CDX Automotive Engine Performance, Sample Chapter 12 by Jones & Bartlett Learning

MASTER AUTOMOTIVE TECHNICIAN SERIES

SAMPLE CHAPTER 12

Automotive Engine Performance

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Education Foundation Lorem ipsum

Nicholas Goodnight Kirk VanGelder

CDX MASTER AUTOMOTIVE TECHNICIAN SERIES Designed for advanced-level instruction, the CDX Master Automotive Technician Series covers the latest technological advances with expert guidance from an extremely experienced diverse group of authors.

Automotive Engine Repair Nicholas Goodnight | Kirk VanGelder

Automotive Drivetrain & Manual Transmissions Keith Santini | Kirk VanGelder

Automotive Braking Systems Nicholas Goodnight | Kirk VanGelder

Automotive Automatic Transmission and Transaxles Keith Santini | Kirk VanGelder

Automotive Steering and Suspension John F. Kershaw, Ed.D. | Kirk VanGelder

Automotive Electricity and Electronics David M. Jones | Kirk VanGelder

Advanced Automotive Electricity and Electronics

Automotive Heating, Ventilation, and Air Conditioning

Michael Klyde | Kirk VanGelder

Joseph Wagner | Kirk VanGelder

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Light Vehicle Diesel Engines

Automotive Engine Performance

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Nicholas Goodnight | Kirk VanGelder

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CONTENTS

Chapter 1

Strategy-Based Diagnostics

Chapter 2

Engine Condition Diagnostics

Chapter 3

Engine Operation Diagnostics

Chapter 4

Engine Noise, Vibration, and Fluid Intrusion

Chapter 5

Compression Ignition and Spark Ignition Engines

Chapter 6

Fuels

Chapter 7

On-Board Diagnostics and Vehicle Control Modules

Chapter 8

Diagnosing Engine Faults with Diagnostic Trouble Codes

Chapter 9

Network Communication

Chapter 10

Introduction to Diagnosing with a Scan Tool

Chapter 11

Oscilloscopes

Chapter 12 Diagnosing with Oscilloscopes Chapter 13

Ignition System Fundamentals

Chapter 14

Types of Ignition Systems

Chapter 15

Ignition System Service

Chapter 16

Oscilloscope Testing Ignition Systems

Chapter 17

Electronic Fuel Injection System Supply Components

Chapter 18

Electronic Fuel Injection System - Inputs

Chapter 19

Electronic Fuel Injection System - Outputs

Chapter 20

Vehicle Emissions Systems

Chapter 21

Gasoline Direct Injection

Chapter 22

Variable Valve Timing

Chapter 23

Supercharger and Turbocharging

CHAPTER 12

Diagnosing with an Oscilloscope Learning Objectives ■ ■ ■

12-01 Analyze electrical component current draw. 12-02 Use a DSO for diagnosis. 12-03 Analyze magnetism and induction waveforms.

■

12-04 Explain the use of transducers and inductive pickups with an oscilloscope. 12-05 Use current ramping and an amp clamp to aid in diagnostics.

ASE Education Foundation Tasks See Appendix A to view the 2017 ASE Education Foundation Automobile Accreditation Task List Correlation Guide.

You Are the Automotive Technician A 2014 Chevy Tahoe is in your bay because the vehicle has a noisy fuel pump when the vehicle is running. The customer doesn’t understand that the noise could be caused by a worn-out fuel pump. How would you explain to the service advisor that the customer needs to replace the fuel pump?

1. Tell the customer that this must be done to maintain vehicle reliability. 2. Using an amp clamp, the technician could create a waveform that shows the customer where the motor is dying. 3. Tell the customer that they can’t just wait for the fuel pump to die and then replace it.

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▶▶ Analysis 12-01 Analyze electrical component current draw.

▶▶TECHNICIAN TIP When motors and pumps work properly, they are usually able to exceed the demands placed on them. As they gradually wear, their output, although reduced, continues to produce satisfactory results most of the time, making diagnosing a pump/motor fault difficult when using traditional testing methods. For example, a fuel pump may exhibit a concern only at high temperatures and under heavy load conditions. Using an amp clamp to monitor the pump’s speed and waveform can help spot the potential of a fault before it becomes a problem or help find an intermittent fault. As pumps and motors wear, their speed slows, current flow drops, and the waveform may show consistent dropouts.

of Motor Current Draw

Most 12-volt motors rely on contact between a set of brushes contacting the rotor to enable current flow. Over time, due to friction, the brushes and rotor slip rings wear. Most of the wear occurs in the soft conductive carbon/metal graphite brushes. As the brushes wear, they continue to contact the rotor due to springs on the back side that keep the pressure on the brushes. The segments of the motor’s commutator also wear (FIGURE 12-1). The wear between the commutator and brushes eventually exceeds the spring tension, causing intermittent contact and arcing from the bouncing brushes. When the brushes are not contacting the rotor’s commutator, current flow drops, motor speed slows, pump flow drops, and the waveform shows erratic and reduced humps (a hump is the waveform amplitude or the amount of power that is required to cause the commutator to continue to operate) when scoped (FIGURE 12-2). Motor analysis involves monitoring the current level and the integrity of the brush-to-commutator connection. Current waveform analysis of a motor includes the following: ■■ ■■ ■■

verifying the proper number commutator segments the frequency of the commutator humps’ motor speed the current draw of the pump/motor

High resistance due to a binding condition in the motor/pump causes the pump to spin slower with increased current draw but lower motor speeds. Excessive voltage drop due to an increase in resistance in the component or circuit will show lower rpm with normal to reduced current levels. With a slower spinning pump, the brushes make better contact with the commutator segments for current flow.

Monitoring the Current Waveform

FIGURE 12-1 The commutator and brushes are the heart of any electrical motor. The brushes transfer the power to the commutator, which causes it to spin.

Good brush-to-commutator contact will display even, uniform humps for all the segments. Although it is impossible to memorize every pattern, for the purposes of a diagnosis, compare the humps (segments) against each other while looking for a difference. If there is poor brush contact, it will be seen as a hump that does not match the others. When checking the hump’s integrity, remember that an open hump will reduce current, whereas a shorted hump will increase current. Poor brush contact is a result of naturally wearing brushes, reduced spring tension on the brush to the commutator, and contamination of the commutator/brushes, increasing wear. A lower peak is a result of excessive resistance between the brush and commutator With poor brush contact, a higher peak is an indication of lower resistance (more amperage) and usually indicates a bad motor winding, such as a possible individual coil winding that is shorting internally. Increased brush-tocommutator resistance will reduce current and slow the motor’s

FIGURE 12-2 A screen capture using a lowcurrent amp clamp to test the fuel pump current draw and waveform. The total fuel pump draw is 6 amps. The average fuel pump draws 5 to 8 amps. The fuel pump waveform shows the current pulse (hump) for each commutator segment. Most fuel pumps have six to eight segments. This waveform shows an eight-segment pump. To determine the number of segments, look for a repeating pattern, insert the cursors, and count the number of current humps. The pattern shows lower current draw for two sectors of the pump that repeats every 360 degrees of rotation.

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TABLE 12-1 Uses for an Oscilloscope in Automotive Diagnostics • Identifying the frequency and amplitude of a signal • The minimum and maximum voltage levels • Input, output, or communication systems • Determining the shape of a signal • DC* square wave (digital), DC analog, AC** sine wave • How a signal changes • Identifying how much noise is in the circuit • Comparing and measuring different signals to and against each other * DC: direct current ** AC: alternating current

speed, and it can create excessive noise in the pattern from the brushes jumping/arching across the commutator as they make and break contact (FIGURE 12-3).

Monitoring Average Current Monitoring the average current of a component allows the technician to see a failure before it leaves the vehicle stranded. Throughout this section different ways of monitoring the current will be discussed and how those different readings can be interpreted. This is a guide and the technician should use this as another source of information when they are determining the failure or potential failure of components.

Decreased Average Current Less current means less work. A lack of resistance to fuel flow decreases fuel pump current. Lower-than-average current draw is usually a sign that the motor is going to fail, commonly due to impeller wear. Other causes of a fast-spinning pump include an empty fuel tank, clogged fuel pump sock, or a stuck open fuel pressure regulator (on rail or in tank).

