A number of tools make diagnostics easier. They allow you to measure electrical activity on circuits and components that would otherwise be undetectable.
Digital Volt-Ohmmeter (DVOM)
A 10-megaohm digital volt-ohmmeter (DVOM) or multimeter, such as essential tool J-39200 (Fluke 87) or equivalent, is useful for testing different aspects of circuit operation. The Fluke 87 Digital Multimeter (figure 6-2) is an example of a digital multimeter.
The digital multimeter has an electronic digital readout of the value measurement being made. This type of meter has electronic circuitry for precise measurements. It is accurate to within .1 percent.
A multimeter with at least 10 megaohms input impedance is needed for use on GM vehicles. This input impedance applies to the meter only when it is used on the voltage scale. This means that the meter resists loading down the circuit being measured with a resistance of 10 million ohms. This high resistance permits measurement of very sensitive circuits without damaging or altering them.
The DVOM can function as a voltmeter to measure the magnitude of voltage. When used as an ammeter, the DVOM accurately measures extremely low current flow. When used as an ohmmeter, the DVOM measures the resistance of a circuit in ohms. These uses are described on the following pages.
FLUKE 87 USAGE
The following terms and symbols appear on the face of the Fluke 87 Digital Mutlimeter and are used in its operation. Their meanings are explained.
MEASURING VOLTAGE
Measuring voltage is usually the first step in testing a circuit. To test for voltage, the circuit being tested must be HOT, or receiving power. Polarity must also be observed.
The negative lead of the multimeter is attached to the negative side of the circuit; the positive lead of the meter is connected to the positive side of the circuit.
Use the following procedure (figure 6-3) to test for voltage:
1. Select Volts DC or 300 mV from the selector.
2. Plug the black test probe into the “Com” input jack of the meter. Plug the red test probe into the “V” input jack.
3. Touch the probe tips to the circuit across a load or power source, as shown, in parallel to the circuit.
4. View the reading. Be sure to note the unit of measurement.
MEASURING RESISTANCE/CONTINUITY
The multimeter measures resistance by applying a test current to a circuit and measuring the voltage drop across the circuit. Resistance is measured in ohms (Q). Resistance measurements must be made with the circuit power OFF to prevent damage to the meter and/or circuit.
The multimeter also allows continuity checking. Continuity distinguishes between an open and closed circuit. Continuity tests determine good or blown fuses, open or shorted conductors, switch operation, and circuit paths.
To perform a resistance or continuity test (figure 6-4):
1. Turn power to the circuit OFF.
2. Select resistance (Q) from the multimeter selector.
3. Plug the black test probe of the meter into the “Com” input jack. Plug the red test probe into the “Q” jack.
4. Connect the probe tips across the compo nent or portion of the circuit for which you want to determine resistance.
5. View the reading. Be sure to note the unit of measurement— ohms (Q), kilohms (kQ), or megaohms (mQ).
MEASURING CURRENT
Current measurements are different from the other mea surements made with a DVOM. Current measurements are made in series, unlike voltage or resistance measure ments, which are made in parallel. The entire circuit being measured flows through the meter. Also, the test leads must be plugged into a different set of input jacks on the meter.
To perform a current test (figure 6-5):
1. Turn power to the circuit OFF.
2. Cut or unsolder the circuit, creating a place where the meter probes can be inserted.
3. Select amps DC.
4. Plug the black test probe into the “Com” input jack. Plug the red test probe into the 10 amp or 300 milliamp input jack, depending on the expected value of the reading.
5. Connect the probe tips to the circuit across the break, as shown, so that all current will flow through the meter. This creates a series connection.
6. Turn the circuit power ON.
7. View the reading. Be sure to note the unit of measurement. If the test leads are reversed, a negative (-) sign will show in the display.
