit. That is, electricity must flow from the power source (the battery + terminal, in the case of most cars) through any switches and relays, through the device that's being operated (a lamp, for instance) and then to ground (the negative terminal of the battery or the car's metal chassis, since it's grounded directly to the battery's negative terminal.) Look at the first photo again. All three meters are, in fact, completing a circuit that goes from the battery positive through the meter itself and back to the battery negative post.
That means the electrons have a closed circuit (path) in which to flow. If any item in the circuit is disconnected, corroded or otherwise "breaking" the circuit, no electricity will flow and the device won't work. Therefore, just measuring for battery voltage doesn't mean that the circuit is intact. You have to measure for voltage at every point in the circuit!
The photo below shows a simple circuit. The battery positive is connected to one terminal on this headlight. Current flows through the light's filament to the other terminal and back to the battery negative post.
Note: In positive ground systems, electrical flow is from the negative battery terminal through the devices and back to the positive terminal. Either way, you have a complete circuit.
Here's a simple electric circuit.
Let's Do Some Measuring And Troubleshooting
All meters have a red and black wire. These are the "test leads" and are colored for use in DC circuits so that the proper polarity (direction of current flow) will be maintained and allow the meter to be read correctly (AC measurements don't care which color leads are used.) Because most DC current is considered to flow from positive to negative, red test leads always are put on the positive voltage source and black on the negative. Notice, in the photo, that the red meter lead is on the battery positive and the black on the switch terminal.
The photo below shows the circuit consisting of the battery positive connected to a switch, then to the light's terminal, then from the other terminal to the battery negative. The meter is measuring battery voltage up to the switch's terminal, but since the switch is in the off position the light isn't on.
When the switch in the circuit is turned on the light glows. The meter's black lead has been moved to the output terminal of the switch, showing that current is flowing through the switch itself. Notice that the voltage has dropped to 10.8, indicating the battery is discharging as it powers the lamp. The photo below shows the circuit with the switch in the "on" position.
We could use the meter to troubleshoot anywhere along the circuit path by measuring voltage into — and out of — each component. Where the voltage measurement stops we know there is no current flow.
The meter can help you interpret what's wrong with your electrical system in many, many ways. Here's one of them:
Suppose your engine won't crank. The headlights and accessories work, but the starter motor won't turn. All it does is click. You're not sure whether the battery is good or not, or whether the battery cable might be bad. Put the meter leads on the battery and measure the voltage (probably close to 12 V). Then, have someone attempt to crank the engine. If the voltage drops to about 7,8 or 9 Volts, the battery is sufficiently discharged, so much so that it can't develop enough current to crank the starter motor. If the voltage stays high, your battery cable(s) might be corroded or broken, the starter solenoid bad or the motor itself bad. See how easy this is when you get used to using a meter?
Suppose, however, a wire is routed through areas that we can't see or the headlight above doesn't glow? Is there a way to check to see if electricity will flow at all? Is there a way to measure without turning on the power? Yes, There Is!
It's called continuity, and we use the resistance scales in the meter to utilize it. Take look at our digital meter. We've set the button at the top on DC and the one on the bottom on OHMS, or resistance. The buttons down the middle are for setting particular ranges of voltage, resistance, etc., but for our purposes it really doesn't matter. All we want to know is if we have a path for current to flow.
The way this works is that the battery inside the meter provides a little voltage to the test leads. When they aren't touching at the ends the resistance measurement is "infinity," shown in this photo as a "1" with an infinite number of zeros next to it (the meter manufacturer chooses to just show blank spaces and a decimal point.) Photo at left shows the meter set on the resistance scale, the meter is reading infinity
If we touched the test lead ends together we would complete a circuit, wouldn't we? After all, the battery inside the meter would be providing current flow from its positive end out through the red lead and then back through the black lead to the negative end. Since the leads are relatively fat wires and there's no device in the circuit to use current, the resistance reading will approach zero. Zero resistance means we have continuity.
In the next photo we have connected the meter leads onto both the input and output sides of the switch, then turned the switch on. Since the circuit is now complete, we have zero resistance or continuity.
Below, we can easily see the switch is in the "on" position. If it were off we'd read infinity.
That's how we can trace wires from one point to another, verify that a wire isn't internally broken, check out filaments in bulbs (if the headlamp's filament were burned out and we put the meter's leads on the back terminals we'd have read infinity) and make measurements of all kinds.
Suppose you want to find out if the red wire under the dash is the one that goes to the horn relay under the hood. Using the resistance setting you can connect one test lead to one end of the wire and the other lead (use a spare piece of wire to extend the reach, of course) to the end in the engine bay. If the resistance is zero, or close to zero, you've found for certain that it's the same wire.
Go out and buy yourself a meter. Take your pick from a wide variety of styles and prices, but don't spend over $20 unless the meter is "self-protected" against hooking it up wrong. Then spend an afternoon playing with it and get used to tracing some simple circuits, moving on to more complicated ones. Once you get used to what a meter can do for you, you'll wonder why you took so long to buy one!