Causes of Lower-Than-Average Current Low voltage to the fuel pump, high circuit resistance, or a failing control module or relay could cause a lower-than-average current situation. Increases in pump motor speed from a pump that cannot build pressure will show the waveform with a decrease in peak current, and the fuel pump speed would also increase.

FIGURE 12-3 Testing a fuel pump for an intermittent stall and a hard-start concern when hot.The vehicle ran in the bay for over an hour before the

screenshot of the waveform seen on the left was taken.The yellow channel, Channel 1, is a side-by-side comparison of a known-bad fuel pump current waveform on the left compared to a known-good waveform on the right. Waveform 2 is the current trace after replacing the fuel pump. Compare the smooth and consistent waveform humps in Waveform 2 to the irregularity in the signal and the dropout of the current humps in Waveform 1.The green trace shown is fuel pump voltage.The voltage does not vary between screenshots, indicating that the voltage supply is not the cause.

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Chapter 12 Diagnosing with an Oscilloscope

FIGURE 12-4 Two signals combined on a scope for comparison or, as is being done here, to speed up diagnosis for a no-start. Channel A is measuring ignition system primary voltage due to the high voltage readings expected in the primary ignition circuit; a 10:1 attenuator is installed in Channel A to protect the scope. A high-voltage inductive kick is induced into the injector windings from the rapidly collapsing magnetic field when the injector(s) are turned off. The induced high voltage is how an ignition coil primary circuit works. The high voltage has to be regulated to protect the scope. The maximum safe voltage the scope is designed to handle is 200 volts. Using an attenuator prevents damage to the DSO.

Causes of Higher-Than-Average Current Typically, high current is a result of a restriction to fuel flow. Examples include a clogged fuel line, restricted fuel filter, stuck closed fuel pressure regulator, and anything that makes the pump work harder to deliver fuel. The causes of pump motor speed slowing is either a mechanical fault or excessive fuel pressure buildup. As this happens, the pump speed decreases and the amplitude in the waveform increases in peak current. This is an unstainable situation that will ultimately lead to pump failure.

Connecting the Scope to Measure Dual Signals Unlike digital multimeters (DMMs), which can measure only one signal at a time, digital storage oscilloscopes (DSOs) with more FIGURE 12-5 Integrating information into an oscilloscope can help than one channel can measure multiple signals at the same time, the technician to install the leads in the correct terminals so that a providing more information with more detail. Measuring current waveform can be produced. Having this information on hand helps to and voltage simultaneously can speed up diagnostics by offering speed up diagnosing the failure. a comparison of voltage to amperage for a single component or different components (FIGURE 12-4). The voltage waveform identifies circuit concerns, ▶▶TECHNICIAN TIP including poor ground connections, low voltage, poor switching, and a reduced inductive A new fuel pump waveform may appear spike. Current waveforms can point to shorted coil windings, high resistance/faulty to indicate a problem until the brushes grounds, low available voltage, poor switching, and reduced performance. seat. If a pump has been recently installed, let the brushes seat before condemning the pump based only on the waveform.

Additional Embedded Information Additional benefits of an automotive-based DSO include specific screen presets; testing procedures, including scope-to-component connections; and embedded diagnostic information, such as connector pinouts, descriptions of components, and the operation of the component and known-good waveforms for comparison. Modern advances in laptop computer–based oscilloscopes have further integrated data with the tools that are used to perform the test (FIGURE 12-5). Along with embedding the service information with the DSO is the ability to include any common failures or quick fixes to help increase the productivity of the technician. Common tests are programmed into the DSO to increase the speed and the accuracy of diagnosing the faults on the vehicle.

▶▶ Using

a Digital Storage Oscilloscope for Diagnosis

12-02 Use a DSO for diagnosis.

Using a scope requires more than just learning how to set it up and connect it. Viewing and interpreting various signal waveforms is essential to getting the most out of any scope.

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FIGURE 12-6 A DC voltage showing waveform that shows on time and off time and one cycle of a ground-sidecontrolled event.

Using basic pattern recognition will help in discerning the difference between what is good and what is bad. Continued practice with the scope will help users familiarize themselves with some of the basic waveforms. Most of the electronic waveforms happen at regular repetitive intervals. The majority of these waveforms conform to a known shape. There are some characteristics that a trained eye can use to differentiate good from bad.

Analyzing a Waveform One of the primary benefits of a scope is that it draws a picture—as opposed to just showing numbers as on a DMM. Scopes also display the current voltage of a signal, along with the minimum and maximum voltage levels on the screen over time. When a signal repeats itself, it is called a wave. Displaying the wave as a graphical image on a scope’s screen is a waveform. (FIGURE 12-6) Sensors and signals have a characteristic waveform (shape). Interpreting the signal as good or bad is the challenge of using a scope. There are, however, signal characteristics that once learned can be beneficial for analysis. It is important to understand what to look for when using an oscilloscope.

Waveform Characteristics Consider the following factors when studying a signal: ■■

■■

Amplitude: Check the consistency of the signal over time, watching for any changes. Verify that the voltage level is within the expected specifications of the circuit and component. •• For a signal whose amplitude should remain constant—e.g., a Hall-effect/optical crank or cam sensor—monitor the waveform over an extended period of time. Watch for any changes, typically a drop-off or missing output, either of which indicates a fault. •• Testing the amplitude of a magnetic pickup/permanent magnet (PM) generator crankshaft position (CKP) or camshaft position (CMP) sensor is different from the same for a digital sensor. Expect to see the voltage level increase with rpm and decrease as rpm decreases. In addition to monitoring the voltage change, observe the signal for dropouts, missing peaks, and reduced peak-to-peak voltage. Shape: Repetitive waveforms should be symmetrical. Any difference seen in the trace would indicate a problem. For example, if the traces are printed out on two separate sheets of paper and laid on top of each other, they should be identical. Rising and falling edges: These are especially important when measuring square waves and pulses. The rising and falling edge of the signal greatly effects the timing of a digital circuit (injectors and ignition coils on time/off time). To help identify a fault, the T/Div setting may need to be decreased. Reducing the time base to a faster setting will increase the resolution of the signal, thus showing any edge irregularities. Repeating waveform periods: Use the scope’s cursors to evaluate each period over time. Compare the signal during each period against each other, looking for any changes or inconsistencies. Momentary fluctuations: Brief transient changes have many potential causes. Poor connections, faulty components, voltage surges or sags are generally best found by using the scope’s buffer/record function.

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Chapter 12 Diagnosing with an Oscilloscope ■■

Drift: Signal drift is a gradual change in voltage over time. Drift can be hard to diagnose since it may occur so slowly. Temperature changes from thermal expansion (component, connections, wiring, or ambient), aging components, or faults in the voltage supply are the primary causes of drift. To find a concern resulting from excessive drift, monitor the signal’s value over an extended time (by increasing the T/Div setting). Use the following to help determine whether a signal is good or bad:

■■ ■■ ■■ ■■

■■

Rely on personal experience from using the scope on known-good vehicles. Keep a log of recorded vehicles and recorded patterns for future use. Look up known-good waveforms included on some scopes, such as Pico and Snap-on. Check the manufacturer’s service information: some manufacturers, such as Toyota and Hyundai, provide reference waveforms. Consult reference books.

Voltage Characteristics The type of voltage (AC or DC), the voltage range, the amplitude of the signal, the on/off (pulse) of a digital signal, and the frequency of a changing signal can be seen from the waveform. ■■

■■

■■ ■■

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Alternating current (AC): a flow of electrons that reverse their flow in a conductor. When viewed on a lab scope, the waveform alternates back and forth between a positive and negative polarity across the 0-volt line. Direct current (DC): the movement of electrons in the same direction, from a high potential to a low potential. A DC signal is linear and does not cross over the 0-volt line. Amplitude: the peak voltage of a signal, when measuring either AC or DC voltage. •• Peak-to-peak amplitude: a measurement of the highest voltage point in comparison to its lowest value. •• Peak amplitude: a measurement of how high or low a signal is past 0 volts. Pulse: a signal that results from rapidly turning a DC voltage on and off. Waveform: when viewed on a lab scope, the waveform alternates between a positive and negative polarity. When measuring an AC voltage, a cycle is the change in the sine wave from zero to a positive peak, back to zero, then to a negative peak, and finally back to zero (FIGURE 12-7). Pulse trains: a DC voltage that turns on (high) and off (low) in a series of pulses. Pulse trains, unlike an AC signal, do not go below zero. AC voltage goes above and below zero voltage.

