Test equipment must be used by the service tech because electrical signals cannot be seen or felt and therefore must be measured. This chapter reviews basics of electrical measurement and provides background for measuring components and circuits in the remaining chapters.
A test light is simply a light bulb with two wires attached. It is used to test for low-voltage level (6 to 12 volts). Battery voltage can’t be seen or felt and can be detected only with test equipment.
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.
More than one tech has misdiagnosed a problem because the test equipment did not work correctly. Examples include the following:
Test light bulb burned out . A test light bulb can be easily checked by regularly testing the test light bulb across a battery. A burned out test light bulb will not be able to detect voltage.
Meter fuse blown . If an ammeter fuse is blown (open), the meter will simply read 0.0 amperes (A) and not conduct current in the circuit. If an inline fuse is used in the meter leads, all functions of the meter will not work. Volts will read 0.0 all the time and often will read OL (over limit) on all ohmmeter scales.
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.
Self-Powered Test Lights A self-powered test light , also called a continuity light , is similar to a test light but includes a battery. A self-powered light will light when connected to both ends of a wire that has continuity or that is not broken.
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
Led Test Light Another type of test light uses an LED instead of a standard automotive bulb for a visual indication of voltage.
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.
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.
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
A logic probe must be first connected to a power source (vehicle battery). This connection powers the probe and gives it a reference low (ground). Most logic probes also make a distinctive sound for each high- and low-voltage level, which makes troubleshooting easier when probing connectors or component terminals. A sound (usually a beep) is heard when the probe tip is touched to a voltage source that is changing. The changing voltage also usually lights the pulse light on the logic probe. Therefore, the probe can be used to check components such as pickup coils, Hall-effect sensors, magnetic sensors, and many other circuits.
Digital multimeter (DMM) and digital volt-ohm-milliammeter (DVOM) are terms commonly used for electronic high-impedance test meters , which have a high internal resistance. Most digital meters have 10 megohms (MΩ) (10 million ohms) or more of internal resistance. Analog (needle-type) meters are almost always lower than 10 megohms and should not be used to measure any computer circuit . A high-impedance meter can be used to measure any automotive circuit within the ranges of the meter. See Figures 34–6 through 34–8
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.
An ammeter measures the flow of current through a complete circuit in units of amperes. The ammeter has to be installed in the circuit (in series) so that it can measure all the current flow in that circuit, just as a water flow meter would measure the amount of water flow (cubic feet per minute, for example).
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.
Digital meters require that the meter leads be moved to the ammeter terminals. Most digital meters have an ampere scale that can accommodate a maximum of 10 A. Many ammeters are the inductive type, meaning the meter probe surrounds the wire(s) carrying current and measures the strength of the magnetic field that surrounds any conductor carrying a current.
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
Most digital meters include an ammeter capability. When reading amperes, the leads of the meter must be moved from volts or ohms ( V or Ω ) to amperes ( A ) or milliamperes ( mA ) or microamperes ( µA ). A common problem may occur the next time voltage is measured. Although the tech 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 0 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. To solve this problem, simply solder an inline blade fuse holder into one meter lead.
Do not think this fuse is necessary only for beginners. Experienced techs 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.
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.
An AC/DC clamp-on DMM is a useful meter for automotive diagnostic work. See Figure 34–11. The major advantage of the clamp-on-type meter is there is no need to break the circuit to measure current (amperes). Simply clamp the jaws of the meter around the power lead(s) or ground lead(s) of the component being measured and read the display. Most clamp-on meters can also measure AC, which is helpful in the diagnosis of a generator (alternator) problem. Volts, ohms, frequency, and temperature can also be measured with the typical clamp-on DMM.
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
A voltmeter measures the pressure or potential of electricity in units of volts, and is connected to a circuit in parallel. All voltmeters have a large, built-in resistance so that the current flow through the meter will not affect either the circuit being tested or the meter. Most digital meters have an internal resistance of 10 MΩ (10,000,000 ohms) or more on the voltmeter scale only. This is called the impedance of the meter and represents the total internal resistance of the meter circuit due to internal coils, capacitors, and resistors. A typical analog voltmeter has only about 12,000 ohms of internal resistance. Although this may sound like a lot of resistance, it is too low for electronic and computer circuit measurement.
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.
Remember, the resistance of any meter is only effective when the meter is set on the voltmeter scales .
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.
T-pins are commonly found in discount stores and craft shops. These low-cost pins are extremely helpful in gaining access to signals without doing any harm. T-pins are commonly found in 1.5-in. and 1.75-in. lengths.
