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All Info About Auto Repair: Digital Multimeters
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Digital Multimeters

A Digital Multimeter is the best weapon you have in fighting those electrical and electronic gremlins that sneak into your vehicle.
» Part 1: DMMs
» Part 2: Using a DMM
» Part 3: Volts & Ohms
» Part 4: Wire Checking

An ohmmeter uses its own battery to conduct a resistance test. Therefore there must be no power in the circuit being tested or the ohmmeter will become damaged.

When you test a component you put the red lead on the positive side and the black lead on the negative side. Current from the battery will flow through the component and the meter will determine the resistance by how much the voltage drops. If the component has an open the meter will flash "1.000" or "OL" to show an open or infinite resistance. A reading of 0 ohms indicates that there is no resistance in the component and it is shorted. If a component is supposed to have 1,000 ohms of resistance and a test shows it has 100 ohms of resistance, which indicates a short. If it reads infinite, then it is open.

Analog ohmmeters will need to be calibrated before they are used. There is an "ohms adjust" screw on the meter used to do the calibration. To calibrate the ohmmeter, you touch the red and black leads together and turn the adjusting screw until the needle is at 0. You should do this each time you use the ohmmeter and each time you change scales. DMMs do not need to be calibrated since they will self calibrate themselves. Holding the two leads together will confirm that they are, indeed, calibrated.

To check a wire in a harness you connect one lead at one end of the wire and the other lead to the other end of the wire. If the wire is good you will get a reading. If it is broken, you will get an infinite reading. This is useful in determining why a particular component is not getting power. Just be sure you isolate the wire from the circuit so your ohmmeter does not get damaged.

These are the three basic functions of all DMMs. Some DMMs will have many other features such as averaging where it will take a reading over a period of time and average it out. Some have a MIN/MAX feature that will hold the highest/lowest reading. Some will do specific diode tests, measure injector pulse times and even have thermometers. Meters like this can be many hundreds, or even thousands, of dollars.

Whatever kind of meter you have it can be the most useful tool you have when it comes to tracking down an electrical or electronic problem. Make sure you know what your meter can and cannot do. And don't push it past its limits. With a good wiring diagram, a decent DMM and a little common sense there won't be any problem you can't track down.

» Part 1       » Part 2       » Part 3       » Part 4
Additional Information provided courtesy of
ALLDATAdiy.com and Warranty Direct
© 2000-2007 Vincent T. Ciulla

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