1. CHAPTER 8 Circuit Testers and Digital Meters

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

3. KEY TERMS AC/DC clamp-on DMM Continuity light DMM DVOM
High-impedance test meter
IEC
Inductive ammeter
Kilo (k)
LED test light
Logic probe
Mega (M)
Meter accuracy
Meter resolution
Micro (μ)
Milli (m) OL
RMS
Test light

4. FUSED JUMPER WIRE DEFINITION
A fused jumper wire is used to check a circuit by bypassing the switch or to provide a power or ground to a component.
A fused jumper wire, also called a lead, can be purchased or made by the service technician.
It should include the following features.
Fused
Alligator clip ends
Good-quality insulated wire

5. FUSED JUMPER WIRE DEFINITION
FIGURE 8–1 A technician-made fused jumper lead, which is equipped with a red 10 ampere fuse. This fused jumper wire uses terminals for testing circuits at a connector instead of alligator clips.

6. FUSED JUMPER WIRE USES OF A FUSED JUMPER WIRE
A fused jumper wire can be used to help diagnose a component or circuit by performing the following procedures.
Supply power or ground

7. TEST LIGHTS NONPOWERED TEST LIGHT USES OF A 12 VOLT TEST LIGHT
CONTINUITY TEST LIGHTS
HIGH-IMPEDANCE TEST LIGHT
FIGURE 8–2 A 12 volt test light is attached to a good ground while probing for power.

8. TEST LIGHTS
FIGURE 8–3 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.

9. TEST LIGHTS
FIGURE 8–4 A continuity light should not be used on computer circuits because the applied voltage can damage delicate electronic components or circuits.

10. TEST LIGHTS
FIGURE 8–5 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.

11. LOGIC PROBE PURPOSE AND FUNCTION
FIGURE 8–6 A logic probe connected to the vehicle battery. When the tip probe is connected to a circuit, it can check for power, ground, or a pulse.
A logic probe is an electronic device that lights up a red (usually) LED if the probe is touched to battery voltage.
If the probe is touched to ground, a green (usually) LED lights.

12. LOGIC PROBE USING A LOGIC PROBE
A logic probe must first be connected to a power and ground source such as the vehicle battery.
This connection powers the probe and gives it a reference low (ground).

13. DIGITAL MULTIMETERS TERMINOLOGY MEASURING VOLTAGE MEASURING RESISTANCE
MEASURING AMPERES
FIGURE 8–7 Typical digital multimeter. The black meter lead always is placed in the COM terminal. The red meter test lead should be in the volt-ohm terminal except when measuring current in amperes.

14. DIGITAL MULTIMETERS

15. DIGITAL MULTIMETERS
FIGURE 8–8 Typical digital multimeter (DMM) set to read DC volts.

16. DIGITAL MULTIMETERS
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)

17. DIGITAL MULTIMETERS
FIGURE 8–10 Using a digital multimeter set to read ohms (Ω) to test this light bulb. The meter reads the resistance of the filament.

18. DIGITAL MULTIMETERS
FIGURE 8–11 Many digital multimeters can have the display indicate zero to compensate for test lead resistance. (1) Connect leads in the V Ω and COM meter terminals. (2) Select the Ω 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.

19. DIGITAL MULTIMETERS
FIGURE 8–12 Measuring the current flow required by a horn requires that the ammeter be connected to the circuit in series and the horn button be depressed by an assistant.

20. How Much Voltage Does an Ohmmeter Apply?
Most digital meters that are set to measure ohms (resistance) apply 0.3 to 1 volt to the component being measured. The voltage comes from the meter itself to measure the resistance. Two things are important to remember about an ohmmeter.
The component or circuit must be disconnected from any electrical circuit while the resistance is being measured.
Because the meter itself applies a voltage (even though it is relatively low), a meter set to measure ohms can damage electronic circuits. Computer or electronic chips can be easily damaged if subjected to only a few milliamperes of current, similar to the amount an ohmmeter applies when a resistance measurement is being performed.

21. Fuse Your Meter Leads!
Most digital meters include an ammeter capability. When reading amperes, the leads of the meter must be changed from volts or ohms (V or Ω) to amperes (A), milliamperes (mA), or microamperes (μA).

22. Fuse Your Meter Leads!
A common problem may then occur the next time voltage is measured. Although the technician may switch the selector to read volts, often the leads are not switched back to the volt or ohm position. Because the ammeter lead position results in zero ohms of resistance to current flow through the meter, the meter or the fuse inside the meter will be destroyed if the meter is connected to a battery. Many meter fuses are expensive and difficult to find.

