Last week, we showed you how to use a multimeter to measure voltage—or, more accurately, to verify that voltage is present—which is the most common reason you’d grab the meter and probe around in your car’s wiring. Now, we’ll tackle the second-most common use of a multimeter in a car—measuring resistance and verifying continuity.
As we discussed several weeks ago, resistance is the property of an electrical conductor that opposes the flow of current. In a load device like an electric motor or a light bulb, resistance is a good thing because it is actually taking the flowing charge and turning it into something useful, like a waterwheel in a river. In most wiring itself, however, you want the resistance to be as slow as possible to let the current flow through it without impeding it.
With that said, when we talk about measuring resistance with a meter, it isn’t the dynamic circuit resistance that we’re measuring; it’s the static resistance of a portion of the circuit.
Let me say that again in a different way. When a circuit is live, the voltage applied to the circuit, together with the total resistance of all of the components in the circuit, causes a certain amount of current to flow. You can measure the voltage and the current of a live circuit and use those figures to calculate the resistance (Ohm’s Law), but you can’t actually measure the resistance of a live circuit. For a number of reasons, you need to turn the power off and measure the resistance of individual pieces of the circuit. Or, to use the language we offered last week, a resistance measurement is taken with the circuit unpowered, in series with a portion of the circuit.
And, really, most of the time we’re not interested in the resistance value itself anyway. Instead, we’re usually interested in verifying continuity. (There are exceptions, such as testing a temperature sensor whose resistance varies with temperature, or verifying the correct resistance of a coil or a ballast resistor.)
So what is the difference between resistance and continuity? Think of it this way: Continuity is a binary version of resistance. If the resistance of the thing we’re testing—the wire we want to make sure isn’t broken, the connection we want to be certain actually goes to ground, the switch we want to know works—is low (like less than 1 ohm), we say that it has continuity.
Okay, let’s do a resistance measurement.
Configure the multimeter to measure resistance. There are three configuration steps:
- Put the black probe in the socket labeled “COM” for “common,” meaning it’s common to all measurements. Once it’s there, it’ll never need to be moved.
- Put the red probe in the socket labeled with the Greek Omega symbol (Ω) for resistance. It’s almost certainly also the same socket with the V for voltage. This means that you can leave the probe leads in the same sockets for the voltage and resistance measurements. You only need to change which socket the read probe lead plugs into if you need to measure current.
- Turn the big rotary dial to the setting for resistance, which is the one with the Omega symbol (Ω). If you don’t have an autoranging meter, select the most sensitive resistance setting. It’s really not going to matter much if you’re just looking for continuity. The meter should say “OL,” which stands for “over limit,” meaning that, with the probe tips not touching, there’s an infinite amount of resistance.
A multimeter configured to measure resistance (red probe in the “VΩ” socket, rotary dial turned to the resistance setting)
Configure the audible beep. If you’re testing continuity (and you almost always are), the “beep” is really handy, as it allows you to test without even looking at the meter. How to turn it on varies meter to meter. On some meters, it’s a separate setting on the rotary dial. On others, such as my old Fluke 85, it’s a button above the dial with a symbol that looks something like increasing sound waves or a megaphone.
The setting for the audible beep for continuity (red rectangle) varies meter to meter. On this one, it’s a push button
Test the meter. Now, touch the probe tips together. The resistance reading should drop from “OL” to near zero (meaning less than an ohm), and the audible beep should sound. This is what you should see when you put the probes on something that has continuity such as an intact wire or a closed switch.
The sub-one-ohm reading indicating continuity
Turn off the power! A resistance measurement must be performed with the power off. The way that a meter measures resistance is that it actually puts a small current across the probes and measures the resulting voltage. The resistance reading is meaningless if there is already voltage on the thing you’re measuring.
Isolate the thing whose resistance or continuity you want to test. For example, if you’re measuring the resistance between the “+” and “-” terminals on a coil, take all the wires off them first. That way you can be certain that you’re testing the resistance of the coil and not the wires running through the rest of the car that may be connected to other devices and to ground. If you’re verifying continuity between a terminal on a device and ground, it’s good practice to disconnect the wire from the device and connect the multimeter to the disconnected wire. Plus, that way, if the circuit is actually turned on without your realizing it, disconnecting the wire breaks the circuit and makes sure you get a valid resistance reading.
Here are some specific examples. The first is the one we just mentioned: Testing the resistance across an ignition coil. Note that we’ve removed the wires to ensure we’re not getting spurious readings from the rest of the wiring in the car.
The resistance of this ignition coil is 1.3 ohms
Next, we verify that the ground wire to a headlight is actually a valid ground (that it’s really connected to the body of the car, and then from there to the battery) by using the red lead to probe the ground wire on the headlight’s connector, and connecting the black probe to the negative battery terminal. The sub-one-ohm reading—and the beep—indicate continuity to ground.