To use a T-pin, carefully push the sharp end alongside the wire and push toward the connector. The sharp end of the T-pin will slide alongside the insulation and contact the metal terminal inside the plastic connector. The T shape of the pin makes it easy to attach scope probes or meter alligator clips to it.
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.
An ohmmeter measures the resistance in ohms of a component or circuit section when no current is flowing through the circuit. An ohmmeter contains a battery (or other power source). When the leads are connected to a component, current flows through the test leads and actually measures the difference in voltage (voltage drop) between the leads, which the meter registers as resistance on its scale. See Figure 34–15. Zero ohms mean no resistance between the test leads, indicating that there is continuity or a continuous path for the current to flow in a closed circuit. Infinity means no connection, as in an open circuit.
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.
To summarize open and zero readings:
0.00 Ω = zero resistance
OL = an open circuit (no current flows)
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.
Beginner technicians often confuse the meaning of the display on a digital meter. When asked what the meter is reading when OL is displayed on the meter face, the response often heard is “nothing.” Many meters indicate OL on the display to indicate over limit or overload . Over limit means that the reading is over the maximum that can be displayed for the selected range. Eg: OL is displayed if 12 volts is being read, but the meter has been set to read a maximum of 4 volts.
Auto ranging 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 while measuring resistance (ohms). Therefore, OL means infinity while measuring resistance or an open circuit is being indicated. The meter reads 00.0 if resistance is zero.
“ OL ” Does Not Mean the Meter Is Reading “Nothing” - Part 1
“ Nothing” in this case indicates continuity (zero resistance), whereas OL indicates infinite resistance.
“ 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.
Sometimes a circuit conducts so little current that a smaller unit of measure is required. Small units of measure of 1/1,000 are called milli (m). The micro is represented by the Greek letter mu (µ). One microampere is one-millionth (1/1,000,000) of an ampere. To summarize:
mega ( M ) = ‑ 1,000,000 (decimal point six places to the right = 1000000.)
kilo ( k ) = ‑ 1,000 (decimal point three places to the right = 1000.)
milli ( m ) = ‑ 1/1,000 (decimal point three places to the left = 0.001)
micro ( µ ) = ‑ 1/1,000,000 (decimal point six places to the left = 0.000001)
Most digital meters that are set to measure ohms (resistance) apply a voltage of from 0.3 to 1.0 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:
How Much Voltage Does an Ohmmeter Apply?
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.
These prefixes can be confusing because most digital meters can express values in more than one prefix, especially if the meter is auto ranging. For example, an ammeter reading may show 36.7 mA on auto ranging. When the scale is changed to amperes (“A” in the window of the display), the number displayed will be 0.037 A. Note that the resolution of the value is reduced.
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.
Getting to know and use a digital meter takes time and practice. Use of the meter usually involves the following steps:
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.
Place the meter leads into the proper input terminals .
The black lead usually is inserted into the common (COM) terminal, and stays in this location for all meter functions.
The red lead is inserted into the volt, ohm, or diode check terminal usually labeled “V Ω,” when voltage, resistance, or diodes are being measured.
When current flow in amperes is being measured, most digital meters require that the red test lead be inserted in the ammeter terminal, usually labeled “A” or “mA.”
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.
A voltage drop is being measured. The specifications indicate a maximum voltage drop of 0.2 volt. Meter reads “AUTO” and “43.6 mV.” This reading means that the voltage drop is 0.0436 volt, or 43.6 mV, which is far lower than the 0.2 volt (200 mV). Because the number showing on the meter face is much larger than the specs, many beginner technicians are led to believe that the voltage drop is excessive.
HINT: Pay attention to the units displayed on the meter face and convert to base units. Continued
Interpret the reading This is especially difficult on autoranging meters, where the meter itself selects the proper scale. The following are two examples of different readings.
A spark plug wire is being measured. The reading should be less than 10,000 ohms for each foot in length if the wire is usable. The wire being tested is 3 ft long (maximum allowable resistance is 30,000 ohms). The meter reads “AUTO” and “14.85 KΩ.” This reading is equivalent to 14,850 ohms.
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.
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.
