How To Use a Multimeter

How To Use a Multimeter

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Easily one of the most common questions we receive here at Pro Street is from customers or users wanting to know how to use a multimeter to test their vehicle’s components or sensors. After all, the multimeter or voltmeter is a mechanic’s best friend when it comes to diagnosing problems, installing parts or troubleshooting OBDII trouble codes. Today we’ll be taking a look at the Multimeter, it’s many functions and what you need to know on how to use a multimeter.

What’s the difference between analog and digital meters?


There’s two different types of readout voltage meters that you can use when it comes to troubleshooting or testing your sensors. Regardless of problem or issues, the diagnosis of your vehicle’s electrical system will lead you to using a voltmeter, ammeter, or ohmmeter to help pinpoint the exact location of the problem. As previously stated there’s two types of meters you can use—analog and digital. Analog meters use a needle and calibrated scale to indicate values. Digital meters display those values on a digital display.

voltmeter

Our How to Use a Multimeter Guide will show you how to use these meters, as well as understand how these meters work. We’ll also be covering the the advantages and disadvantages of each type of meter.

Before you use any kind of meter, digital or analog on your vehicle, it’s a good idea to read the manufacturer’s operating instructions. Because most analog meters require you to do some math internally, it’s a good idea to have a firm grasp on how your analog meter works. For instance, a meter might have three voltage ranges: 4.0 V, 20 V and 40 V, but only two scales: 4.0 V and 20 V. In order to use the 40 V range, you need to multiply the needle reading on the 4.0 V scale by 10 (or for that matter, the 20 V scale by 2).

Multimeter
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Digital meters are easier to use and read because most will automatically make the adjustments for you depending on where you have the switch or mode set to. These auto-ranging style meters are great for the average everyday shadetree mechanic. Most digital meters require the operator to select the proper range. If you are going to learn how to use a multimeter, it’s a good idea to understand the units of measure dictated by the multimeter in question.

The electrical units of measure symbols are:

  1. M for mega or million
  2. K for kilo or thousand
  3. m for milli or one-thousandth
  4. u for micro or one-millionth

What’s the difference between a digital Multimeter and an Analog one?


 

There’s three different types of meters that you can use to test circuits and electrical connections in your vehicle. There’s voltmeters, ammeters and ohmmeters and all of these testers or multimeters can connect to the circuits and reads them in different ways.

how-to-test-lm7-maf41

All three types of multimeter have different functions and purposes, let’s jump into all three and explain what they do.

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What’s a Voltmeter?


 

Voltmeters measure voltage or voltage drop in a circuit when connected to it. Voltage drop can be used to locate excessive resistance in the circuit which could cause poor performance or improper operation. Lack of voltage at a given point may indicate an open circuit or ground, or even a break in the wire or harness.

Because voltmeters must be connected in parallel with the device or circuit so that the meter can tap off a small amount of current, they aren’t always ideal for testing. When testing your automotive circuit, it’s always a good idea to use the positive or red lead to the circuit closest to the positive side of the battery. While the negative or black lead is connected to ground or the negative side of the circuit.

If a voltmeter is connected in series, its high resistance would reduce circuit current and cause a false reading. Because voltmeters are always hooked to a circuit in parallel, they become part of the circuit and reduce the total resistance of the circuit. This is important to remember depending on the kind of testing you will be performing to your circuit. Make sure to read the manufacturer’s specifications in their manual to determine if your voltmeter has a resistance that is too low for your circuit.

volt

False readings can result from a voltmeter that isn’t properly matched to the resistance in your circuit. The false reading can occur when the meter changes the circuit by lowering the resistance, which increases the current flow in the circuit. The effect a voltmeter has on the circuit to which it is attached is sometimes referred to as “loading effect” of the meter. The loading effect a voltmeter has on a circuit is determined by the total resistance of the circuit in relation to the impedance of the voltmeter

What is Impedance and why is it important?


Every voltmeter has an impedance, which is the meter’s internal resistance. The impedance of a conventional analog voltmeter is expressed in “ohms per volt.” The amount of resistance an analog voltmeter represents to the circuit changes in relation to the scale on which it is placed.

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Digital voltmeters, on the other hand, have a fixed impedance which does not change from scale to scale and is usually 10 M ohms or more. Impedance is the biggest difference between analog and digital voltmeters. Since most digital voltmeters have 50 times more impedance than analog voltmeters, digital meters are more accurate when measuring voltage in high resistance circuits.

What’s a Ammeter?


 

Ammeters measure amperage, or current flow, in a circuit and provide information on current draw as well as circuit continuity. High current flow indicates a short circuit, unintentional ground or a defective component. Some type of defect has lowered the circuit resistance. Low current flow may indicate high resistance or a poor connection in the circuit or a discharged battery. No current indicates an open circuit or loss of power to the circuit or the sensor you are testing.

Ammeters can be tricky to use because they must always be connected in series with the circuit, never in parallel. That means that the sensor circuit current must travel through your meter in order to be read. This kind of test is most common when you have to test a pin of a sensor, like the TPS or MAF circuit.

how-to-test-mustang-maf
How To Test a Mustang MAF

The Ammeter is another multimeter that would serve you well to read the manufacturer instructions. Knowing what the maximum rating of your ammeter and how much internal resistance in present in the ammeter will ensure that you don’t overload or blow the meter and / or damage the sensor or circuit you are working on.

When it comes to the digital vs the analog debate on ammeters, there’s not much reason or difference between the two. Digital meters are often capable of measuring smaller currents, all the way down to microamps and often times have failsafe mechanisms built into them. Like most digital meters, they are easier to use because they give a specific value, eliminating the need to interpret the analog meter’s needle on its scale.

What’s a Ohmmeter?


An ohmmeter is powered by an internal battery that applies a small voltage to a circuit or component and measures how much current flows through the circuit or component. It then displays the result as resistance. Ohmmeters are used for checking continuity and for measuring the resistance of components.

Zero resistance indicates a short while infinite resistance indicates an open in a circuit or device. A reading higher than the specification indicates a faulty component or a high resistance problem such as burnt contacts, corroded terminals or loose connections. Ohmmeters, because they are self- powered, must never be connected to a powered circuit as this may blow a fuse in the meter and damage its battery.

Digital Ohmmeters are more convenient and do not require calibration like their analog cousins do. Because of the internal battery, analog ohmmeters must be calibrated frequently because of it’s constantly changing state of charge for the internal battery.

These kinds of meters are not good for testing low resistance, high current components like the starter or alternator. Because they only use a small amount of power from the internal batteries, locating points of high resistance is impossible using a Ohmmeter.

That does it for our How To use a Multimeter primer, for more information, DIY and How To’s subscribe to My Pro Street today.

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