Verifying continuity to ground
Lastly, we use the meter to check that a switch actually works by verifying that, in the “off” setting, there’s infinite resistance:
And that, in the “on” setting, there’s continuity (less than one ohm resistance, and a beep):
Now, you may find it surprising to learn that just because a meter verifies continuity, that doesn’t mean that the wire or the switch is capable of carrying enough current for the circuit to function. We’ll learn about that next week when we talk about measuring current.
Rob Siegel has been writing the column The Hack Mechanic™ for BMW CCA Roundel Magazine for 30 years. His new book, Ran When Parked: How I Road-Tripped a Decade-Dead BMW 2002tii a Thousand Miles Back Home, and How You Can, Too, is availablehere on Amazon. In addition, he is the author of Memoirs of a Hack Mechanic and The Hack Mechanic™ Guide to European Automotive Electrical Systems. Both are available fromBentley Publishers and Amazon. Or you can order personally inscribed copies through Rob’s website: www.robsiegel.com.
- First insert the black test lead into the COM jack.
- Then insert the red lead into the VΩ jack. ...
- With the circuit de-energized, connect the test leads across the component being tested. ...
- The digital multimeter (DMM) beeps if a complete path (continuity) is detected.
- Measure Volts. Turn the selection knob to either AC or DC volts, depending on what you're testing. ...
- Check Continuity. If your multimeter has a dedicated continuity setting, turn the selection knob to continuity. ...
- Measure Resistance. ...
- Measure Amps.
Multimeters measure resistance by injecting a small current into the circuit, and then measuring the voltage drop across those points in the circuit. The known current, and the resulting voltage drop are then used to calculate the resistance using Ohm's Law, V=I^2*R.What is resistance and continuity? ›
Resistance is the difficulty which an electrical current has in travelling along a wire or other conductor. This may be due to the thinness of the wire or because of the material which makes up the wire. Insulators have an extremely high resistance to electricity. Continuity is the integrity of the current path.What is the first step in using a multimeter? ›
To begin, make sure no current is running through the circuit or component you want to test. Switch it off, unplug it from the wall, and remove any batteries. Plug the black probe into the COM port on your multimeter. Plug the red probe into the VΩmA port.How do you verify that resistance? ›
From this, we conclude that the resistance of a conductor is directly proportional to its length (l) for constant cross-sectional area. Therefore, R∝L. We notice that the resistance was more when the cross - section area was less. Therefore , R∝AL.What are the 3 forms of resistance? ›
There are three types of resistance, Logical/Rational, Psychological/Emotional, and Sociological.How do you use a multimeter to measure resistance in a series or parallel circuit? ›
In a series circuit, each circuit element has the same current. So, to measure current in a circuit, you must attach the multimeter in series. In a parallel circuit, each circuit measurement has the same voltage. So, to measure voltage in a circuit, you must attach your multimeter in parallel.What are the 3 main function of a multimeter? ›
A typical multimeter can measure voltage, resistance, and current, in which case it is also known as a volt-ohm-milliammeter (VOM), as the unit is equipped with voltmeter, ammeter, and ohmmeter functionality. Some feature the measurement of additional properties such as temperature and capacitance.How do you use a resistance meter? ›
Set your multimeter to the highest resistance range available. The resistance function is usually denoted by the unit symbol for resistance: the Greek letter omega (Ω), or sometimes by the word “ohms.” Touch the two test probes of your meter together. When you do, the meter should register 0 ohms of resistance.
For a function to be continuous at a point, it must be defined at that point, its limit must exist at the point, and the value of the function at that point must equal the value of the limit at that point.What is the symbol for continuity? ›
Continuity: Usually denoted by a wave or diode symbol. This simply tests whether or not a circuit is complete by sending a very small amount of current through the circuit and seeing if it makes it out the other end.What is called continuity? ›
: uninterrupted connection, succession, or union. … its disregard of the continuity between means and ends … Sidney Hook. : uninterrupted duration or continuation especially without essential change.What are the 2 basic tools can be used to check continuity in a circuit? ›
Two primary tools include the continuity tester and the multimeter.When using a multimeter what should be done before use? ›
Before you take a measurement with your multimeter, you should visually inspect it first. Check the meter, test probes and accessories over for signs of physical damage. Make sure all plugs fit securely and keep an eye out for exposed metal or any cracks in the casing.What is the first step in measuring resistance? ›
- Turn power to circuit OFF. ...
- Turn digital multimeter dial to resistance, or ohms, which often shares a spot on the dial with one or more other test/measurement modes (continuity, capacitance or diode; see illustration below). ...
- First insert the black test lead into the COM jack.
