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How to Choose an Automotive DMM

An Automotive Digital Miltimeter is an essential tool when troubleshooting vehicle problems.

An Automotive Digital Miltimeter is an essential tool when troubleshooting vehicle problems.

Advances in On-Board Diagnostics (OBD) technology and the capabilities of scan tools in the last few years have gradually reduced the use of the Automotive Digital Multimeter (DMM) in many shops. Yet, the DMM remains a practical, versatile and capable tool often overlooked.

DIYers, specially, shouldn't leave it out of their toolbox.

For one, professional-grade scan tools cost hundreds of dollars. On the other hand, DMMs have greatly improved in quality, accuracy, and number of functions in the last decade and cost much less than a professional scan tool. This means that DIYers can now use a good automotive multimeter — by itself or along with an aftermarket scan tool — for car repairs at home without spending a fortune in high-tech diagnostic tools.

The question now is what type of automotive DMM you should buy? What features and functions you should look for to make the most out of your diagnostic tool?

Often, the answer to this question comes down to what type of auto repairs you want to do and, of course, your car maintenance budget.

You can certainly buy a professional grade, fancy multimeter that gives you the most features possible, if you have the money to spend. Most DIYers, however, can get by very nicely with a multimeter that offers a key number of functions without the need to spend a lot of money.

Use a DMM to check the ignition and many other vehicle systems.

Use a DMM to check the ignition and many other vehicle systems.

Here, we take a look at the most useful features and functions you should be looking for to help you find the best automotive DMM for you.

1. Manual or Auto-ranging

Many automotive DMMs now come with auto-ranging capacity as a standard feature. And for a good reason. You don't have to worry about selecting the range for the electrical value you're trying to measure.

This feature really shines when troubleshooting circuits or components with unknown values, or with values far off their intended range. However, when working with millimetric units, some of these meters may not correctly "feel" the small values and fail to give an accurate reading. Make sure you get a quality multimeter that gives accurate readings even at very low values.

On the other hand, manually selecting the range pretty much eliminates this problem. And quality DMMs with preset ranges cost less. Having said that, some DMMs offer both autoranging and manual presets. So, keep your eyes open.

2. Amperage

Most automotive DMMs will come with a top 10 Amps measurement capacity. But with the introduction of new technology in modern vehicles, it's a good idea to look for a multimeter with up to 15 or 20 Amp in current range.

3. AC-DC Voltage

Besides being able to measure voltages in alternating current (AC) and direct current (DC), look for a meter with a DC volts top range that suits your needs. For example, a multimeter with a capacity of 1000 DC voltages — usually referred to as CAT III rating — is ideal for work on some modern vehicles. Some newer car models, and those with hybrid systems, require hundreds of voltages to operate. Otherwise, an end range of 600 DC Volts gives you a good coverage to work on most vehicles.

Automotive Digital Multimeters

4. Resistance and Car Computer Safety

Verify that the DMM you select comes with a 10 Meg Ohm impedance rating protection. You need this protection to prevent the multimeter from drawing too much current from circuits that can damage the car computer and other delicate electronics as you troubleshoot devices.

5. Diode Tests

This function comes standard in most automotive multimeters. You may need it when checking an alternator rectifier, charging system or some other control system in your vehicle.

6. Dwell and RPMs

You may not have the need for the dwell feature at all during your repair and maintenance tasks, unless you have a 70s or older model. These vehicles incorporated breaker points inside the distributor for the ignition system, and required frequent adjustment with a feeler gauge or dwell meter to prevent missing, starting issues, and other ignition system problems.

Usually, dwell meters would come with a tachometer as well. The dwell meter would measure current flow through the ignition coil while the tachometer or RPM meter would check engine speed as the contact points were adjusted for proper ignition timing.

Some old car models with the breaker point ignition system are still rolling down the road. But perhaps more common is to find the GM feedback carburetor with the mixture control solenoid where a dwell meter will come in handy.

On modern vehicles, an electronic system or the car computer controls dwell, but you'll still find the RPM function useful when diagnosing sensors and other devices that require troubleshooting at designated RPMs.

7. Frequency

Your vehicle uses many electrical switches in a variety of circuits that are constantly being energized and de-energized including solenoids, fuel injectors, relays and other similar type of devices. A frequency function helps check the number of cycles per second of a particular device including barometric pressure and manifold absolute pressure sensors.

Important Automotive DMM Features and Functions

AC-DC Voltage and Current

10 Meg Ohm impedance

Diode

Dwell

RPM

Frequency

Duty cycle

Pulse width

Audible testing tone

Temperature

8. Duty Cycle and Pulse Width

This is a must have feature in an automotive DMM. The duty cycle is an alternative frequency function that let's you test the percentage (ratio) of On time versus Off time per second of a particular component. For example, you may need to test a fuel injector's On-time ratio and compare this to the specification in your repair manual during a troubleshooting procedure.

Additionally, some multimeters also incorporate a Pulse Width function that translates this On-Off cycle as a length of time in milliseconds (ms). With this feature you can test the operating condition of any device in your vehicle that works through On-Off cycles like some solenoids (fuel mixture control solenoid, canister purge solenoid), switches, relays, small motors (idle air control motor), and other similar components.

9. Testing Tone

More than once you'll find yourself deep under the dashboard or with your hands buried inside the engine compartment trying to check continuity on a wire, electrical connector or sensor, and unable to watch your meter. That's why most — if not all — automotive DMMs come with a convenient audible tester that allows you to check for continuity without having to look at the meter's display. Although this is a standard feature on most DMMs now, make sure to look for it when choosing a meter.

10. Temperature

Not a deal breaker, but being able to measure temperature with your meter will save you a lot of time. You will need this feature from time to time when a particular troubleshooting procedure calls for you to check for engine, oil, cooling system, a device, a sensor, or battery temperature. So, if at all possible, choose a DMM with a temperature function as well.

The Temperature function on a DMM saves you time when troubleshooting the cooling system.

The Temperature function on a DMM saves you time when troubleshooting the cooling system.

These are the most useful features and functions that you ought to be looking for when choosing an automotive DMM. You'll find more standard features in many multimeters today like auto shut-off, reverse polarity protection (auto-polarity), and a large display; and other not so common, nice to have, functions that can make your job easier like graphical readouts for changing value trends, minimum and maximum values measured, hold measurement of last value, vacuum, pressure, self-calibration, plus an assortment of accessories to go with the tool.

Your specific budget and the types of car maintenance and repairs you want to do will influence your final decision. However, even if you are operating under a small budget, a careful search may lead you to a decent meter that incorporates most, if not all, features and functions listed above without having to sacrifice quality.

As with many other products on the market, you'll find that one or two particular brands in hand-held DMMs have the best reputation in terms of quality and accuracy, specially within the field of automotive repair. But, unless you intend to use your DMM in a production environment, don't overlook other brands that may give you more for your money and are perfectly suited for the DIYer.

And once you make your purchase, go through the tool's manual carefully so that you know your DMM capabilities. Then, when the time comes, you'll know you have the tool to deal with that electrical circuit or component that might be giving you trouble.

Test your Knowledge of Digital Multimeters

For each question, choose the best answer. The answer key is below.

  1. What do you use an audible testing tone for?
    • Check for unsafe impedance
    • Check for voltage loads
    • Check for continuity
    • Check for AC currents
    • Check for unusual frequencies

Answer Key

  1. Check for continuity

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