Chapter 34

[object Object],[object Object],[object Object],OBJECTIVES: After studying Chapter 34, the reader should be able to: Continued

[object Object],[object Object],OBJECTIVES: After studying Chapter 34, the reader should be able to:

[object Object],[object Object],[object Object],[object Object],KEY TERMS: Continued ×

[object Object],[object Object],KEY TERMS:

TEST LIGHTS ,[object Object],Continued Figure 34–1 A 12-volt test light is attached to a good ground while probing for power. A test light can be purchased or homemade. A purchased test light could be labeled as a 6- to 12-volt test light. Do not purchase a test light designed for household current (110 or 220 volts). It will not light with 12 volts.

[object Object],[object Object],[object Object],Always Test Your Equipment

Figure 34–2 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. Continued CAUTION: The use of a continuity test light is not recommended on any electronic circuit. Because a continuity light contains a battery and applies voltage, it may harm delicate electronic components.

[object Object],Figure 34–3 A continuity light should not be used on computer circuits because the applied voltage can damage delicate electronic components or circuits. Continued

[object Object],Figure 34–4 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 0.025 amperes (25 milliamperes) from the circuit being tested. This low current draw helps assure the tech that the circuit or component being tested will not be damaged by excessive current flow. Continued An LED test light requires only about 25 mA (0.025 A) to light, so it can be used on electronic circuits and on standard circuits.

LOGIC PROBE ,[object Object],Figure 34–5 A logic probe connected to the vehicle battery and relay used to check for power, ground, or a pulse. A logic probe can “sense” the difference between high- and low- voltage levels, thus the term logic . A typical logic probe can also light another light (a “pulse” light) when a change in voltage levels occurs. Continued

DIGITAL MULTIMETERS ,[object Object],Continued

Figure 34–6 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 VW terminal. Continued

Figure 34–7 Common abbreviations used on the display face of many digital mutimeters. See the charton Page 342 of your textbook. Continued

Figure 34–8 A summary chart indicating what measurement type may be used to test which vehicle system. See the chart on Page 343 of your textbook.

MEASURING AMPERES ,[object Object],Continued CAUTION: An ammeter must be installed in the circuit to measure the current flow in the circuit. If a meter set to read amperes is connected in parallel, such as across a battery, the meter fuse will blow or the meter itself may be destroyed by the current available across the battery.

[object Object],Figure 34–9 An inductive ammeter uses a clamp that measures the current through the wire by using the strength of the magnetic field surrounding the wire. Continued

[object Object],Fuse Your Meter Leads - Part 1

[object Object],Fuse Your Meter Leads - Part 2 Figure 34–10 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. If the meter is measuring very low resistance, touch the two leads together and read the resistance Subtract the resistance of the leads from the resistance of the component being measured. 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.

AC/DC CLAMP-ON DIGITAL MULTIMETER ,[object Object],Continued

Figure 34–11 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. Continued

MEASURING VOLTAGE ,[object Object],Continued

Figure 34–12a 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. (a) Manufacturers specify that a high-impedance digital meter be used. When a voltmeter is connected to measure voltage, the meter itself becomes part of the circuit. Continued

Figure 34–12b 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. (b) The high internal resistance has little effect on the circuit or component being measured. Continued

Figure 34–13 Typical digital multimeter (DMM) set to read DC volts.

[object Object],NOTE: The input impedance of any meter can be measured by using another meter set to read ohms and measuring the resistance of the test meter set to the voltmeter scale. This is the reason most automobile manufacturers recommend testing voltage at selected points instead of resistance or current.

[object Object],[object Object],The T-Pin Advantage Figure 34–14 A typical T-pin. After scope or voltage measurements have been completed, the T-pin can be removed without damaging the environmental seal.

MEASURING RESISTANCE ,[object Object],Continued

Figure 34–15 Using a digital multimeter set to read ohms ( Ω ) to test this light bulb. The meter reads the resistance of the filament.

