Understanding the DVOM's abilities with vehicle electrical systems

Basics help you get the most out of this fundamental tool.

DVOM digital volt-ohmeter digital multimeter electrical electrical systems vehicle electrical systems electrical 101 automotive electrical work basic electricity electricity automotive aftermarket Today's technician needs to know three things to be successful in repairing electrical faults. One, he (and I mean that generically, ladies!) needs to understand the fundamentals of how electrical circuits work (Electrical 101, January 2009 Motor Age). Two, he needs to be competent in reading wiring schematics (The Electrical Roadmap, February 2009 Motor Age). And three, he needs to know how to test these circuits. The third starts with becoming comfortable in using that fundamental electrical testing tool: the Digital Volt-Ohm Meter (DVOM), also referred to as a Digital Multimeter.

DVOM Basics

In the old days, we used analog multimeters for electrical testing. Analog meters are capable of measuring voltage, resistance and amperage, and do so "live." Digital meters are slightly different. They, too, can measure the basics of voltage, resistance and amperage, but they do so by sampling the live readings and then averaging those samples to provide the display you see on the screen.

This is both a curse and a blessing. A curse, in that some readings happen at speeds too fast for the meter to read, and a blessing in that it provides testing abilities the analog meter can't match. DVOMs can measure pulse width signals, signal frequency, engine rpm, duty cycles and more. But I have to be honest with you; I never use the majority of these extra capabilities!

When choosing a DVOM, stick with a name you know. A good meter is an investment, not just a cost of doing business. The sample rates available vary quite a bit, from 40 to more than 400 samples per second. The faster the sample rate, the more capable the tool is of catching those faster signals and providing you with more accurate readings. As a practical example, a meter with a slow sample rate will not be able to measure AC bleed in a charging system, while a higher end meter can.

Make sure the meter you choose has the full range of basic measurements, of course. Voltage will get the most use, followed by amperage and then resistance. Most meters are auto ranging, meaning that the meter will adjust the display to accommodate the measurement being taken. That's OK; just make sure there is a manual ranging feature to go along with it. Your meter of choice should have the ability to record, allowing you to detect the minimum and maximum readings of any measurement taken over time.

There is always some measure of inaccuracy, and generally these inaccuracies won't affect your troubleshooting as long as you are aware of them. Some are inherent to the meter itself, and some inaccuracy is generated by the test leads. Look for the Relative feature in your meter that allows you to compensate for inaccuracy caused by the test leads. If hybrid electrical repair is in your future, make sure the meter and test leads are rated Cat III or better and that the meter is rated at voltage inputs of at least 750 volts.

All meters use a battery, usually 9 volts, to power the meter and provide the current needed for static resistance measurements. Weak batteries can affect meter accuracies before the low battery warning comes on in some meters, so keep the battery fresh. There are fuses to protect the meter from damage should you accidentally connect the meter incorrectly. Replace them with "fast burn" fuses of the same rating. Always make sure you set the meter for the measurement you want to take before you connect it, and that the test leads are in the correct jacks. Treat your meter with care and it will last you a lifetime.

Let's get into a few tips on getting a little more out of the DVOM.

Taking Temperatures

Of the extra features many meters offer, the one I use the most is the temperature function. Using a contact probe, this allows the meter to display the temperature in either Fahrenheit or Celsius. It is great for cooling system troubleshooting.

Insert the probe of the temperature sensor in the radiator fins as close to the inlet side (where coolant from the thermostat enters) of the radiator as possible. Connect your scan tool and monitor the reading from the Engine Coolant Temperature (ECT) sensor, and start the engine. As the engine warms, you should see ECT increasing and the DVOM temperature remaining stable or increasing only slightly. If the DVOM reading increases quickly, the thermostat is not closed and may be sticking or damaged.

Continue to monitor the DVOM reading as the ECT approaches the temperature specification for thermostat opening. As soon as the thermostat opens, the DVOM temperature will climb rapidly, letting you know it did, and the reading on the scan tool should be near that specification. Continue to let the engine run while watching the DVOM reading. On systems with electric cooling fans, you should see the fans come on when the reading approaches 210 to 220 degrees Fahrenheit and shut off when the temperature reading drops roughly 50 degrees Fahrenheit.

