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OBJECTIVES After studying Chapter 8, the reader should be able to:
Prepare for ASE Electrical/Electronic Systems (A6) certification test content area “A” (General Electrical/Electronic System Diagnosis). Explain how to set a digital meter to read voltage, resistance, and current. Explain meter terms and ratings. Interpret meter readings and compare to factory specifications. Discuss how to properly and safely use meters.

TEST LIGHTS A test light is simply a light bulb with a probe and a ground wire attached. FIGURE 8-1 A 12-volt test light is attached to a good ground while probing for power.

TEST LIGHTS FIGURE 8-2 A test light can be used to locate an open in a circuit. Note that the test light is grounded at a different location than the circuit itself.

TEST LIGHTS Continuity Test Lights
A continuity light illuminates whenever it is connected to both ends of a wire that has continuity or is not broken. FIGURE 8-3 Self-powered test lights contain a battery, clamp, and probe. This type of test light should not be used on any computer-controlled circuits because the applied voltage can damage delicate electronic components or circuits.

TEST LIGHTS LED Test Lights
An LED test light uses an LED instead of a standard automotive bulb for a visual indication of voltage. FIGURE 8-4 High-impedance test light. An LED test light can be easily made using low-cost components and an old ink pen. With the 470 ohm resistor in series with the LED, this tester only draws ampere (25 milliamperes) from the circuit being tested. This low current draw helps assure the technician that the circuit or component being tested will not be damaged by excessive current flow.

TEST LIGHTS Logic Probe
A logic probe is an electronic device that lights up a red (usually) LED if the probe is touched to battery voltage. FIGURE 8-5 A logic probe connected to the vehicle battery and relay used to check for power, ground, or a pulse.

DIGITAL MULTIMETERS Digital multimeter (DMM) and digital volt-ohmmilliammeter (DVOM) are terms commonly used for electronic high-impedance test meters.

DIGITAL MULTIMETERS FIGURE 8-6 Typical digital multimeter. The black meter lead always is placed in the COM terminal. Except when measuring the current in amperes, the red meter test lead remains in the V terminal.

DIGITAL MULTIMETERS FIGURE 8-7 Common abbreviations used on the display face of many digital multimeters. (Courtesy of Fluke Corporation)

DIGITAL MULTIMETERS FIGURE 8-8 A summary chart indicating what measurement type may be used to test which vehicle system. (Courtesy of Fluke Corporation)

DIGITAL MULTIMETERS Measuring Voltage
A voltmeter measures the pressure or potential of electricity in units of volts. A voltmeter is connected to a circuit in parallel.

DIGITAL MULTIMETERS Measuring Voltage
FIGURE 8-9 A typical autoranging digital multimeter automatically selects the proper scale to read the voltage being tested. The scale selected is usually displayed on the meter face. (a) Note that the display indicates “4,” meaning that this range can read up to 4 volts. (b) The range is now set to the 40 volt scale, meaning that the meter can read up to 40 volts on the scale. Any reading above this level will cause the meter to reset to a higher scale. If not set on autoranging, the meter display would indicate OL if a reading exceeds the limit of the scale selected. (Courtesy of Fluke Corporation)

DIGITAL MULTIMETERS Measuring Voltage
FIGURE 8-10 Typical digital multimeter (DMM) set to read DC volts.

DIGITAL MULTIMETERS Measuring Resistance
An ohmmeter measures the resistance in ohms of a component or circuit section when no current is flowing through the circuit.

DIGITAL MULTIMETERS Measuring Resistance
FIGURE 8-12 Typical digital multimeter showing OL (over limit) on the readout with the ohms () unit selected. This usually means that the unit being measured is open (infinity resistance) and has no continuity. FIGURE 8-11 Using a digital multimeter set to read ohms () to test this light bulb. The meter reads the resistance of the filament.

DIGITAL MULTIMETERS Measuring Resistance

DIGITAL MULTIMETERS Measuring Resistance
FIGURE 8-13 Many digital multimeters can have the display indicate zero to compensate for test lead resistance.(1) Connect leads in the Vand COM meter terminals.(2) Select the scale.(3) Touch the two meter leads together.(4) Push the “zero” or “relative” button on the meter.(5) The meter display will now indicate zero ohms of resistance. (Courtesy of Fluke Corporation)

DIGITAL MULTIMETERS Measuring Amperes
An ammeter measures the flow of current through a complete circuit in units of amperes.

DIGITAL MULTIMETERS Measuring Amperes
FIGURE 8-14 In this digital multimeter set to read DC amperes, note that the red lead is placed in the far left-hand socket of the meter. The meter is displaying the current flow (4.18A) through the electric fuel pump on this General Motors 3800 V6 engine.

DIGITAL MULTIMETERS Measuring Amperes
FIGURE 8-15 An inductive ammeter clamp is used with all starting and charging testers to measure the current flow through the battery cables.

AC/DC CLAMP-ON DIGITAL MULTIMETER
An AC/DC clamp-on digital multimeter (DMM) is a useful meter for automotive diagnostic work and uses a Halleffect sensor to measure current.

AC/DC CLAMP-ON DIGITAL MULTIMETER
FIGURE 8-16 A typical mini clamp-on-type digital multimeter. This meter is capable of measuring alternating current (AC) and direct current (DC) without requiring that the circuit be disconnected to install the meter in series. The jaws are simply placed over the wire and current flow through the circuit is displayed. FIGURE 8-17 An AC and DC current clamp such as the one shown can be used with a regular digital multimeter. The amp probe contains a separate battery and electronic circuit that converts the amperage reading into a millivolt (mV) signal.