Figure 34–16 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. ,[object Object],[object Object],[object Object],With a closed circuit (low ohms), maximum current from the built-in battery causes a low reading, whereas an open circuit prevents current from flowing. Different meters have indicate infinite resistance different ways, or a reading higher than the scale allows. Most meters read OL, meaning “ over limit ,” whereas others may show a number 1 or 3 on the left side of the display.

[object Object],[object Object],“ OL ” Does Not Mean the Meter Is Reading “Nothing” - Part 1

[object Object],“ OL ” Does Not Mean the Meter Is Reading “Nothing” - Part 2 Figure 34–17 Many digital multimeters can have the display indicate zero to compensate for test lead resistance. (1) Connect leads in the V Ù and COM meer terminals. (2) Select the Ù 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. Here are examples of how the meter should be attached to read voltage, current (amperes), and resistance (ohms). When talking with another tech about a meter reading, make sure you know exactly what the reading on the face of the meter means.

Figure 34–18 Summary of test meter hookup.

ELECTRICAL UNIT PREFIXES ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],Continued

Figure 34–19 Always look at the meter display when a measurement is being made, especially if using an autoranging meter. Continued

[object Object],[object Object],[object Object],[object Object],[object Object],Continued

[object Object],How Much Voltage Does an Ohmmeter Apply? ,[object Object],[object Object]

[object Object],HINT: Lowercase m equals a small unit (milli), whereas a capital M represents a large unit (mega). HINT: Always check the face of the meter display for the unit being measured. To best understand what is being displayed on the face of a digital meter, select a manual scale and move the selector until base units appear, such as A for amperes instead of mA for milliamperes.

HOW TO READ DIGITAL METERS ,[object Object],Continued Select the proper unit of electricity for what is being measured : volts , ohms (resistance), or amperes (amount of current flow). If the meter is not autoranging, select the proper scale for the anticipated reading. If a 12-volt battery is being measured, select a meter reading range higher than the voltage but not too high. A 20- or 30-volt range will accurately show the voltage of a 12-volt battery.

[object Object],Continued ,[object Object],[object Object],[object Object],CAUTION: If the meter leads are inserted into ammeter terminals, even though the selector is set to volts, the meter may be damaged or an internal fuse may blow if the test leads touch both terminals of a battery.

[object Object],[object Object],[object Object],Continued If a 12-volt battery is measured with an autoranging meter, the correct reading of 12.0 is given. “AUTO” and “V” should show on the face of the meter.

[object Object],HINT: Pay attention to the units displayed on the meter face and convert to base units. Continued

[object Object],[object Object],HINT: When converting from kilohms to ohms, make the decimal point a comma. Because this reading is well below specified maximum allowable, the spark plug wire is usable.

[object Object],Purchase a Digital Meter That Will Work for Automotive Use ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],If working on older-model vehicles, select a meter that includes : Additional features for advanced automotive diagnosis include : ,[object Object],[object Object],[object Object],[object Object]

[object Object],Figure 34–20 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. RMS and averaging are two methods used to measure the true effective rating of a signal that is constantly changing. Continued

[object Object],Think of Money

Figure 34–21 This meter display shows 052.2 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.

[object Object],Continued ,[object Object],[object Object],[object Object],[object Object]

[object Object],— high = 12.61 low = 12.59 — high = 12.63 low = 12.57 — high = 12.66 low = 12.54 — high = 12.73 low = 12.47 ± 0.1% ± 0.25% ± 0.50% ± 1.00%

Analog Versus Digital Storage Oscilloscope ,[object Object],Continued

Figure 34–22 (a) On an analog scope, the voltage measured at the throttle position signal wire is displayed on a horizontal line at about 0.5 volts. (b) As the throttle is opened, the horizontal line representing the voltage increases. (c) At wide-open throttle (WOT), the horizontal line indicates about 4.5 volts. ,[object Object],(a) (b) (c) Continued

Figure 34–23 The display on a digital storage oscilloscope (DSO) displays the entire waveform from idle to wide-open throttle and then returns to idle. The display also indicates the maximum reading (4.72V) and the minimum (680 mV or 0.68V). The display does not show anything until the throttle is opened, because the scope has been set up to only start displaying a waveform after a certain voltage level has been reached. This voltage is called the trigger.