A Down and Dirty Charging System Test

This test is ideal in providing a little extra service to your customers. It is a fast, simple test that will reveal any weaknesses in the battery or charging system, and is an easy step to add to any general inspection you may already do for them.

Connect your DVOM test leads to the battery with the engine off. A reading close to 12.6 volts means the battery has sufficient charge to continue. Lower readings indicate a discharged battery that should be recharged and retested, while higher readings are surface charges that can be bled off by turning the headlights on for a minute or two. To continue the test, turn on your meter's Min/Max or Record feature, and then start the car three times in succession, leaving it run for a few minutes after the third start before shutting it off.

The minimum reading recorded by your meter is the result of your load test on the battery. After all, what is the biggest, real, load the battery is going to face? Any reading under 9.50 volts is enough to condemn the battery. For readings above 9.50 volts but below 10.50 volts, follow up with a battery capacitance test before signing off on its condition.

The maximum reading recorded is the charging system voltage output. This reading is normally between 13.50 volts and 14.50 volts, but be careful here. Many late model vehicles are starting to incorporate "on demand" charging system controls that may cause this value to be higher or lower than what you are used to (see February 2009, Motor Age). Before condemning the charging system, advise your customer of your preliminary results, then follow up by researching the operation of that particular system in your service information system.

Amperage Testing

Amperage is the measurement of current flow in a circuit. Most meters are capable of measuring only 10 to 15 amps directly. Trying to measure any more than what the meter is capable of could result in a fried meter, even with the fuses acting as protection.

Current measurements using the DVOM require that the meter be connected "in series" to the circuit that you want to test. That means the circuit has to be opened and one test lead attached to one side of that opening while the other lead is attached to the other side. It helps to think of your meter as being wired in the same way you would an inline fuse.

For that reason, I prefer the use of an accessory tool called an Amp Clamp (see August 2008, Motor Age). This tool provides higher measuring ranges with no risk of damage to the meter and no requirement to open the circuit.

But one tip that I learned from our own Jim Garrido (April 2008, Motor Age) bears repeating here. Using the DVOM amperage function, you can test a variety of solenoids (purge control, injectors, etc.) for intermittent failures. First, check the schematic to see how the solenoid is controlled. Is it sent battery voltage to turn it on or off, or is the ground path completed? Whichever side of the circuit contains the control is the side of the circuit you will connect your meter to.

The idea here is to use the meter as the control device, either sending power to a solenoid that is always grounded, or supplying ground to one that always receives power. The advantage in doing so is the current measurement you get at the same time. Using Ohm's Law, you can tell if the resistance of the solenoid is within specification and can test the solenoid for actual operation. And because most solenoids fail only when heated, you can repeat the test several times quickly, looking for changes in current flow that mean changes in internal resistance.

Voltage Drop Testing

When taking any voltage measurement, there is one thing to keep in mind. This reading is a measurement of the voltage potential between the two test leads, and your meter is doing the math for you. Don't believe me? Just turn the leads around and see what happens to your reading.

Voltage is the force used to overcome resistance in an electrical circuit, and that fact makes measuring the voltage on a live circuit much more accurate in locating circuit faults than taking static resistance measurements. The reason is all voltage will be used to overcome the resistance in a circuit. If there is more than one resistance, the available voltage will be split proportionally among them.

The primary resistance in any circuit is the load, or device that actually performs the work in that circuit. A light bulb is a good example. For the load to work properly, the current flowing through it must be correct. And that current flow will only be correct when full system voltage is reaching the load, and it has a solid path back to the ground side of the source, the battery.

This is a powerful testing technique, but unfortunately that is a topic for a future issue of Motor Age.

Pete Meier is an ASE CMAT, member of iATN, and full-time tech in Tampa, Fla. His experience reaches back more than 30 years, and his contributions to Motor Age reflect a wide variety of experience with almost every make and model. You can contact Meier directly at www.autoservicetech.com.