A multimeter is a hand-held tool that’s typically capable of measuring voltage, current, resistance and continuity. Voltage is measured in volts. Current is measured in amps. Resistance and continuity are measured in ohms. A multimeter can give you readings for each of these.
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Features of a Multimeter
Let’s start by looking at some of the features you can expect to see on a typical multimeter. This is the display, where you’ll see the reading. Depending on the multimeter, this may be analog, which has a moving dial and printed numbers, or could be a digital readout.
The selector, which allows you to choose what you’ll be measuring.
Two probes, typically one red and one black that you’ll use to make connection to what you’re testing, and ports where you’ll plug in the probes. The only actual difference between the red and black probes is the color.
Aside from ports that are labeled V for voltage, A for amperage, and Omega for ohms, you might also see the following symbols indicating smaller or larger measurements:
- µ (micro) = one millionth
- m (milli) = one thousandth
- k (kilo) = one thousand
- M (mega) = one million
What Current, Voltage, and Resistance Mean
Current is the rate at which electricity flows through a circuit, and is measured in Amperes, or Amps.
Voltage is the pushing force behind current that determines how quickly it flows through the circuit, measured in Volts.
Resistance is a force which throttles or lessens the current, increases voltage, and is measured in Ohms.
Imagine a reservoir of water. The water funnels down to a tube where the water can escape.
- The water itself would be current that flows through the circuit.
- The voltage would be the pressure pushing the water through the hose and out of the reservoir.
- The resistance is the size of the hose the water escapes from.
With more water (or current), pressure (or voltage) is also increased, but as it runs out (or loses charge), the voltage lessens. The size of the hose increases or decreases the resistance of the flow of water. A larger hose will allow the water to escape more quickly, while a smaller hose will restrict the flow of water and it will take longer to drain.
The two simplest ways a circuit can be connected are in series and in parallel.
In a series circuit, all the components share the same amount of current, but voltage is different across each component. This is because the components are wired in a single path from the supply, and as it passes through the resistors (the squiggly lines in the diagram) the voltage will drop at each component connected farther down the circuit.
In a series circuit, if a single component fails, the rest of the circuit will lose connection to the source.
In a parallel circuit, all components will take the same amount of voltage but may draw a different amount of current.
As you can see in this diagram, the current from the source will be divided among the multiple paths all connected to the source directly. But because each resistor is attached to the source, the voltage will be the same going into each resistor or other component connected to the source.
Because each component is pulling from the same source, if a single component fails, the remaining components are still able to function.
Measuring Voltage
Voltage tests are important to auto repair in a number of ways. Voltage drop testing is a great way to test to see if a component is getting adequate voltage and whether the component itself is failing. The voltage coming in should be approximately 12 volts depending on the component and circuit leading to it. Measuring the voltage coming in versus the voltage on the other side of the component will give you an idea of the amount of voltage it takes to run the component.
For example, imagine a corroded wiring connector. If you know your voltage before the connector from the source is approximately 12 volts, the voltage on the other side of the connector should be very close to 12 volts as well. Since voltage is a measurement of the difference between two points, the multimeter display should show 0. If the number is much higher than 0 when you take your reading, something between those two points is creating resistance in the circuit. Corrosion makes it harder, if not impossible, for electricity to flow between two points, and this voltage drop would indicate that the two sides of the wiring harness are no longer making a complete connection.
If you’re measuring DC voltage, it’s represented by the V with a straight line. DC stands for direct current, and this type of voltage is common to automobiles and other battery-powered devices. DC power can only flow in one direction through the circuit, from source to ground.
If you’re measuring AC voltage, you’ll want the V with a wavy line. Alternating current, or AC voltage, is more common to home wiring and electronics. It is called alternating current because the flow can periodically change directions. Your vehicle’s alternator produces AC voltage, which is converted to DC by the voltage regulator before it is supplied to the vehicle’s accessory systems or used to charge the battery.