OSCILLOSCOPE DISPLAY GRID ,[object Object],Continued NOTE: These numbers represent the metric dimensions of the graticule in centimeters. Therefore, the display would be 8 cm (80 mm or 3.14 in.) high and 10 cm (100 mm or 3.90 in.) wide.

Figure 34–24 An automotive oscilloscope (scope) is of the same construction as a cathode ray tube (CRT) or television screen. An automotive oscilloscope is a visual voltmeter. The higher up a trace (line) on the scope, the higher the voltage. The scope illustrates time from left to right. The longer the horizontal line, the longer the amount of time. ,[object Object],Continued

SETTING THE TIME BASE ,[object Object],See the chart onPage 350 of yourtextbook. NOTE: Increasing time base reduces the number of samples per second. Continued

[object Object],Continued ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

SETTING THE VOLTS PER DIVISION ,[object Object],[object Object],Continued

AC COUPLING ,[object Object],CAUTION: 110 V AC can cause bodily injury. Always touch the rubber or plastic portions of the scope probes when making measurements of any circuit that exceed 30 volts (AC or DC). Also, some scopes, such as the MODIS, cannot handle household or industrial AC voltages. Continued

PULSE TRAINS ,[object Object],Continued Frequency Number of cycles per second, measured in hertz is known as frequency . Engine rpm signal is an example that can occur at various frequencies. At low engine speed, ignition pulses occur fewer times per second (lower frequency) than at higher engine speeds (rpms). Duty Cycle Percentage of on-time of the signal during one complete cycle is the duty cycle . As on-time increases, the amount of time the signal is off decreases. Also called pulse width modulation ( PWM ) and can be measured in degrees Pulse train signals can vary in several ways:

Figure 34–25 A pulse train is any electrical signal that turns on and off, or goes high and low in a series of pulses. Igniter and fuel-injector pulses are examples of a pulse train signal. Continued

Figure 34–26 (a) Scope representation of a complete cycle showing on-time and off-tIme. (b) A meter display indicating the on-time duty cycle in percent (%). Note the trigger and negative (-) symbol. This indicates that the meter started to record the percentage of on-time when the voltage dropped (start of on-time). ,[object Object],(a) (b) Continued

Figure 34–27 Most automotive computer systems control the device by opening and closing the ground to the component. ,[object Object],Continued

Figure 34–28 (a) A symbol for a positive trigger—a trigger occurs at a rising (positive) edge of the signal (waveform). (b) A symbol for a negative trigger—trigger occurs at a falling (negative) edge of the signal (waveform). ,[object Object],The scope display indicates both positive and negative slope symbols. See the remaining chapters for examples of scope usage. (a) (b)

USING SCOPE LEADS ,[object Object]

[object Object],MEASURING BATTERY VOLTAGE WITH A SCOPE Figure 34–29 Battery voltage is represented by a flat horizontal line. In this example, the engine was started and the battery voltage dropped to about 10 V as shown on the left side of the scope display. When the engine started, the generator (alternator) started to charge the battery and the voltage is shown as climbing. An easy things to measure on a scope is battery voltage. Lower voltage can be observed on the display as the engine is started and a higher voltage after the engine starts.

GRAPHING MULTIMETER ,[object Object],Figure 34–30 This shows a typical graphing multimeter.

[object Object],Meter Usage on Hybrid Vehicles - Part 1

[object Object],[object Object],[object Object],[object Object],For best personal protection, use only meters and meter leads that are CAT III or CAT IV rated when measuring voltage on a hybrid vehicle. Meter Usage on Hybrid Vehicles - Part 2

Figure 34–32 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. ,[object Object],Figure 34–31 Be sure to only use a meter that is CAT III-rated when taking electrical voltage measurements on a hybrid vehicle. Meter Usage on Hybrid Vehicles - Part 3 A CAT III 600-volt meter is safer than a CAT II 1,000-volt meter.

[object Object],( cont. ) Continued

SUMMARY ,[object Object],[object Object],[object Object],[object Object],Continued

SUMMARY ,[object Object],[object Object],( cont. )

Chapter 34

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Chapter 34