What is a DMM Anyway?
Analog meters, the ones with needles that swing from one side of the meter to the other, have been around for years. They work well for most measurements but prove delicate in the shop and have several limitations. Digital equipment, on the other hand, with numbers that flash on a display, is designed - using the latest technology - for delicate components found in automotive computerized systems. This new technology allows the meter to make accurate measurements on a circuit without affecting the circuit or any of the readings. Analog meters would rob power from the circuit and actually change the results of the test, and could cause excessive current flow through delicate circuits. Digital Multimeters (DMMs) provide the latest in measurement technology for all of your electrical testing needs, and at 10 Meg ohm input impedance the circuit hardly knows they are there.

But what should we look for in a meter? After all, there are quite a few on the market, with a lot of bells and whistles.

Normal Functions of a DMM
DMMs integrate a wide range of test functions in a single package. Common measurements such as volts, the electrical pressure in a circuit, are found on most meters. A DC volt selection is the main test function used around a vehicle. DC stands for direct current, flowing in one direction only. Whatever leaves the battery will also return. AC stands for alternating current and is primarily found around the shop or in the home. However, alternators produce AC voltage and current, both of which are rectified using a set of diodes. There is normally a small amount of leakage in diodes, but if this leakage is great enough the battery could discharge and the electrical system could be affected.

To test for excessive ripple voltage, connect the red lead from the meter to the BAT terminal on the back of the alternator and ground the black lead. Run the vehicle at approximately 2500 rpm (with electrical loads applied) and measure the amount of leakage. Any amount over 500mV AC could be cause for concern. By the way, do not connect the leads to the battery, as the battery acts like a capacitor and could absorb the AC, which could lower your reading.

Most accessories designed for use with multimeters convert measurements into a millivolt output. For instance, when using a temperature adapter, the output is 1mV for every degree of temperature measured. If measuring current with a clamp, depending on the scaling factor, each amp equals 1mV. For these reasons, most DMMs have a millivolt setting on the rotary selection knob.

Continuity, used to determine if a circuit is open or shorted, and ohms tests are common on most DMMs.

When buying a meter, make sure to find out what accuracy level and measurement ranges the meter is capable of, and - in addition - determine if the meter is manual or auto ranging. Auto Ranging will automatically determine the best range for the measurement and adjust the meter accordingly. Some meters use an integrated chip to ensure compatibility and accuracy of all functions. Remember, you are not just buying an instrument; you are buying years of research, experience and product knowledge from that company too.

Advanced Functions of a DMM

•  Data Recording Functions
What is your time worth, anyway? Good question. Some circuits on the vehicle are designed to turn on and off over time. If these circuits do not turn off when they should, excessive current drain could cause the battery to run down. Min/Max is a data recorder designed to record when a reading has reached a minimum or maximum level. This function will also indicate what the average measurement is, enabling a fast look at oxygen sensor trends. Min/Max operates at different speeds, depending on the equipment used. The Fluke 88, for instance, will record signals at a speed of 100 milliseconds. The Fluke 87 Series III will record signals at a speed of 250 microseconds.

So what's the difference between a millisecond, microsecond or nanosecond? In one word: speed. The millisecond is equal to 1x10-3, or .001, and represents mechanical switching speeds. Injectors and relays fall into this category. The microsecond is equal to 1x10-6, or .000001, and represents electrical switching speeds. Transistors inside the PCM operate in the microsecond time domain. The nanosecond is equal to 1x10-9, or .000000001, and represents electrical noise. If you use unshielded test leads and wiring, electrical noise from discharging coils can cause malfunctions.

Min/Max is like having another set of hands. Set the meter to make the measurement (current, voltage, temperature, resistance - it doesn't matter), and then walk away. After returning to the meter, simply replay the results. If there were a current drain causing the battery to run down it would show up on the display.

•  Touch Hold Feature
How many times have you tried to make connections in awkward locations, making operation of the meter and holding the test leads at the same time next to impossible? If this is a frequent problem, you may want to look for a Touch Hold function that can be entered into at any time and will freeze the results on the display. For example, to operate Fluke's Touch Hold feature, rotate the rotary knob to the desired measurement and press the Touch Hold button. Set the meter aside and touch the test leads to the connection points, and then remove the test leads. The display will capture the reading.

•  Relative or Zero Modes
Some components found on today's cars have extremely low resistance. If the measurement is made without subtracting test lead resistance the component could accidentally be deemed defective. Relative or Zero will automatically subtract one reading from another, and display the result on the instrument. Set the rotary knob to measure resistance and short the test leads together. Press the Relative or Zero button and the display will indicate zero (the actual reading will show up on the analog bar graph). Now the actual resistance of any item under test will be displayed.

•  Frequency
Frequency is the number of cycles that take place per second. The more cycles that take place in one second, the higher the frequency reading. Frequencies are measured in Hertz, which is the number of cycles per second. There are a variety of sensors on the vehicle that create either an AC or DC frequency.

The AC frequency is typically created from a magnetic-type sensor, one with two wires. It does not require any outside power to operate and will increase signal strength, depending on the speed of the trigger wheel cutting through a magnetic field. Air gap will also affect the output signal strength; the greater the gap, the lower the output voltage.

