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Digital Multimeters


Digital Multimeters

   This week I'd like to talk about using a Digital Multimeter (DMM). With engine management systems being electronic it is one of the most powerful and versatile tools at your command. With a good wiring diagram and a good multimeter, you can find the cause of almost any problem be it electrical or electronic.

   There are two basic types of multimeters, digital and analog. Analog multimeters have a needle and the digital has an LCD or a LED display. With today's vehicles it makes more sense to have a DMM but an analog multimeter still has its uses.

   In the old days a meter was used mostly to check batteries and dwell angles. A mechanic might have gone days without taking his multimeter out. Today that has changed. Today a good multimeter is an essential part of a mechanics toolbox. Today there's hardly a day goes by where I don't use my meter at least once.

   A DMM will have many functions built into it, and since there are many brands and types of DMMs on the market, I will only go into the basics that they all have in common. As with any tool or piece of equipment make sure you read and follow the instructions and cautions that come with it. This will protect you and your equipment.

   All DMMs will test for voltage, current and resistance. These are the three functions you will use most when trying to diagnose a problem. When you purchase a DMM, one of the most important things to look at is the meter's impedance, which is the meter's operating resistance. Most DMMs have very high impedance. Since the meter is part of the circuit being tested, its resistance will affect the current flow through that circuit.

Typical Amperage Test

   If a DMM has a very high impedance or resistance it will cause a slight increase in the circuit's current. This becomes a concern when you test electronic systems because the increased current draw can damage the components being tested or, at the very least, alter the readings or change a sensor signal. You want to get a meter that has an impedance of at least 10 megaohms. That is to say their current draw is so low it becomes invisible.

   Before you use your DMM to perform a test, you need to know what you are testing and what kind of results you are looking for. If you are looking for volts, you will need to select the proper range for the test. If you are looking for a 12-volt result, select a meter range higher than 12 volts. For example, a 0 to 25 volt range would be best. A range of 0 to 500 volts will not yield an accurate result.

   Almost all DMMs have an "auto-range" features that will automatically select the proper range. Some DMMs will let you override this feature and let you manually select the range you want. Some DMMs do not have this option and must be set manually. Check the documentation that came with your DMM and make sure you know and understand the different ranges it is capable of.

   Most DMMs that have an auto-range will have the setting either before or after the reading. Ohms are measured in multiples of ten and given the designation 'K' or 'M' with 'K' standing for 1,000 ohms and 'M' standing for 100,000,000 ohms. Amps would be displayed as mA, milliamps or 1/1000 of an amp or A for full amps. Volts will also be displayed as mV or volts. When you take a reading with a DMM that has auto-range, be sure you note at what range the meter is on. You could mistake 10 mA as 10 amps.

Typical Voltage Test

   Most DMMs that have auto-range will show the reading with a decimal point. A reading of 1.2 amps will be 12 amps if you ignore the decimal point.

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