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
Digital meter displays can often be confusing. A battery being measured as 12.5 volts would be displayed as 12.50 V, just as $12.50 is 12 dollars and 50 cents. A 0.5-volt reading on a digital meter will be displayed as 0.50 V, just as $0.50 is half of a dollar. Low-value displays can be even more confusing. For example, if a voltage reading is 0.063 volt, an autoranging meter will display 63 mV or 63/1,000 of a volt or $63 of $1,000. (It takes 1,000 mV to equal 1 volt.) Millivolts are like one-tenth of a cent with 1 volt being $1.00. Therefore, 630 mV is equal to $0.63 of $1.00 (630 one-tenths of a cent or 63 cents). To avoid confusion, manually range the meter to read base units (whole volts). If the meter is ranged to base unit volts, 63 mV would be displayed as 0.063 or maybe just 0.06 depending on the display capabilities of the meter
Resolution, Digits, and Counts Resolution refers to how small or fine a measurement the meter can make. Knowing resolution of a DMM helps determine if the meter measures down to 1 volt or to 1 millivolt (1/1,000 of a volt). You would not buy a ruler marked in 1-in. segments (or in cm) if you had to measure down to .25 in. (or to 1 mm). A thermometer that measures only in whole degrees is of little use when your normal temperature is 98.6°F. You need a thermometer with 0.1° resolution . The terms digits and counts describe a meter’s resolution. DMMs are grouped by number of counts or digits they display. A 3.5-digit meter can display three full digits ranging from 0 to 9, and one “half” digit which displays only a 1 or is left blank. A 3.5-digit meter will display up to 1,999 counts of resolution.
A 4.5-digit meter can display up to 19,000 counts of resolution. It is more precise to describe a meter by counts of resolution rather than by 3.5 or 4.5 digits. Meters with more counts offer better resolution. For example, a 1,999-count meter cannot measure down to a tenth of 0.10 volt when measuring 200 volts or more. See Figure 34–21. A 3,200-count meter will display 0.10 volt up to 320 volts. Digits displayed to the far right of the display may at times flicker or constantly change, called digit rattle, a changing voltage being measured on the ground (COM terminal of the meter lead). High-quality meters are designed to reject this unwanted voltage.
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.
Accuracy Largest allowable error that will occur under specific operating conditions. An indication of how close the displayed measurement is to actual value of the signal being measured. Accuracy usually expressed as a percent of reading. An accuracy of ±1% of reading means for a displayed reading of 100.0 V, actual voltage could be anywhere between 99.0 to 101.0 volts. A lower accuracy percentage is better.
An oscilloscope ( scope ) is a visual voltmeter with a timer (clock) that shows when voltage changes. An analog scope uses a cathode ray tube ( CRT ) similar to a TV screen to display voltage patterns. The scope screen displays the electrical signal constantly. A digital scope commonly uses a LCD, but a CRT may also be used on some digital scopes. It takes samples of the signals that can be stopped or stored; hence the term digital storage oscilloscope ( DSO ). Analog scopes display all voltage signals and do not take samples, so they cannot miss an occurrence. A digital scope can miss glitches between samples; A DSO with a high “sampling rate” is preferred.
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.
For example, if a throttle position sensor was to be tested on an analog scope and a DSO, the results would be as shown here and in Figure 34-23.
(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.
A typical scope face has 8 divisions vertically (up and down) and 10 divisions horizontally. The grid lines on the scope screen are used as a reference scale, which is called a graticule . This arrangement is commonly called an 8 × 10 display .
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.
Voltage is displayed on a scope as a line vertically from the bottom. The scope illustrates time left to right as shown here.
Most scopes use 10 divisions, left to right on the display. The time base indicates how much time will be displayed in each division. The time base should be set to an amount that allows two to four events to be displayed. Sample time is milliseconds per division (indicated as ms/div) and total time includes:
See the chart onPage 350 of yourtextbook. NOTE: Increasing time base reduces the number of samples per second. Continued
Horizontal scale is divided into 10 divisions. If each represented 1 sec of time, total time period displayed will be 10 sec. Time per division can vary greatly in automotive use: Fuel injector —2 ms per division ( 20 ms total ) Throttle position ( TP ) sensor —100 ms per division ( 1 sec total ) Oxygen sensor —1 sec per division ( 10 sec total ) T ime per division is selected so several events of the waveform are displayed. This allows comparisons to see if the waveform is changing. Multiple waveforms shown on the display allow for measurements to be seen more easily.
The vertical scale has eight divisions. If each division is set to equal 1 volt, the display will show 0 to 8 volts. This is acceptable in a 0- to 5-volt variable sensor such as a TP sensor. The volts per division (V/div) should be set so the entire anticipated waveform can be viewed. Examples include:
Throttle position ( TP ) sensor — 1 V/div ( 8 V total ) Battery, starting and charging — 2 V/div ( 16 V total ) Oxygen sensor — 200 mV/div ( 1.6 V total )
Note from the examples that the total voltage to be displayed exceeds the voltage range of the component being tested. This ensures that all the waveform will be displayed. It also allows for some unanticipated voltage readings. For example, an oxygen sensor should read between 0 and 1 V (1,000 mV). By setting the V/div to 200 mV, up to 1.6 V (1,600 mV) will be displayed.