Time Characteristics ■■

Frequency: how often the signal repeats in one second, which is measured in hertz (Hz). •• The more cycles that take place in one second, the higher the frequency.

FIGURE 12-7 Pulse width is a measurement of on time, measured in milliseconds (thousandths of a second). Using the cursors to measure the pulse width shows an 8.02 ms pulse width. When the cursors are used, a pop up box shows the measurement of each cursor and the difference, also known as delta, between them. The triangle (∆) symbol represents delta. Fuel injector on time is measured in milliseconds (ms).

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FIGURE 12-8 Measuring the frequency of a zirconia oxygen sensor. This sensor is switching at 2 Hz. Set the cursors to 1 second and count the transitions or cycles that occur between the cursors. The voltage range should vary between 0.2 volts and 0.8 volts; 0.2 volts indicates a lean exhaust and 0.8 volts a rich exhaust. A good sensor will switch from low to high and high to low at 1 Hz (one time per second). If the switching rate slows, suspect a failing sensor.

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•• Hertz can be measured in AC or DC voltage circuits (FIGURE 12-8). Period: how long it takes in seconds for a waveform to repeat. Duty cycle: the percentage of on time of a signal compared to how long the signal is off is one complete cycle. •• As on time increases off time decreases. The total of the on and off time equals 100%. •• A duty cycle can be either the time the signal is high, a pull-up circuit, or the time the circuit is low, a pull-down circuit. •• A 50% duty cycle means the signal is on and off for an equal amount of time. A 70% duty cycle indicates the signal is on 70% of the time and off 30%. Conversely, a 30% duty cycle means the signal is on 30% of the time and off 70%. •• Dwell is also used to measure on time, which is measured in degrees.

AC Voltage Signals AC sine waves have several characteristics to look for when using a scope to check their operation. The AC sine wave signals are very easy to produce because they need to show only a change from positive to negative, but they are not very good when the module requires a definite on or off (FIGURE 12-9). Because a sine wave has a large on and off portion, the sensor can say it is on longer than necessary and can indicate that it’s off for a longer period of time.

Permanent Magnet Generator Sensor Permanent magnet sensors are simple and reliable devices that can turn mechanical motion into an electrical signal. They use magnetism and generate electricity to generate a signal. To generate electricity, electrons need to be forced to move by intermittently passing a metal reluctor near the magnet, which creates an AC waveform. A metal reluctor is a toothed wheel that excites the magnet to generate a waveform (FIGURE 12-10). This waveform changes based on the speed of the reluctor moving past the magnet. The faster the reluctor moves past the magnet, the more closely the waveform moves together. The sensor is reliable and has minimal failures because it has no moving parts and isn’t powered up from an external source. The reluctor is a simple toothed wheel that does not physically touch the sensor, which means wear does not affect it. The major issue with the PM sensor is the air gap between the sensor and the reluctor (FIGURE 12-11).

Voltage Amplitude

0 sec

Frequency Time Period

Peak–Peak Voltage 1 sec

Time

2 sec

FIGURE 12-9 AC voltage amplitude and frequency should change with

rpm/speed in PM generator sensors. As rpm increases, the frequency and value of the voltage should increase. PM generators produce a voltage that goes above and below the 0-volt line. When operating properly, the positive and negative peaks should be about equal. Verify that the scope’s zero line is set in the middle of the screen, to monitor the peak-to-peak voltage changes.

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FIGURE 12-10 The reluctor excites the PM sensor to generate a

FIGURE 12-11 The air gap is crucial for a PM sensor to operate, because

waveform that the computer can understand.

it works off the change from ground to off ground, which generates an AC signal that is usable for the electronic control module (ECM).

FIGURE 12-12 A magnetic rpm sensor

waveform at 1200 rpm. While the idle and higher rpm patterns look the same at first glance, the transition time and voltage levels are different. Increasing the rpm causes the voltage output from the sensor to go higher and lower (peak-to-peak) and change faster compared to low/idle rpm.The peak-to-peak voltage level has increased to 8.54 volts.The time it takes for the voltage to change also occurs faster.The rapid switching of the signal requires a reduction in the time base setting to see the same amount of data compared to idle.The T/Div is set to 10 ms/Div instead of 20 ms/Div at idle.

The Basics of Measuring a Permanent Magnetic Sensor Generator Scope ■■ ■■ ■■

■■ ■■ ■■ ■■ ■■

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The higher the speed, the more spikes appear in the pattern (FIGURE 12-12). As speed increases, the sine wave pattern occurs more frequently and increases in height. The module monitors the frequency (Hz) of the sensor to calculate the speed of the component (FIGURE 12-13). 1 Hz equals one cycle per second. When the motors speed increases, the PM generator sensor’s AC voltage output increases. A PM generator’s output is affected by the air gap between the sensor’s tip and the reluctor. The correct air gap is essential for proper operation. All PM generators must be adjusted/installed correctly; some sensors are adjustable, but others are not. Corrosion that forms under the sensor pushes the sensor away from its mounting location, increasing the air gap on both adjustable and nonadjustable sensors. An excessive gap will have a significant effect on operation, especially at lower speeds. Excessive air gap may cause the voltage peaks to be so small that the module cannot read them, which will affect low-speed operation first. Increasing the air gap decreases the sensors AC voltage output. Low signal output with the correct air gap can be a result of a cracked sensor.

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FIGURE 12-13 A magnetic pickup/ PM generator signal during cranking. At low rpm, the voltage amplitude (peaks) are relatively low. Permanent magnet generators increase amplitude and frequency with rpm. As rpm increases the amplitude, the height of the output voltage and the number of events on the screen will increase. If the voltage does not get high enough during cranking, the vehicle will not start. Notice the V/ Div setting at 5 volts; the T/Div setting is 20 ms. Increasing the time sweep shows multiple events on the screen, including Cylinder 1 ID missing tooth. The time for one cycle is 89.61 ms.

FIGURE 12-14 When a knock happens,

the sensor generates an AC voltage signal that is read by the computer to indicate that the engine is knocking. This is the result of spark happening when it is not supposed to, and if it is allowed to continue, it could damage the engine. ■■

■■

If the magnet on the end of the sensor cracks, then instead of having one large magnet, there will be two smaller magnets. Smaller magnets produce less voltage than a larger magnet, due to the smaller ones having a weaker magnetic field.

Knock Sensor Knock sensors are piezoelectric devices that generate an AC voltage when sensing a knock. A knock sensor has a rapid response rate, requiring a low (fast) T/Div setting. Here the scope is set to 50 ms/Div and 5 volts (FIGURE 12-14). The amplitude of the signal depends on the severity and frequency of the knock. Knock sensors are “tuned” to produce a voltage that corresponds to a particular frequency, typically 15 kHz. The powertrain control module (PCM) responds by retarding ignition timing. Knock sensors are best tested by removing them from the engine, if applicable, and tapping them with a small hammer. If access to the sensor is restricted, use a tool that can be placed as near to the sensor as possible, then tap near the sensor while monitoring the waveform.

Alternator Ripple AC coupling can be used to measure alternator ripple. AC coupling removes the DC voltage, leaving only an AC voltage waveform. The voltage scale shown is not meant to measure the alternator output; instead, it is set to show the minimum and maximum voltage (min/max) of the alternator ripple (FIGURE 12-15). The amplitude of the waveform will vary depending on the battery’s charge. A fully charged battery produces a flatter pattern. A discharged battery will show a higher amplitude that gradually decreases as the battery charges.

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▶▶TECHNICIAN TIP A degraded or dead battery can cause undue harm to the electrical system and potentially create an engine drivability issue. When diagnosing an engine performance problem, make a habit of beginning by checking the battery to determine whether it is in operational condition. FIGURE 12-15 Known-good AC ripple waveforms on the left and abnormal ones on the right.

AC ripple is measured to check for excessive electromagnetic interface (EMI)/radio frequency interference (RFI), which can affect engine operation. Normal scope settings for AC ripple are 50 mV/Div and 2 ms/Div, with the scope AC coupled.