While most service manuals suggest testing the AC voltage signal on ABS wheel sensors, there is no specific relationship between the voltage level and wheel speed. Air gap, wheel speed and magnet strength all can affect the voltage level.

That's why the ABS computer doesn't actually read the voltage level; it reads the AC signal frequency. The frequency is directly related to the wheel speed, and increases as the wheel speed increases. The ABS computer compares the frequencies from the wheel sensors, and uses them to maintain the wheel speeds while braking.

DC frequency is created from a pulsed DC signal. There is a number of DC frequency producing sensors in cars today; from Hall Effect sensors to Karman Vortex airflow meters. These sensors vary their signal frequency: Voltage and dwell readings remain constant, for the most part.

•  Duty Cycle
Duty cycle is a measurement comparing the signal on time to the length of one complete cycle. As on time increases, off time decreases. Duty cycle is measured in percentage of on time: A 60 percent duty cycle is a signal that is on 60 percent of the time, and off 40 percent of the time. Another way to measure duty cycle is dwell, which is measured in degrees instead of percent.

• Feedback Carburetor

•  Pulse Width
Pulse width is the actual on time of a signal, measured in milliseconds. Fuel injectors and EGR systems use pulse width modulation (a constantly changing pulse width) to vary on time. With pulse width measurements, off time doesn't really matter - the only real concern is how long the signal's on.

• Fuel Injector Measurement

•  Temperature
Some DMMs have built-in temperature measurement capabilities. For the ones without this feature built in, you can get a separate temperature module that converts a temperature measurement into a millivolt output. This voltage can be measured from most pieces of handheld equipment.

Temperature is a great aid in diagnostics.

• Exhaust runners can be measured and compared to find a cool running cylinder, pointing to an ignition, fuel or compression problem.

• If the brakes are pulling from one side to the next, use a temperature probe to determine if a sticking caliper is a problem.

• Wondering if a bearing or u-joint is going south? Use an infrared probe to gain non-contact temperature information. A dry joint or bearing will typically run a little hotter than others; compare your readings.

• To find out if a heater core is working well, compare the inlet to outlet temperature. Even a restricted radiator will show up very quickly.

•  Current
Many DMMs have the ability to measure current directly, but only up to 10 amps (20 amps for 30 seconds). This portion of the meter is usually fused to protect against excessive current loads. If high current measurements are required, such as starting or charging circuit tests, current clamps are available. Similar in operation to the temperature modules, current clamps use Hall Effect technology to convert the current measurement into a voltage read by the meter.

Current clamps are designed to operate within a certain “range of operation.” Low current clamps have narrow jaws and great electrical shielding to ensure accuracy at the low measurement range. Other clamps have the ability to measure as high as 1000 amps, but may only measure down to 2 or 3 amps.

Before determining what type of clamp you should buy, think about your application. Do you work with low current circuits and perform a lot of diagnostics? If the answer is yes, then buy one that will measure from 50 milliamps up to 100 amps, with 10mA resolution. If you need a clamp to measure starting currents, look for a clamp with a higher upper range. You may find that more than one clamp is required to fit all of your measurement needs.

It's important to understand that not all clamps are the same. Don't buy one strictly on price. The speed of signals is increasing every day. Consider that most crankshaft position sensors are designed to produce a single TDC signal every revolution. If the engine were running at 3000 rpm, this sensor would produce a 50 Hz signal (cycles per second). (To find the cycles or revolutions per second, divide the cycles per minute by 60). What if your sensor generated 360 pulses for every crankshaft revolution? At 3000 rpm, this sensor would produce one million eighty-thousand pulses per minute, or an 18 kHz signal (18,000 cycles per second).

The range of frequencies over which AC voltage and current measurements can be made within the specified accuracy is the frequency response. If you are measuring a signal above or below the meter's or current clamp's specified bandwidth, the meter may not produce accurate measurements.

This is especially important if you're using a low current clamp to look at the brush and armature signatures of an in-tank electric fuel pump. Also, in such applications as measuring the output of audio amplifiers, audio signals typically run as high as 20 kHz (20 thousand cycles per second!). High-performance DMMs typically have bandwidth specs of 20 Hz to 20 kHz, some as high as 100 kHz.

Conclusions
Bottom line when looking for a meter: Raise the hood and take a look inside. Talk to your fellow technicians and find out what works for them. You will probably use the meter for the next several years, so purchase one with more power than you think you need. You will grow into the capabilities as time goes on, and you will be glad you did. Look for a product with the best warranty and service. If something does happen to your meter, you don't want to be without it for very long.

Corey Glassman is the automotive program manager for Fluke Corporation. Glassman is past president of the Automotive Training Managers Council (ATMC); ASE Triple Master certified; a member of the Society of Automotive Engineers (SAE); and co-author of Advanced Engine Performance Diagnosis, published by Prentice Hall. He has wrenched for a living and taught at Denver Institute of Technology in Colorado.

RATE THIS ARTICLE What do you think of this article? Your input will help AutoInc. develop additional articles on this subject. Share your thoughts! Your name Your e-mail address

MOST ACCESSED ARTICLES MOST E-MAILED ARTICLES