Use your selector to choose the type of voltage you want to measure. Automotive current will come off the battery, which is direct current. The battery itself can also be measured, which is the basis of testing an alternator or your battery’s condition and ability to hold a charge.
Testing Your Vehicle’s Battery
When your vehicle is not running, with a fully charged battery, it should read 12.6 volts. With the vehicle running and the alternator charging the battery, that reading will be higher, usually between 13.7-14.7 volts.
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Do It Right: Your selector has ranges of voltage to choose from. You will want to select the highest voltage that you might expect in the system. |
For a vehicle battery, which is usually 12 volts, you’ll typically set the selector to measure from zero to twenty volts, since two volts would not be sufficient to measure a 12 volt system.
Your red probe will need to be plugged into the port with a “V” next to it. The black probe will be plugged into the COM port.
Connect the red probe to the positive, or source side of the component. This is where the power is coming from. Connect the black probe to the other side of the component.
The voltage will appear on the display. If you accidentally switch the red with the black, it won’t hurt anything, but the reading will be negative instead of positive. This is a good way to tell which side is positive if you’re not sure.
The voltage will appear on the display. If you accidentally switch the red with the black, it won’t hurt anything, but the reading will be negative instead of positive. This is a good way to tell which side is positive if you’re not sure.
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Do It Right: If your selector gives you voltage ranges to choose from, but you’re not sure about the range of the component, you may need to try several. |
If the voltage of your component is greater than the range you’ve selected, your display may read “1” or “OVER” or “OL”. On older analog style multimeters, this may blow a fuse inside the meter that will need to be replaced. And if the voltage of your component is less than the range you’ve selected, your display may provide a voltage reading, but it won’t be as accurate as when the proper range is selected. If the multimeter reads “0”, the range you’ve selected is probably too high for it to register a reading.
Current: What It Is and How to Measure It
Current is the rate at which power flows through the circuit. If you’re going to measure current, you’ll need to plug the red probe into the Amp port, which is usually marked with an “A”. There could be more than one port for amps, with labels like 10A or mA.
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Do It Right: It’s a good idea to start with a port that can measure a larger current than what you need if you aren’t sure. |
Select the appropriate setting for current on your multimeter.
To get a reading, you’ll touch both the red and black probes to component leads in a series circuit. The multimeter itself will complete the circuit, and should give you a reading of the current.
Resistance: What It Is and How to Measure It
Resistance, in the way electricity flows, is like kinking a hose. Though the current and voltage (like water volume and pressure) remain the same on one side of the “kink”, or resistor, higher resistance causes less current (or water) to flow, and with lower pressure (or voltage) after the resistor.
A resistance measurement must be performed with the power off. The way a multimeter measures resistance is to send a small current across the probes and measure the resulting voltage. The resistance reading is meaningless if there is already voltage on the thing you're measuring.
To measure resistance, you’ll plug the red probe into the “V” port.
Use your selector to choose resistance, which will be a range with an omega sign Ω.
Like before, choose the setting on your selector that is directly above what the resistance should be. High resistance values where they’re not expected may be a sign of an open circuit or a failing component. Knowing where in the circuit a resistor may have been added from the factory can help you understand why certain components only receive a certain amount of voltage and current.
Measuring Continuity
Current is the rate at which electricity flows through a circuit, and is measured in Amperes, or Amps.
Continuity is also measured in ohms, so you’ll use the same “V” port you used for resistance, but in testing continuity you’re more likely testing whether two points are connected at all. This can be helpful when tracing broken wires or assuring that a solid connection has been made with an electrical connector. Testing continuity must be performed with the power off to the circuit you are testing. Simply turning the vehicle off does not guarantee the power will be off. Either use the multimeter to check for power or disconnect the battery to be sure.
You’ll set the selector to the icon that looks like a speaker or sound wave. For continuity readings, many voltmeters have a speaker built in that will emit a tone if the reading is below a certain amount of resistance. This tells you that the two points are connected. If the resistance reading is too high, the circuit is read as an open circuit, or a circuit that is not electrically connected, and no tone is emitted.