The alternator must be charging to maximize the effectiveness of this test. Raise the rpm and turn on as many electrical loads as possible, or use an adjustable carbon pile to load the alternator. Note: Resistive loads, such as lights and the rear window defrost grid, are preferred over blower motors and inductive loads that may generate electrical noise in the vehicle’s wiring harness and therefore the waveform. The even, high amplitude on the pattern indicate that the alternator diodes and stator windings are good. A faulted generator stator winding or diode will show a repetitive fault. If the alternator has a shorted diode or phase, it will show a pattern like the one above but with the base peak voltage exceeding 1 volt. It is not uncommon when a phase or diode shorts for the amplitude of the current to be so high that the waveform goes off the top and bottom of the screen. A diode fault will reduce the current output of an alternator by about one-third (33%). For example, a 150-amp alternator’s output will be reduced to approximately 100 amps.

Potentiometer Waveform Diagnosis:Throttle Position Sensor/Analog Voltage Signal A potentiometer is a sensor that states a position of a mechanical device in electrical terms that the ECM can understand. Most potentiometers have a 5–8 V reference voltage on one pin, a ground wire, and a signal wire. The position changes in the sensor based on the mechanical change in the component. Position changes move the sweep arm in the sensor, and this movement changes the voltage from zero to the maximum voltage in the circuit to indicate the position of the mechanical device (FIGURE 12-16). The ECM changes its strategy based on the mechanical position of the component. The throttle position sensor (TPS) is one of these potentiometers. It tells the PCM where the throttle blade is, which allows the PCM to change the fuel and ignition system’s parameters to meet what the driver is requesting. To perform a TPS sweep test, proceed through the following steps. TPSs produce an analog DC voltage that increases as the throttle opens. The scope is set at 5 volts since the maximum expected output voltage is under 5 volts. The time base is set to 200 ms/Div (0.2 seconds total sweep). Slowly open and close the throttle (FIGURE 12-17). A fault with the wiper or contact strip would be shown by a sudden drop in voltage. It is important to open PCM

Throttle Position (TP) Sensor

416 GRY

417 DK BLU

452 BLK

5 Voltas Reference TP Sensor Signal Sensor Ground

FIGURE 12-16 When a technician uses an oscilloscope, if they probe the signal wire while moving the component,

it will produce a changing waveform that the PCM uses to determine the amount of throttle the driver is requesting.

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FIGURE 12-17 When viewing a

potentiometer’s waveform, no dropouts or glitches should appear in the signal. If the signal is not smooth and steady, suspect that the potentiometer is faulty.

and close the throttle as slowly as possible to maximize the effectiveness of the test. Faulty TPSs cause multiple drivability faults, including stalling, a hesitation on acceleration, and shift timing or missed gear errors on electronically controlled transmissions. Vehicles with electronic throttle control use two TPSs and accelerator pedal position (APP) sensors that operate as a TPS. A fault in any of the sensors can cause the PCM to limit the driver’s throttle control. Electronic throttle body (ETB) defects typically produce sudden surges, dangerous stalls, a hesitation on acceleration, and the PCM limiting the throttle opening, reducing power, known as limp-home mode. Limp-home mode limits acceleration and engine power to a predetermined level set by the manufacturer.

Hall-Effect/Digital Speed and Position Sensor These sensors require an input voltage to operate. Hall-effect sensors are three-wire sensors consisting of signal (output), power (supply voltage), and ground circuits. Magnetoresistive sensors may have two or three wires. Two-wire magnetoresistive sensors use either the power supply or ground wire as a dual-purpose circuit that also carries the signal. Different manufacturers use different operating voltage levels. The most common are 5 volts, 8 volts, and 12 volts (system voltage). The height, amplitude, of the waveform, does not change with rpm; it remains constant at whatever the input voltage is. Unlike PM sensors, Hall-effect/ magnetoresistive do not develop their own voltage. The PCM monitors the change in frequency to determine speed. Increasing engine rpm speeds up the frequency of the signal (FIGURE 12-18). The time base may need to be changed if the signal is checked at various rpm, since the pattern will get faster or slower with engine speed.

FIGURE 12-18 When viewing a Hall-

effect sensor’s waveform, the technician should look for clean transitions from on to off. Also, the waveform should reach the full voltage of the sensor to determine whether it is operating correctly.

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Chapter 12 Diagnosing with an Oscilloscope

Fast, clean, sharp switching events are characteristic of a good Hall-effect/magnetoresistive waveform. The amplitude of the signal should reach the power supply voltage and 0 volts on the ground for proper operation (FIGURE 12-19). The PCM needs to observe these voltages to determine whether the signal is switching from high to low and low to high. Failure to reach the correct minimum and maximum voltage levels can confuse the PCM, causing improper operation. Amplitude problems include a decrease in voltage at the start of a waveform, known as droop or failing to reach the full amplitude. For example, a CMP sensor, sometimes referred to as a cylinder identification sensor (CID), is a digital signal with varying patterns (signal notches) to identify the location of a component. The square wave goes up and down from 5 volts to 0 volts. Hall-effect/magnetoresistive sensors will not move below the 0-volt line. The on time and off time of the component is measured on the square wave waveform so that the technician can understand at what duty cycle the component runs. Duty cycle is expressed as a percentage of on time versus off time of the component. For example, 50% duty cycle means the component is on half of the time (FIGURE 12-20). By running the component half of the time, it increases the life of the component because it is being used half of the normal time. The ECM controls the duty cycle based on the operation of other systems in the vehicle. The following are some special rules for digital signals: ■■

The ground must be 0 to +0.5 volts max. Any voltage difference over 0.5 volts may cause modules or computers not to respond appropriately.

FIGURE 12-19 The waveforms above are screenshots from a GMC 2500 delivery van with an intermittent stalling concern and no stored

diagnostic trouble codes (DTCs). Using a DSO resulted in finding a failing CKP sensor, as seen by the Hall-effect square wave shown on the left. The upper and lower sections of the trace are rounding badly, failing to reach the required source voltage value or zero volts (ground) intermittently. As the operating temperature increases, the signal continues to deteriorate, matching the complaint that the stalling condition was worse after extended driving. The screenshot to the right is the CKP pattern after replacing the sensor. Notice the uniformity of the trace and the evenness of the events at source voltage and ground.

FIGURE 12-20 Duty cycle of a pull-

down circuit. In this example, the purge valve was commanded by a scan tool at a 50% duty cycle. System voltage (12 V) is applied 50% of the time, and the signal is pulled to ground by the PCM driver 50% of the time. During purge valve operation, the higher the duty cycle, the more vacuum that is applied to the carbon canister.

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369

The measured source voltage must be +0.5 volts of the supply voltage; otherwise, the module s and computers may not react correctly.

The Basics of Measuring a Hall-Effect/Magnetoresistive Sensor Scope ■■

■■ ■■

■■

■■ ■■ ■■ ■■ ■■

■■ ■■ ■■ ■■

Voltage remains the same regardless of speed, because unlike a PM generator, the Hall-effect switches the 5-volt reference signal only on and off. When measured with a voltmeter, a 5-volt signal will show 2.3 to 2.4 volts due to averaging. The frequency of the signal changes as speed changes. The cycles are not affected by voltage output. As speed increases, the square wave pattern occurs more frequently but does not increase in height, because the sensor is switching on and off. The module looks at the frequency of the sensor to calculate the speed of the component. When speed increases, the Hall-effect sensor’s DC voltage stays the same: at source voltage. When the speed increases, the Hall-effect sensor’s frequency follows and increases. A Hall-effect sensor is not greatly affected by small changes in the air gap. When increasing the sensor’s air gap, the frequency and voltage output will stay the same, within reason. If the gap becomes excessive, the sensor may not switch. Hall-effect sensors create a digital DC voltage. The voltage is always positive and is either on or off. Hall-effect sensors are used as CMP sensors, speed sensors, and location sensors.

▶▶ Waveforms

from the Ignition Coil and the Fuel Injector Operation

An ignition coil, like a fuel injector, uses the principles of magnetism and induction to generate spark. The ignition coil operates by creating a magnetic field when the controller (known as the PCM, ignition module, or igniter) grounds the primary windings (FIGURE 12-21). When the module releases the ground, the magnetic field collapses, inducing a voltage into the secondary windings of the coil. The high voltage then travels to the spark plug, jumping the gap and thus igniting the air-fuel mixture. Using the properties of magnetism, induction allows power to be increased to operate the component more efficiently. Looking for which part of the component that has failed or is failing can help the technician to diagnose the problems with the circuit. A fuel injector is similar to an ignition coil in that it uses a low voltage (12 V) to power a coil to produce another action. In this case, the other action is opening the pintle with the coil inside the injector. Because it is an electrical coil, it can be viewed on an oscilloscope (FIGURE 12-22).

