Digital Multimeters (Part 1)

Objectives

  • Inspect and set up a digital multimeter (DMM)
  • Perform common circuit measurements
  • Demonstrate the correct procedure to test solenoids

Common Measurements

Here are the most common measurements made by mechanics who are troubleshooting, ranked in order of how frequently they're used:

  1. Voltage
  2. Resistance
  3. Current

Why this order?

Voltage Measurement

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.

Resistance Measurement

A resistance measurement is NEVER made in a live circuit; an ohmmeter is a source of voltage and acts much like an additional power supply.

The device being tested must be isolated as the current from the ohmmeter will find all possible paths.

Current Measurement

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.

Be extremely cautious when using an ammeter to measure current because the ammeter acts as a short circuit!

DMM Inspection and Initial Testing

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:

  • Inspect the case before using the meter. Look for cracks or missing plastic. Do not use the meter if it is damaged.
  • Pay particular attention to the test leads. Look for damaged insulation or exposed metal.
  • Check the condition of the battery and fuses, if applicable.
  • The test leads can add error to resistance measurements. To test the leads, touch the probe tips together and read the resistance of the leads. Remember to factor this resistance into your measurements.
  • You can use the relative (REL) mode to automatically subtract resistance of the test leads.
  • Make sure that the measurements you are planning are within the operational range of that DMM.

Voltage Measurement

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.

A perfect voltmeter has an infinite resistance. This is only hypothetical, of course - in the real world a functioning voltmeter will offer a very high resistance, but it can't be infinite. For example, when you are using a Fluke 87V DMM, the internal resistance (or input impedance) will be 10 MΩ.

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