DC coupling allows the scope to display both AC and DC voltage. A flat horizontal display line at 12 volts indicates DC voltage signal, the starting and charging voltage measured at the battery. When the starter motor is energized, a load is applied to the battery and the battery voltage drops to about 10.5 volts, again seen as a horizontal line, but now at a level lower. When the engine starts, the generator starts to charge the battery and voltage increases to about 14.5 volts. Again, a horizontal line, but with a higher level than before. Generator (alternator) ripple voltage caused by leaking diodes would be displayed as an AC signal on top of the DC waveform.
AC coupling allows the scope to read alternating current ( AC ) voltage signals and ignore any direct current ( DC ) voltage present in the circuit. This setting allows the tech to view ripple voltage of the charging circuit without seeing the 14 volts DC signal. As an example, connect the scope probes to display 120 volts AC household voltage at an outlet.
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
An AC voltage rises and falls, above and below the zero level. To display several repeating continuously variable voltage signals, the proper time per division must be selected. Household electricity is 60 hertz (60 cycles per second) where 1 cycle requires 18 ms (0.018 sec). One complete cycle displays as 0.2 ms/div (10 divisions × 2 ms/div = 20 ms). Because it is best to view two or three cycles, set the time base to 4 or 6 ms per division. Setting volts per division is a little tricky. A 120 V AC signal actually goes over 120 volts positive and down more than 120 volts negative. To view the entire waveform, total voltage range on the display must be greater than 240 volts. With eight vertical grids, a setting of 50 volts per division would allow the scope to display 400 volts, 200 positive, 200 negative.
A DC voltage that turns on and off in a series of pulses is called a pulse train . See Figure 34–25.
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).
An fuel pump in an electronic return-less fuel system is an example of a PWM pulsed on and off with a variable duty cycle. Solenoid activation is changing 10 times per second (10 Hz), but on-time varies. In this example, the duty cycle is measured in degrees.
(a) (b) Continued
Figure 34–27 Most automotive computer systems control the device by opening and closing the ground to the component.
Pulse Width The pulse width is a measure of the actual on-time measured in milliseconds. Fuel injectors are usually controlled by varying the pulse width.
External Trigger An external trigger occurs when a signal is received from another (external) source. An external trigger comes from the probe clamp around cylinder 1 spark plug wire to trigger the start of an ignition pattern, for example. Trigger Level A scope will not display a voltage signal until it is triggered. The trigger level must be set to start display. In the example, the pattern is to start at 1 volt, the trace will begin displaying on the left of the screen after the trace has reached 1 volt. Trigger Slope The trigger slope is the voltage direction required to start display. Most often, the trigger is taken from the signal itself.
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).
Besides trigger voltage level, most scopes can be adjusted to trigger only when the voltage rises past the trigger-level voltage, called a positive slope , or by the voltage falling past the trigger level, it is called a negative slope .
The scope display indicates both positive and negative slope symbols. See the remaining chapters for examples of scope usage. (a) (b)
Most scopes, both analog and digital, normally use the same test leads. These leads usually attach to the scope through a BNC connector , a miniature standard coaxial cable connector named after its inventor, Baby Neil Councilman. BNC is an international standard that is used in the electronics industry. Each scope lead has an attached ground lead that should be connected to a good clean, metal engine ground. The probe of the scope lead attaches to the circuit or component being tested.
An analog scope displays rapidly and cannot be set to show or freeze a display. Though an analog scope shows all voltage signals, it is easy to miss a momentary event (glitch) on an analog scope.
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.
A graphing multimeter , ( GMM ) is a cross between a digital meter and a digital storage oscilloscope. It displays voltage levels on a display and a digital readout. Usually not capable of capturing short duration faults likely captured with a digital storage oscilloscope.
Figure 34–30 This shows a typical graphing multimeter.
Many hybrid (gasoline and electric motor) powered vehicles are equipped with a generator that can exceed 400 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 leaks 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 are ratings for over voltage protection and are rated CAT I , CAT II , CAT III , and CAT IV . The higher the category, the greater the protection against voltage spikes. Under each category there are various voltage ratings.
CAT I ‑ Typically a CAT I meter is used for low-voltage measurements such as voltage measurements at wall outlets in the home. Meters with a CAT I rating are usually rated at 300 to 800 volts.
CAT II ‑ A higher-rated meter that would be typically used for checking voltages at the fuse panel in the home. Meters with a CAT II rating are usually rated at 300 to 600 volts.
CAT III ‑ The minimum-rated meter that should be used for hybrid vehicles. This category is designed for voltage measurements at the service pole transformer. CAT III meters 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 .
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.
Always use the highest CAT rating meter, especially when working with hybrid vehicles.
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.