12-03 Analyze magnetism and induction waveforms.

FIGURE 12-21 Understanding what is

happening in the waveform allows the technician to target specific areas as they diagnose the system. For example, if the source voltage is making it to only 8 volts, the technician might start their diagnosis at the power source for this ignition coil. If the power source is not 12 volts, then the rest of the waveform will not reach the voltage that it should for proper operation.

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Chapter 12 Diagnosing with an Oscilloscope

FIGURE 12-22 A typical port

fuel injector pattern. Injectors are electromechanical solenoids that use system voltage to operate. The PCM opens the injector by pulling the voltage low, creating a magnetic field that moves the injector’s pintle. The amount of time the injector is open depends on inputs from various engine sensors.

Ignition Primary ■■

■■

Source voltage: This is the system voltage available to the positive side of the coil before it is grounded. This voltage should be within 0.5 volts of system voltage. If the technician is viewing an ignition coil that is not limiting current, the voltage level may be difficult to read due to the high scaling required to capture the inductive kick from the coil. Dwell period: There is no current flow in the coil until the dwell period begins. The dwell period is when the primary voltage is grounded. When the voltage is grounded, current flows through the primary windings of the coil, building a magnetic field. This is known as coil saturation time. The primary windings must be grounded long enough to create a sufficient magnetic field that when it collapses (ground removed) generates an intense spark capable of igniting the spark plug. Typically, the amount of current flow in most ignition coils is 6 to 8 amps. If the coil saturation (dwell) time is too short, the spark will be weak and may result in a misfire, especially under load. Induced voltage: This is the voltage created when the magnetic field collapses. This event is known as magnetic induction. Removing the ignition primary at precisely the right time induces a voltage into the secondary coil windings to fire the spark plug. Burn time: Spark is jumping the gap between the spark plug electrodes, igniting the air-fuel mixture. Expect the burn time to be one to two milliseconds. A lack of burn time is a sign of excessive resistance in the ignition primary circuit, coil spark plug wire, or spark plug. If the burn time is excessively long, look for something that would lower the primary circuit resistance. Low resistance faults can include a fouled spark plug, narrow plug gap, rich fuel mixture, or a plug wire/coil boot arching to ground.

Analyzing the waveform consists of the following: ■■ ■■

■■

System voltage, the beginning voltage level at the injector, must be monitored. The voltage is being pulled low by the PCM as it grounds the coil, creating a magnetic field. This voltage needs to reach 0 volts. If the voltage does not reach 0 volts, suspect a fault control wire or weak/failing PCM driver (transistor). The PCM has turned the injector off by removing the ground. The time from number 2 to number 3 is the on time of the injector and is measured in milliseconds. The collapsing magnetic field in the solenoid generates a voltage known as an inductive kick. The height of the voltage spike varies by vehicle. Some manufacturers limit the voltage from 35 to 50 volts by using a diode or clamping resistor to clamp the voltage. If the height of the voltage spike does not reach at least 35 volts, suspect weak/deteriorating

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■

371

coil windings in the injector or excessive voltage drop, reducing available amperage to the injector. The height of the inductive kick can be used for comparison, to find a weak injector on vehicles that do not regulate the voltage. The pintle hump shows that the injector pintle is moving (opening). The lack of a hump would indicate a stuck injector pintle.

Using the Trigger Mode for Intermittent Conditions There are times when manipulating the trigger mode is beneficial to diagnosis. Setting the trigger level to record only when desired is useful for intermittent conditions. Advanced users may set multiple channels to record only when a failure occurs. Advanced trigger techniques should be used only by a user who thoroughly understands and is comfortable with basic trigger operation and function. The technician must know the normal operating range of the sensor or component to perform this test. Select Normal mode for the trigger, and set the trigger voltage level higher or lower than the normal operating range of the component. If the signal remains within specifications, the scope’s screen will continue to be blank since the voltage did not cross over the trigger level. If the signal fails and passes over either of these settings, the scope will start drawing the waveform. Using or selecting the wrong trigger mode or level at the wrong time can prevent the fault from being found. For example, when attempting to diagnose a stall while under driving conditions, the list of probable causes is quite long. A brief list of possibilities includes a loss of ignition due to a failing crank sensor or loss of fuel pressure. Setting the trigger voltage to draw a pattern for any of these components only when a signal is out of its normal operating range can pinpoint the fault. Using a multichannel scope equipped with extended test-drive leads can zero in on the failure during a test drive. Connect one channel to the CKP sensor, one channel to the fuel pump power circuit, one to the fuel pump control, and another to a pressure transducer to measure fuel pump pressure. Test-drive while monitoring the signals and using the trigger to start drawing the waveform when the signal goes outside of its intended range, determined by the selected setting.

Using the Scope to Catch an Intermittent Glitch One of the most useful assets of a DSO is its storage ability, also known as a buffer. The buffer allows users to look back at different screens when a fault has occurred. The amount of data or samples stored varies by the scope and its settings. For example, a Pico Scope will store a maximum of 32 million samples of data. As soon as the scope displays the pattern on the screen, the buffer will begin to capture and store the information. Stopping (freezing) the display and then restarting clears the buffer and resets the counter. Typically, by the time fault is felt or noticed and the waveform is frozen on the screen, the glitch will already be gone. Using the buffer provides a searchable library of data, leading up to and including the glitch, for analysis (FIGURE 12-23). Once the glitch has been found, the waveform can be zoomed in on to find a fault. The number of events recorded depends on how long the scope was running for and its storage capacity.

Peak Detect Mode Peak Detect mode is available on some scopes to capture glitches that may be missed due to the scope’s settings or sampling and update rate. When using this mode, the scope saves the minimum and maximum value sample points taken during two waveform intervals and then uses these samples as two consecutive waveform points. The scope then connects the samples (dots), showing the glitch. DSOs with peak detect run the analog-to-digital converter at a fast sample rate even when setting the scope to a slow T/Div settings. Remember that slow time base settings result in long waveform

FIGURE 12-23 The sample rate of a DSO varies with the time base

settings. The slower the T/Div setting, the slower the scope samples the signals. Selecting too slow of a time base can cause the scope to miss a glitch if it falls in between samples, as seen above. To overcome missed faults, some DSOs include a Peak Detect mode, which captures fast transients (glitches) at slow sweep speeds.

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Chapter 12 Diagnosing with an Oscilloscope

sample intervals. The Peak Detect mode can capture a fast-changing signal that would occur between the normal waveform sampling points when in sample mode (non-peak detect). Another use for Peak Detect mode is for measuring narrow signal pulses that are spread far apart in time. Widely spaced signals simulate a long time base (T/Div) setting, slowing down the scope’s update rate.

Using the Sample Rate Manipulating the sample rate of a scope is an advanced function for experienced scope users. The default sample rate selected by the manufacturer is where the scope will operate most of the time. All scopes have a limited amount of data storage in their buffer. The amount of detail, known as the sample rate, and the time base settings combine to determine the amount of the data being stored. Either increasing the sample rate or extending the time base will use the storage capability, decreasing the amount of stored data available for review (FIGURE 12-24). By changing the sample rate or the time base, the scope can be manipulated to find a fault. However, increasing or reducing the rate excessively can result in the scope missing and failing to record the event. Altering the sample rate changes how often (interval) the scope takes a “picture” of the signal and displays it on the screen. The sample rate, the number of pictures taken per second of the signal, is adjustable on some scopes. Varying the sample rate, if available, on the scope being used may help locate an intermittent concern. For instance, increasing the number of samples records and displays the signal more often for glitch capture while providing more clarity and definition in the waveform. However, if it set too high, it decreases the number of “pages” of data the scope can record. By reducing the number of pages of data, the scope may miss the fault if it occurs when the scope is not recording. Lowering the sample rate increases the number of pages of data captured but also reduces the definition of the signal. If the rate is decreased too much, the waveform will become choppy and inconsistent. The resulting pattern will likely have poor definition, making analyzing the waveform to find a glitch difficult. Also, when the sample rate is set too low, the glitch can occur in between snapshots, and the scope may not be recording when the fault occurs. Increasing the T/Div or total sweep time also affects the storage buffer. The slower the update rate, the more the samples are spread out. The size of the gap between samples depends

FIGURE 12-24 Screenshots showing the difference in quality when the sample rate is reduced. The waveform to the left is

smooth and consistent even when zoomed 20×. The scope was set to 1 MS (one million samples), resulting in a sample rate of 20 MS/s (20 million samples per second) at a 50 ns. (nanosecond) interval. The screenshot to the right is of reduced quality. The scope was set at 10 kS (one thousand samples). The sample interval slows to 200 kS/s or 200,000 samples per second, with an interval between samples of 5 µs. (microseconds). Notice that when reducing the sample rate, the trigger has a difficult time locating the signal due to the inconsistencies in the pattern from the extended capture time, so the waveform runs on the screen. Observe the trigger and the waveform above. With a high sample rate on the left, the pattern lines up with the trigger. The reduced sample rate on the right shows that the trigger and the waveform are not aligned.