The most important thing to remember is that the voltmeter should be connected in parallel with the light bulb. That means you can always connect the voltmeter last. You do not have to disconnect any circuit elements in order to properly add a voltmeter to a circuit.What is the one hand rule when using a multimeter? ›
The trick to that answer is you don't hold both leads. Using one hand (place the other behind you or in your pocket, or use it to hold the meter), clip a lead onto the first measurement point (preferably not the one that's hot). Then pick up the other lead and hold it (or clip it) to the other reference point.What is multimeter with diagram? ›
The part of the circuit diagram of Multimeter, which can be used to measure DC voltage is shown in below figure. The above circuit looks like a multi range DC voltmeter. The combination of a resistor in series with PMMC galvanometer is a DC voltmeter. So, it can be used to measure DC voltages up to certain value.What is multimeter example? ›
A multimeter or a multitester, also known as a volt/ohm meter or VOM, is an electronic measuring instrument that combines several measurement functions in one unit. A typical multimeter may include features such as the ability to measure voltage, current and resistance.
The parts of a multitester are a scale, a needle or pointer, an adjustment screw, a zero-ohm selector, a range selector knob, some ports and test probes.What is resistance and how is it measured? ›
Resistance is a measure of the opposition to current flow in an electrical circuit. Resistance is measured in ohms, symbolized by the Greek letter omega (Ω). Ohms are named after Georg Simon Ohm (1784-1854), a German physicist who studied the relationship between voltage, current and resistance.What are the five types of resistance? ›
- Passive change resistance. ...
- Active change resistance. ...
- Attachment change resistance. ...
- Uncertainty change resistance. ...
- Overload change resistance.
Resistance is defined as a refusal to give in or to something that slows down or prevents something. An example of resistance is a child fighting against her kidnapper. An example of resistance is wind against the wings of a plane.What are the four types of resistance? ›
- objects with mass.
- inertia and momentum.
- gravity and air resistance.
Answer: There is 2 combinations are possible. The combination number of two equal resistors is 2. which is equal to 2r.
The formula to calculate the resistance using Ohm's Law is given as follows: R = V I. where, R is the resistance of the resistor R in ohms (Ω) V is the voltage drop in the resistor in volts.What is a resistor symbol? ›
The value of a resistor is measured in ohms and represented by the Greek letter capital omega (Ω). The resistance value is specified in ohms, the standard symbol is “R” or Ω. Resistor values are often stated as “k” (kilo, or times 1,000) or “M“, (meg, or times 1,000,000) for convenience.How do you find current? ›
Ohm's Law denotes the relation between voltage, current, and resistance. You ought to be familiar with the equation V = IR. From the equation, you can easily get the value of the current by dividing the voltage with the resistance or: I = V/R.Is measuring resistance the same as continuity? ›
The two are not the same! An Ohmmeter is used to measure the resistance to electrical flow between two points. The Ohmmeter is most commonly used to check continuity. Continuity is not a “measurement” as much as it is a yes / no statement.
A continuity test is a quick check to see if a circuit is open or closed. Only a closed, complete circuit (one that is switched ON) has continuity. During a continuity test, a digital multimeter sends a small current through the circuit to measure resistance in the circuit.Is checking for resistance and continuity the same? ›
Continuity testing is the act of testing the resistance between two points. If there is very low resistance (less than a few Ωs), the two points are connected electrically, and a tone is emitted. If there is more than a few Ωs of resistance, than the circuit is open, and no tone is emitted.What is the difference between a continuity test and an ohms test? ›
Using a DDM to preform an ohm test will display the resistance value measured between the probes of the meter. A continuity test will cause the meter to beep if this reading is below a certain value indicating a short between the two meter probes.Why do we measure continuity? ›
A continuity test is an important test in determining the damaged components or broken conductors in a circuit. It can also help in determining if the soldering is good, if the resistance is too high for flow of current or if the electrical wire is broken between two points.What is the measure of continuity? ›
A continuity test is like a simplified resistance/ohms measurement. A basic method is to apply a voltage across the resistor and measure the current OR apply a current and measure the voltage. Then through R = V/I you can calculate the resistance.What is a good reading for continuity? ›
Full Continuity - Short Circuit
The meter is displaying 0.2 ohms, the residual resistance of its test leads. For almost every common household purpose, any reading less than 1.0 ohms is sufficiently low to be considered excellent conductivity.
The readout should stay level around one ohm. Two or three ohms is still acceptable, but if one of your cables shows much higher readings than the others (of the same length), you should check that all conductors of that cable really make optimal contact with the terminals in each plug.How many ohms is considered a short? ›
The resistance is usually zero in the event of a short circuit. There are zero ohms in a short circuit. A short circuit is also indicated by the resistance of fewer than two ohms.