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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.
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» Part 1: DMMs
» Part 2: Using a DMM
» Part 3: Volts & Ohms
» Part 4: Wire Checking
 

DMMs do have a limit on how much current they can measure. Usually this limit is printed at the point where the red lead plugs into the meter. If it says, "10 Amps Max" then there is a 10-amp fuse inside the meter that will blow if the current is above 10 amps. If you take out the 10-amp fuse and put in a 20-amp fuse, you will burn out the meter beyond repair. I would suggest buying a DMM that will handle at least 20 amps for automotive testing.

Some DMMs have an inductive pickup that clamps around the wire being tested. These ammeters measure amperage based on the magnetic field created by the current flowing through the wire. DMMs that have an inductive pickup usually will read higher current and have a higher limit. Since this type of meter doesn't become part of the circuit you do not need to disconnect any wires to get a reading.

Voltmeters are usually connected across a circuit. You can perform two types of tests with a voltmeter. If you connect it from the positive terminal of a component to ground, you will read the amount of voltage there is to operate the component. It will usually read 0 volts or full voltage. If you test a component that is supposed to have 12 volts, but there is 0 volts, there is an open in the circuit. This is where you will have to trace back until you locate the open.

The other test is a voltage drop test. With a light bulb circuit, if a voltmeter reading indicates full voltage at the light sockets but a light doesn't work, the problem could be the bulb, the socket or the ground connection. If you install a bulb that you know is good, the problem now becomes the socket or ground. Without disconnecting anything, connect the voltmeter to the ground terminal of the socket and a good chassis ground. If the socket is bad, the meter will read zero volts. If the socket is good and the ground connection is dirty or bad, the meter will read very close to battery voltage. Any voltage reading at all indicates a poor ground circuit.

Another useful function of the DMM is the ohmmeter. An ohmmeter measures the electrical resistance of a circuit. If you have no resistance in a circuit, the ohmmeter will read 0. If you have an open in a circuit, it will read infinite.

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» 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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