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on the storage buffer size and sampling rate. Slowing the sample rate down (high number, usually seconds per division) also increases the amount of storage used. If a slow time base and a high sample rate is used, the amount of data stored will be drastically reduced. To extend the time base and maintain the detail of the waveform, the sample rate may need to be reduced. Adjust each setting and recheck the display. If the pattern becomes notchy and loses detail or if the number of stored pages is reduced significantly, adjust one or both back and retest. The belief that if some is good, then more is better does not always hold true when adjusting the sample rate. It’s easy to oversample a pattern, reducing the amount of data stored, which results in missing a glitch. There is a fine line between taking enough samples to catch and record the glitch and still providing enough recorded data and signal clarity. For example, setting the scope to sample the same point on a waveform 10 to 20 times will probably be of little value diagnostically while using substantial amounts of the data buffer, reducing the number of events the scope can capture. Again, manipulating the sample rate is an advanced technique that requires considerable practice. For the novice or inexperienced user, the default setting of the scope will usually be sufficient for most diagnostic scenarios.

Snap-on Scope Method to Glitch Catching Snap-on scopes operate differently than Pico Scopes. First, they do not break the screen up by division. Time and voltage settings are totals for the screen. So, selecting 20 volts and 10 milliseconds means the scope screen will show any voltage under 20 volts and the total sweep time is 10 ms for the entire screen. For most everyday scope use, when using a Snap-on scope rather it is the Verus, Modis, or Vantage start fast, with a 50 or 100 ms screen, then the time base can be adjusted up or down as necessary. To capture glitches when using a Snap-on scope, then, use a different strategy than the one for a Pico Scope. When setting up a Snap-on scope, adjust the time base to a fast capture (Pico uses slow). The time base will be shorter than the signal being looked at. If the scope is set up right, no trends will probably be seen and the time base will appear too fast (FIGURE 12-25). Seeing all the events on the screen is not important with a Snap-on product. By running a faster time base, the scope stores the data in the buffer instead of displaying it on the screen. This is the way the scope is intended to be used, which takes practice since it is different than other scopes. The buffer consists of all the stored data that are then used to analyze the waveform. Running the scope at too slow of a time base slows down the scope’s recording ability. Sampling at slow speeds on a Snap-on scope drops too much of the signal, so the pattern drawn by the scope may or may not be accurate. A slow time base produces false gaps that appear as signal faults in the waveform when zooming, leading to a misdiagnosis. The buffer on a Snap-on scope can store about 260 screens. To zoom, pause the scope first. After pausing, all the data stored are accessible for viewing (FIGURE 12-26). From here,

FIGURE 12-25 Maximum zoom

highlighted and in use in this screenshot. To use the buffer correctly, start with a fast time base, fill the buffer, press pause, and zoom in to find a concern. Slowing the sweep time to see everything on the screen for live viewing will show gaps when zooming on the pattern, creating false gaps. These holes in the data can be mistakenly interpreted as an issue when there is not an actual concern.

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CHAPTEr 12 Diagnosing with an Oscilloscope

▶ TECHNICIAN TIP When using the zoom function on a Snap-on scope, it cannot be zoomed below what the initial time sweep setting. In other words, if the original sweep was 200 ms, then that is as close as it can be zoomed in.To figure the amount of zoom on a Snap-on scope, multiply the zoom selected by the original time selection.

the collected data can be saved, or view the stored waveform can be viewed without saving. Choosing the Save All Frames option stores the entire buffer. After pausing the scope, a magnifying glass icon will appear. The magnifying glass allows the saved screens in the buffer to be manipulated. After capturing the data, the stored data in the buffer can be zoomed out on, for analysis. Again, this operation is the opposite of that for a Pico Scope. When zooming on a Pico, the user zooms in on the waveform. With Snap-on, the user zooms out. The Snap-on records all the data and compresses them. To see details, zoom out, which open up the capture. The maximum zoom (zoom out) takes all the recorded screens and compresses them into one screenshot to see every bit of stored data in the buffer. The zoom bar is displayed on the right-hand side of the screen. Select any zoom level. To view more detail, step down to a lower zoom level (FIGURE 12-27). When changing the zoom level, the buffer bar indicates how much of the buffer is being viewed. The white dotted bar that appears when using the zoom function is the zoom cursor. Every time the zoom level changes, the cursor will be in the center of the screen. Move the cursor near the next area to be zoomed in on, and select lower zoom value. Each zoom level shows a different time on the screen. Continue moving the cursor and zooming until the fault can be viewed in detail.

FIGURE 12-26 Using the Snap-on

Verus lab scope in zoom mode to catch a glitch. The original scope screen was set at 100 milliseconds with the engine running. After noticing the fault, stop the recording. While running, the buffer is recording and storing the data. Select the zoom button on the bottom right of the screen. With the zoom bar displayed on the right-hand side, click on any desired zoom level. After stopping the recording and selecting zoom, the dropout is obvious. Here, the zoom is set to Max. The black area in the middle of the yellow signal indicates a fault.

FIGURE 12-27 Setting the zoom to

16× produces more clarity, making the fault apparent. The white dotted bar is the zoom cursor. Whenever the zoom level changes, the bar will always return or remain in the center. That bar can be moved wherever; just drag it.

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SKILL DRILL 12-1 Frame-by-Frame Snap-on Scope Glitch Detection Step-by-step use of the Zoom function using a Snap-on Vantage. Remember that on any Snap-on scope, set the time base fast to fill the buffer, then use the Zoom feature to isolate the fault. 1. Show the scope set to capture the waveform. Remember on Snap-on scopes to set the time base fast and use the buffer to find the fault. With fast-moving signals, such as this one, being able to view the fault as it’s occurring live is very difficult. Use the tool to help. With the engine running, a wiggle and tap test was performed on the harness and the CMP sensor. The Capture/Pause button is highlighted in blue on the top of the scope in the scope’s banner.

2. Use the capture highlight to capture the icon, and select Yes to freeze the waveform and access the buffer. Scroll the timeline buffer using the arrow to find the fault. In this screenshot, the solid band of yellow indicates that the error is not present on this screen, so continue scrolling through the buffer.

3. Scroll through the buffer to find the black gap in the yellow waveform, showing a signal fault.

Continued

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Chapter 12 Diagnosing with an Oscilloscope

4. Use the Zoom feature, the magnifying glass icon at the top of the screen, and select a zoom setting, narrowing in on the fault. In this example, −16× has been selected.

5. Once the error has been found, continue zooming in on the fault—in this frame, −8× to add clarity to the glitch.

6. The glitch becomes more apparent at −2×.

7. The signal dropout is very evident, and the fault has been found.

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Connecting Transducers

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▶▶ Connecting Transducers Oscilloscopes measure voltage only. Multiple adapters (transducers), such as current probes, temperature, vibration, pressure, and vacuum transducers, increase the scope’s ability to read other signals. Transducers convert a measurement into a voltage signal that the scope can display on the Y axis (vertical) (FIGURE 12-28). Transducers modify the signal differently. Common methods include an inductive/Hall-effect sensor in amp clamps, a strain gauge to convert pressure, thermocouples to modify temperature, and an accelerometer for vibrations. The scope’s screen must be set to match the transducer in use. Most automotive-based DSO software includes presets for frequently used transducers (FIGURE 12-29). If a preset is not included, the setup will require simple math to convert the voltage scale. For example, most low-current amp clamps have a setting for 100 millivolts = 1 amp. High-current clamps may use a conversion of 1 millivolt to 1 amp. Adjust the voltage scale to match the output of the transducer (FIGURE 12-30).

