Here are the most common measurements made by mechanics who are troubleshooting, ranked in order of how frequently they're used:
Why this order?
Voltage will be the #1 measurement because it requires no alteration to the circuit before the measurement is made, and does not affect the circuit operation during the measurement. It's safe to say that approximately 75% of all electrical faults can be isolated through voltage measurements.
A resistance measurement is NEVER made in a live circuit; an ohmmeter is a source of current and acts much like a short circuit.
A current measurement is seldom made because the ammeter must be connected in series with the device being tested. This requires wires to be disconnected or cut.
For your own safety, and for the safety and accuracy of the device, here are a few rules to remember before you use a DMM:
A voltage measurement is made by placing the test leads in parallel with the device or source under test.
Measuring voltage across any electrical or electronic component or device is also referred to as measuring the voltage drop.
Make sure that you dial the mode selector to the proper position to measure volts AC or volts DC.
Be sure to check that your voltmeter is operational before testing a live circuit. This can be done by measuring a known voltage.
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