FUSE YOUR METER LEADS! FIGURE 8-18 Note the blade-type fuse holder soldered in series with one of the meter leads. A 10 amp fuse helps protect the internal meter fuse (if equipped) and the meter itself from damage that might result from excessive current flow if accidentally used incorrectly.

ELECTRICAL UNIT PREFIXES
Electrical units are measured in numbers such as 12 volts, 150 amperes, and 470 ohms. Kilo (k) means 1,000. If the value is over 1 million (1,000,000), then the prefix mega (M) is often used. Small units of measure expressed in 1/1,000 are prefixed by milli (m). The micro unit is represented by the Greek letter mu (μ).

“OL” DOES NOT MEAN THE METER IS READING “NOTHING”
FIGURE 8-19 A summary of the test meter hookup.

HOW TO READ DIGITAL METERS
Getting to know and use a digital meter takes time and practice. Select the proper unit of electricity for what is being measured. Place the meter leads into the proper input terminals. Measure the component being tested. Interpret the reading.

HOW TO READ DIGITAL METERS
FIGURE 8-20 Always look at the meter display when a measurement is being made, especially if using an autoranging meter. (Courtesy of Fluke Corporation)

HOW TO READ DIGITAL METERS RMS versus Average
Alternating current voltage waveforms can be true sinusoidal or nonsinusoidal. A true sine wave pattern measurement will be the same for both root-mean-square (RMS) and average reading meters.

HOW TO READ DIGITAL METERS RMS versus Average
FIGURE 8-21 When reading AC voltage signals, a true RMS meter (such as a Fluke 87) provides a different reading than an average responding meter (such as Fluke 88).The only place this difference is important is when a reading is to be compared with a specification. (Courtesy of Fluke Corporation)

HOW TO READ DIGITAL METERS Resolution, Digits, and Counts
Meter resolution refers to how small or fine a measurement the meter can make. The terms digits and counts are used to describe a meter’s resolution. DMMs are grouped by the number of counts or digits they display.

HOW TO READ DIGITAL METERS Resolution, Digits, and Counts
FIGURE 8-22 This meter display shows AC volts. Notice that the zero beside the 5 indicates that the meter can read over 100 volts AC with a resolution of 0.1 volt.

HOW TO READ DIGITAL METERS Accuracy
Meter accuracy is the largest allowable error that will occur under specific operating conditions. Accuracy for a DMM is usually expressed as a percent of reading. An accuracy of 1% of reading means that for a displayed reading of V, the actual value of the voltage could be anywhere between 99.0 V to V.

HOW TO READ DIGITAL METERS Accuracy
Unacceptable 1.00% Okay 0.50% (1/2%) Good 0.25% (1/4%) Excellent 0.10% (1/10%)

METER USAGE ON HYBRID ELECTRIC VEHICLES
Many hybrid electric vehicles use system voltage as high as 650 volts DC. These categories are ratings for overvoltage protection and are rated CAT I, CAT II, CAT III, and CAT IV.

METER USAGE ON HYBRID ELECTRIC VEHICLES
FIGURE 8-23 Be sure to only use a meter that is CAT III rated when taking electrical voltage measurements on a hybrid vehicle. FIGURE 8-24 Always use meter leads that are CAT III rated on a meter that is also CAT III rated to maintain the protection needed when working on hybrid vehicles.

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SUMMARY Digital multimeter (DMM) and digital volt-ohmmilliammeter (DVOM) are terms commonly used for electronic high-impedance test meters. Use of a high-impedance digital meter is required on any computer-related circuit or component. Ammeters measure current and must be connected in series in the circuit. Voltmeters measure voltage and are connected in parallel. Ohmmeters measure resistance of a component and must be connected in parallel, with the circuit or component disconnected from power. Logic probes can indicate the presence of ground, as well as power.

REVIEW QUESTIONS Explain why most digital meters are called high-impedance meters. Describe how an ammeter should be connected to an electrical circuit. Explain why an ohmmeter must be connected to a disconnected circuit or component.

CHAPTER QUIZ Inductive ammeters work because of what principle? Magic
Electrostatic electricity A magnetic field surrounds any wire carrying a current Voltage drop as it flows through a conductor

CHAPTER QUIZ 2. A meter used to measure amperes is called a(n) _____.
Amp meter Ampmeter Ammeter Coulomb meter

CHAPTER QUIZ 3. A voltmeter should be connected to the circuit being tested _____. In series In parallel Only when no power is flowing Both a and c

CHAPTER QUIZ 4. An ohmmeter should be connected to the circuit being tested _____. In series In parallel Only when no power is flowing Both b and c

CHAPTER QUIZ 5. A high-impedance meter _____.
Measures a high amount of current flow Measures a high amount of resistance Can measure a high voltage Has a high internal resistance

CHAPTER QUIZ 6. A meter is set to read DC volts on the 4 volt scale. The meter leads are connected at a 12 volt battery. The display will read _____. 0.00 OL 12 V 0.012 V

CHAPTER QUIZ 7. What could happen if the meter leads were connected to the positive and negative terminals of the battery while the meter and leads were set to read amperes? Could blow an internal fuse or damage the meter Would read volts instead of amperes Would display OL Would display 0.00

CHAPTER QUIZ 8. The highest amount of resistance that can be read by the meter set to the 2 k scale is _____. 2,000 ohms 200 ohms 200 k (200,000 ohms) 20,000,000 ohms

CHAPTER QUIZ 9. If a digital meter face shows 0.93 when set to read k, the reading means _____. 93 ohms 930 ohms 9,300 ohms 93,000 ohms

CHAPTER QUIZ 10. A reading of 432 shows on the face of the meter set to the millivolt scale. The reading means _____. 0.432 volt 4.32 volts 43.2 volts 4,320 volts

END

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