12-04 Explain the use of transducers and inductive pickups with an oscilloscope.

FIGURE 12-28 An in-cylinder pressure test waveform. A pressure transducer takes a physical quantity

and changes it into an electrical signal that is viewable on the scope’s screen. Pressure transducers can check cylinder compression, fuel pressure, oil pressure, exhaust pressure, crankcase pressure, and cooling system pressure. For the drivability technician, in-cylinder pressure testing is one of the most beneficial and informative types of testing. With practice, a skilled technician can tell the overall mechanical condition of a cylinder, including compression pressure, the difference in compression between cylinders, valve opening, sealing and timing, and cam timing.

FIGURE 12-29 Using a transducer to change a pressure to an electrical waveform allows the

technician to view the change in pressures next to other readings.

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CHAPTEr 12 Diagnosing with an Oscilloscope

Inductive Probes Most DSOs trigger off an incoming signal, as previously discussed. There are times when an external trigger source is needed, and many modern scopes have this option. Inductive ignition probes operate like an amp clamp. Uses for inductive probes include acquiring secondary ignition patterns for diagnosis or substituting them for a sync probe to trigger the DSO off a spark plug wire or coil-on-plug (COP) (FIGURE 12-31). When high voltage passes through the probe, it produces a voltage and sends it to the lab scope. External inductive triggers are known as sync probes. Sync probes that clip around a spark plug wire look like the end of a timing light. Connecting the inductive probe to the scope is the FIGURE 12-30 Available presets for a Snap-on Verus four-channel lab scope. All same as all leads. Select a channel and attach the probe. these options are included in the Verus scope. Select the preset that matches the transducer or ignition probe being used. The scope changes and adjusts the vertical Set that channel as the trigger. Start with the scope in axis (voltage) from voltage to the proper signal, to correlate with the selection. Auto Trigger mode to draw a pattern. Once a waveform is shown, adjust the trigger level to approximately half way up the spark line. Set the Trigger mode to Normal, with a rising (positive) slope (FIGURE 12-32). ▶ TECHNICIAN TIP Most automotive DSOs include a preset for an ignition probe, making the process easier. If a preset When using an inductive probe to check is being used, the scope settings may need to be fine-tuned once the engine is running. a COP, obtaining a signal to display on the The connection on the ignition component varies. The pickup may have an indicator stamped scope is more a result of the coil design on it that faces the spark plug (similar to the arrow on an amp clamp). Commonly, Cylinder 1 is set than the quality of the probe. Not all coils as the sync trigger for a timing reference point to stabilize a waveform, such as an AC waveform will produce a signal. Two-wire coils offer from a crank sensor, or for establishing which pulse is from which cylinder when checking relative the best results. Thus, almost all probes compression, fuel injector or coil amperage, and valve timing. Sync probes are directional. If a can pick up a signal. Picking up a signal clean sync signal isn’t produced when using an inductive sync probe, turn it over and retest. with three-wire coils is hit and miss. FourA trigger signal is a useful aid for waveform analysis in a variety of situations. A trigger wire coils give off virtually no signal to pickup produces a pulse and sends it to the scope. Uses for sync triggers include holding/ produce a waveform, no matter which probe or how expensive it is. Three- and stabilizing a repetitive signal, judging the timing of an ignition event, and evaluating a sigfour-wire coils, as a rule, require using nal to the position of the crankshaft. COP extension leads with an inductive Using the crankshaft sensor signal for a sync trigger does not always work. Many crank probe to produce a usable signal. signals are repetitive without any specific characteristic to trigger from. Using a repetitive signal for a trigger will cause the signal to walk across the screen. Using a trigger pickup stabilizes the waveform for analysis.

Inductive Probe Use

An amp clamp is measuring injector current. Measuring both signals together verifies that the coil is being controlled. Without a primary ignition event, the injector will not be controlled and the waveform will be missing. If a fault exists in the injector or its circuitry, the

FIGURE 12-31 COP adapters are available to connect an inductive clamp to a COP coil for a trigger signal or ignition system diagnosis. To use one,

remove the coil and attach the spark plug wire adapter between the coil and plug, then connect an inductive pickup for a trigger or a capacitivetype pickup for ignition system diagnosis to the wire. Use the ground terminal when the COP is attached with a retaining bolt. Install the ground bolt to the coil, and then ground the lead back to the cylinder head by using the coil hold-down bolt. Failure to use the ground can damage the coil.

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Connecting an Amp Clamp

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FIGURE 12-32 Snap-on Vantage with an inductive sync trigger lead

FIGURE 12-33 Using an inductive ignition probe for a sync signal to

attached. There are two types of ignition probes available: inductive and capacitive. Inductive leads provide a signal for triggering. Capacitive leads provide a signal with detail of the ignition waveform. A good trigger pickup will shape the ignition signal into a clean pulse that can be used on the scope for a trigger.

identify top dead center (TDC) of a cylinder during an in-cylinder pressure test. During cranking, the ignition event should occur at TDC of the pressure waveform. In this waveform, the timing is advanced since the ignition timing is to the left of TDC. If the ignition event were to occur to the right of TDC, the timing would be retarded.

current ramp of the injector will not take place. Using both an inductive probe and a transducer, the inductive pickup can help the user view the transducer at the point that the event happens (FIGURE 12-33). Connecting the pressure spike to the trigger through an inductive pickup helps to identify what may be causing the spike in pressure.

▶▶ Connecting

an Amp Clamp

The most common transducer in use is generally the amp clamp. The amp probe converts 12-05 Use current ramping and an amp clamp to aid in diagnostics. current measurements into a calibrated voltage output for the scope to display. Amp clamps are available in low- and high-current designs. Depending on the amp clamp, low-current probes can generally measure from 30 to 60 amps. Low-current probes may include one or two settings. The low setting generally ranges from 20 to 30 amps and the high setting up to 60 amps. Using a low-current amp clamp to display a waveform is known as current ramping. Current ramping requires two tools: a DSO and a low amp clamp. High-current models generally measure current flow exceeding 30 to 60 amps (FIGURE 12-34). One of the most powerful diagnostic tools a technician can use is current ramping. Analyzing a picture of a waveform can find faults in outputs that voltage and resistance testing fail to catch. Current waveforms provide an indication of the magnetic field strength, the overall circuit resistance, and the integrity of the coil windings, and they can show mechanical movement (fuel injectors). By monitoring current, a technician can determine how much work is being done and the circuit’s and components’ performance. Current ramping is a favorite among technicians in the field for testing fuel injectors, ignition coils, solenoids, and fuel pumps with intermittent conditions. If the windings of the coil are beginning to break down, current ramping can produce a waveform that will show the fault before it becomes bad enough for voltage or resistance testing to find. When connecting an amp clamp, the lab scope does not know it is connected to a current probe. The scope displays a volt- FIGURE 12-34 Selecting the correct amp clamp is necessary to get age signal that is interpreted as a current pattern (FIGURE 12-35). the correct reading on the oscilloscope. When connecting the amp Current probes produce a voltage output that depends on the clamp, remember that the two theories for current flow: electron and amount of current that flows through the probe’s jaws. Remember conventional. Electron theory states that current flows from negative that whenever current flows in a circuit or a wire, a magnetic field to positive. The automotive industry relies on the conventional theory, is generated. The strength of the magnetic field is controlled by which states that current flows from positive to negative.