23. Fuse Your Meter Leads!
To avoid this problem, simply solder an inline 10 ampere blade-fuse holder into one meter lead.
Do not think that this technique is for beginners only. Experienced technicians often get in a hurry and forget to switch the lead. A blade fuse is faster, easier, and less expensive to replace than a meter fuse or the meter itself. Also, if the soldering is done properly, the addition of an inline fuse holder and fuse does not increase the resistance of the meter leads. All meter leads have some resistance. If the meter is measuring very low resistance, touch the two leads together and read the resistance (usually no more than 0.2 ohm). Simply subtract the resistance of the leads from the resistance of the component being measured.

24. Fuse Your Meter Leads!
FIGURE 8–13 Note the blade-type fuse holder soldered in series with one of the meter leads. A 10 ampere fuse helps protect the internal meter fuse (if equipped) and the meter itself from damage that may result from excessive current flow if accidentally used incorrectly.

25. What Does “CE” Mean on Many Meters?
The “CE” means that the meter meets the newest European Standards and the letters CE stands for a French term for “Conformite’ Europeenne” meaning European Conformity in French.

26. INDUCTIVE AMMETERS OPERATION
Inductive ammeters do not make physical contact with the circuit.
They measure the strength of the magnetic field surrounding the wire carrying the current, and use a Hall-effect sensor to measure current.
The Hall-effect sensor detects the strength of the magnetic field that surrounds the wire carrying an electrical current.

27. INDUCTIVE AMMETERS OPERATION
FIGURE 8–14 An inductive ammeter clamp is used with all starting and charging testers to measure the current flow through the battery cables.

28. INDUCTIVE AMMETERS AC/DC CLAMP-ON DIGITAL MULTIMETERS
An AC/DC clamp-on digital multimeter (DMM) is a useful meter for automotive diagnostic work.
FIGURE 8–15 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.

29. Over Limit Display Does Not Mean the Meter Is Reading “Nothing”
The meaning of the over limit display on a digital meter often confuses beginning technicians. When asked what the meter is reading when an over limit (OL) is displayed on the meter face, the response is often, “Nothing.” Many meters indicate over limit or over load, which simply means that the reading is over the maximum that can be displayed for the selected range. For example, the meter will display OL if 12 volts are being measured but the meter has been set to read a maximum of 4 volts.

30. Over Limit Display Does Not Mean the Meter Is Reading “Nothing”
Autoranging meters adjust the range to match what is being measured. Here OL means a value higher than the meter can read (unlikely on the voltage scale for automobile usage), or infinity when measuring resistance (ohms). Therefore, OL means infinity when measuring resistance or an open circuit is being indicated. The meter will read 00.0 if the resistance is zero, so “nothing” in this case indicates continuity (zero resistance), whereas OL indicates infinity resistance. Therefore, when talking with another technician about a meter reading, make sure you know exactly what the reading on the face of the meter means. Also be sure that you are connecting the meter leads correctly.

31. Over Limit Display Does Not Mean the Meter Is Reading “Nothing”
FIGURE 8–16 Typical digital multimeter showing OL (over limit) on the readout with the ohms (Ω) unit selected. This usually means that the unit being measured is open (infinity resistance) and has no continuity.

32. DIODE CHECK, PULSE WIDTH, AND FREQUENCY
Diode check is a meter function that can be used to check diodes including light-emitting diodes (LEDs).
PULSE WIDTH
Pulse width is the amount of time in a percentage that a signal is on compared to being off.
FREQUENCY
Frequency is a measure of how many times per second a signal changes.
Frequency is measured in a unit called hertz, formerly termed “cycles per second.”

33. ELECTRICAL UNIT PREFIXES DEFINITIONS
FIGURE 8–17 Always look at the meter display when a measurement is being made, especially if using an autoranging meter.

34. ELECTRICAL UNIT PREFIXES DEFINITIONS

35. ELECTRICAL UNIT PREFIXES USE OF PREFIXES
The prefixes can be confusing because most digital meters can express values in more than one unit, especially if the meter is autoranging.
For example, an ammeter reading may show 36.7 mA on autoranging.
When the scale is changed to amperes (“A” in the window of the display), the number displayed will be A.
Note that the resolution of the value is reduced.

36. Think of Money
Digital meter displays can often be confusing. The display for a battery measured as 12 1/2 volts would be V, just as $12.50 is 12 dollars and 50 cents. A 1/2 volt reading on a digital meter will be displayed as 0.50 V, just as $0.50 is half of a dollar.

37. Think of Money
It is more confusing when low values are displayed. For example, if a voltage reading is volt, an autoranging meter will display 63 millivolts (63 mV), or 63/1,000 of a volt, or $63 of $1,000. (It takes 1,000 mV to equal 1 volt.) Think of millivolts as one-tenth of a cent, with 1 volt being $1.00. Therefore, 630 millivolts are equal to $0.63 of $1.00 (630 tenths of a cent, or 63 cents).

38. Think of Money
To avoid confusion, try to manually range the meter to read base units (whole volts). If the meter is ranged to base unit volts, 63 millivolts would be displayed as or maybe just 0.06, depending on the display capabilities of the meter.

39. HOW TO READ DIGITAL METERS STEPS TO FOLLOW
STEP 1 Select the proper unit of electricity for what is being measured.
STEP 2 Place the meter leads into the proper input terminals.
STEP 3 Measure the component being tested.
STEP 4 Interpret the reading

40. HOW TO READ DIGITAL METERS STEPS TO FOLLOW

41. HOW TO READ DIGITAL METERS
RMS VERSUS AVERAGE
RESOLUTION, DIGITS, AND COUNTS
ACCURACY

42. Purchase a Digital Meter That Will Work for Automotive Use
Try to purchase a digital meter that is capable of reading the following:
DC volts
AC volts
DC amperes (up to 10 A or more is helpful)
Ohms (Ω) up to 40 MΩ (40 million ohms)
Diode check
Additional features for advanced automotive diagnosis include:
Frequency (hertz, abbreviated Hz)
Temperature probe (°F and/or °C)
Pulse width (millisecond, abbreviated ms)
Duty cycle (%)

43. HOW TO READ DIGITAL METERS
FIGURE 8–18 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 a Fluke 88). The only place this difference is important is when a reading is to be compared with a specification.

44. HOW TO READ DIGITAL METERS
FIGURE 8–19 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.

45. Meter Usage on Hybrid Electric Vehicles
Many hybrid electric vehicles use system voltage as high as 650 volts DC. Be sure to follow all vehicle manufacturer’s testing procedures; and if a voltage measurement is needed, be sure to use a meter and test leads that are designed to insulate against high voltages. The International Electrotechnical Commission (IEC) has several categories of voltage standards for meter and meter leads. These categories are ratings for overvoltage protection and are rated CAT I, CAT II, CAT III, and CAT IV. The higher the category, the greater the protection against voltage spikes caused by high-energy circuits. Under each category there are various energy and voltage ratings.

46. Meter Usage on Hybrid Electric Vehicles
CAT I Typically a CAT I meter is used for lowenergy voltage measurements such as at wall outlets in the home. Meters with a CAT I rating are usually rated at 300 to 800 volts.
CAT II This higher rated meter would be typically used for checking higher energy level voltages at the fuse panel in the home. Meters with a CAT II rating are usually rated at 300 to 600 volts.

47. Meter Usage on Hybrid Electric Vehicles
CAT III This minimum rated meter should be used for hybrid vehicles. The CAT III category is designed for high-energy levels and voltage measurements at the service pole at the transformer. Meters with this rating are usually rated at 600 to 1,000 volts.
CAT IV CAT IV meters are for clamp-on meters only. If a clamp-on meter also has meter leads for voltage measurements, that part of the meter will be rated as CAT III.

48. Meter Usage on Hybrid Electric Vehicles
FIGURE 8–20 Be sure to only use a meter that is CAT III rated when taking electrical voltage measurements on a hybrid vehicle.
FIGURE 8–21 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.

49. SUMMARY
Digital multimeter (DMM) and digital volt-ohm-milliammeter (DVOM) are terms commonly used for electronic highimpedance 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 power, ground, or pulsed signals.

50. REVIEW QUESTIONS
Why should high-impedance meters be used when measuring voltage on computer-controlled circuits?
How is an ammeter connected to an electrical circuit?
Why must an ohmmeter be connected to a disconnected circuit or component?

51. 1. Inductive ammeters work because of what principle?
CHAPTER QUIZ
1. 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

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

53. 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

54. CHAPTER QUIZ
4. An ohmmeter should be connected to the circuit or component being tested ________ .
With current flowing in the circuit or through the component
When connected to the battery of the vehicle to power the meter
Only when no power is flowing (electrically open circuit)
Both b and c

55. 5. A high-impedance meter ________ .
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

56. 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

57. 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

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

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

60. 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