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Chapter 12 Diagnosing with an Oscilloscope

FIGURE 12-35 The arrow (some clamps use a plus sign) on the

amp clamp points in the direction of current flow. The location of the arrow varies by manufacturer ; it will be either external, as shown here, or in between the jaws. If the pattern is upside down or fails to display, the clamp may be installed backward. Flip the clamp so that the arrow points in the opposite direction, and then retest.

the amount of current flowing. More current produces a stronger field. The clamp uses a Hall-effect switch to measure the magnetic field and then output a corresponding voltage. Uses for high-current amp clamps typically include starter and alternator testing. Diagnostic techs have extended the use of the high-current clamp to check relative compression by using the starter’s current draw. Low amp clamp usage includes testing an extensive list of electrical components, such as ignition coils, fuel injectors, fuel pumps, solenoids, parasitic draw, and relative compression (FIGURE 12-36). The advantages to using an amp clamp are that the circuit does not need to be disturbed and that the amp clamp can be placed anywhere in the circuit. Amp clamps display the dynamic operation of a component, identifying faults that may not be detected by using traditional testing methods that measure voltage and resistance. ▶▶TECHNICIAN TIP Current ramping and measuring amperage flow are dynamic and essential diagnostic tools. Nonetheless, measuring current is not always the best test. The following are some of the reasons not to current ramp or measure current: ■■

Sensor circuits: Sensors use voltage changes, not current, to transfer information. Most sensors operate off minimal current that only a microamp clamp can measure.

FIGURE 12-36 Relay cavity jumpers and connector and relay adapters are available to form a current loop or an attaching point for amp clamp

installation. Use only fused jumper wires with the same size of installed fuse that the circuit uses.

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Connecting an Amp Clamp ■■

■■

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High voltage/low current: The combination of high voltage and low amperage is generally best tested by using voltage. Finding a fault with minimal current flow is difficult and requires expensive tooling (microamp clamp). Finding excessive resistance: Voltage drop or circuit load testing are the best ways to find a resistance concern. Voltage drop has an enormous impact on low-current circuits, including communication networks and sensors. Current waveforms indicate overall circuit resistance, but they fail to find the source or location of the high or low resistance. Excessive resistance will decrease current flow, whereas low resistance will increase current flow. Since the current flow in these circuits is minimal to begin with, current change due to resistance is negligible. Current can be used, however, to find internal resistance changes in actuators and outputs due to temperature swings and vibration.

SKILL DRILL 12-2 Connecting a Current Probe to the Scope Once the scope has been set up, connecting the current probe is just like connecting a DMM. 1. Select the electrical circuit for testing.

2. Turn the probe on, verify the strength of the internal battery, and connect the probe to the scope.

3. If equipped, set the scope’s calibration to match the probe.

Continued

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Chapter 12 Diagnosing with an Oscilloscope

4. Preliminary scope settings include the following: • volts per division, if not a preset set to the scale of the probe being used • setting the time base the same as for voltage or slightly slower • setting the Trigger mode on Auto • trigger slope—rising edge/positive • trigger level slightly lower than half the expected output. • If the current level is unknown, set the level just above 0 volts, between 20 and 50 mV.

5. Zero the probe before attaching it to the circuit.

6. Since current is the same anywhere in a simple circuit, choose a location that is the easiest to access. (If the probe is installed with the arrow pointing in the wrong direction, the reading will read opposite of it should.)

7. When selecting a connection point, either determine where the wiring harness goes and provide access to the power wire, or use a loop to tap into the fuse in the fuse box. Note: If the wire feeds multiple loads, the current being measured will include all loads. For example, don’t connect the clamp to the injector or ignition coil power supply, since most manufacturers share this circuit with all the coils or injectors.

Continued

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8. Adjust the scope’s T/Div and V/Div settings to provide the best signal.

9. Analyze the signal and determine what the next step should be in diagnosing the problem with the engine.

Current Probe Scaling Most scopes have presets to match the amp probe being used. Some probes have more than one output setting. Match the selection switch on the amp clamp with the scope channel preset to ensure accurate measurements. Synchronizing the probe to the scope produces a clear, interpretable pattern. Some scopes do not have presets for probes, or they do not list a setting that matches the output range of the probe. When using a scope without preset ranges, set the scope to a DC voltage scale matching the output of the probe. The current clamp’s output for each value of the tool can be found next to the selection switch. Amp clamps output millivolts. When using an amp clamp, the voltage output of the probe must be matched to the scope. For high-current clamps, the scope’s typical input setting is 50 mV/Div to 100 mV/Div, depending on the probe’s output. Low-current probes ordinarily have two calibration settings controlled by a slider switch. The two settings may include a 20-amp and 60-amp setting. Each range requires a different scope setup (FIGURE 12-37). For example, when measuring current up to 20 amps (20-amp setting), set the scope for 1 mV/10 mA (100 mV = 1 A). Moving the clamp’s switch to measure higher current, up to 60 amps, set the scope to read 10 mV/100 mA (10 mV = 1 A).

Current Waveform Analysis Current waveform analysis, like all scope analysis, requires regular practice and known-good waveforms for comparison. To interpret the waveform, a technician will need a basic understanding of how circuit integrity, including the component, affects current flow and therefore

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CHAPTEr 12 Diagnosing with an Oscilloscope

FIGURE 12-37 Setting the scope to a preset that matches the amp clamp. Selecting the 60 A current

clamp in 20 A mode adjusts the voltage values to amperage. Using the scope’s presets prevents the technician from having to do the math of converting the voltage output of the clamp to amperage.

TABLE 12-2 Typical Current Probe Output Settings High-amp-current probe: 1 millivolt/1 amp Low range of a high-amp-current probe or the high range of a low-amp-current probe: 10 millivolts/1 amp Low-amp-current probe: 100 millivolts/1 amp

TABLE 12-3 Current Probe Interpretation 1 millivolt/1 amp scale: • If the current probe measures 1 amp, it outputs 1 millivolt (0.001 V) • If the current probe measures 200 amps, it outputs 200 millivolts (0.200 V) • If the current probe measures 500 amps, it outputs 500 millivolts (0.500 V) 10 millivolts/1 amp scale: • If the current probe measures 1 amp, it outputs 10 millivolts (0.100 V) • If the current probe measures 20 amps, it outputs 200 millivolts (0.200 V) • If the current probe measures 50 amps, it outputs 500 millivolts (0.500 V)

▶ TECHNICIAN TIP Current waveforms can reveal developing problems that other testing methods may not find. A developing fault may or may not result in a current engine performance problem at the time of testing. For example, a fuel pump with low amperage is an indication that it is failing internally. The pump may not present a problem during testing, but an increase in ambient temperatures, coupled with a low fuel level, may result in a future no-start complaint. Experience analyzing waveforms will provide a solid foundation to help determine potential faults.

the waveform. A shorted or open circuit, or anything in between, will pull more or less current than desired (FIGURE 12-38). The resulting change in current affects the signature pattern of the waveform. Interpreting the waveform provides an indication of circuit performance. Monitoring and studying current waveforms require a basic understanding of the component’s operation and basic electrical theory. Current ramping will not find voltage-related issues. Monitor the minimum, maximum, and average current draw to determine the amount of work being done (FIGURE 12-39). Studying the waveform indicates the integrity of the component’s primary coil winding, the current control, the overall primary circuit resistance, and the available energy (wattage) potential. The waveform indicates how long it takes the current to build up (ramp), the peak current level, and how well the waveform transitions. Motors, injectors, ignition coils, and solenoids are the components that are typically current ramped. Electric motor analysis includes determining how much work the motor is doing (average amperage), the speed of the motor, and the integrity of the brush-tocommutator contact (waveform). Ignition coils and fuel injector waveforms start with a current ramp. The slope is due to the gradual buildup of current as the incoming voltage overcomes the induced voltage created by the coil winding. A coil winding with good integrity will slow the incoming rush

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FIGURE 12-38 When current ramping a fuel injector or ignition coil, a slope should be noticeable. In any coil of wire, incoming current must overcome the induced voltage created when current flows through the coil windings. A good coil will slow the initial in-rush of current and display a gradual buildup on the screen. Any change in resistance of the circuit or the coil’s winding will affect the ramp’s waveform.

FIGURE 12-39 Analyzing a coil current trace using an amp clamp. As current flows through the coil

windings, a voltage is created that opposes the incoming current. A good coil will show a gradual buildup of current while the coil saturates, as seen on the left. If the coil windings short together, their resistance will decrease, increasing current flow through the coil windings. Shorted windings will be seen by a rapid rise from 0 amps, a straight line instead of a ramp, and a higher total current draw compared to a good coil. Typical coil amperage draw has a range of 6 to 10 amps. Verify by checking several coils on the same engine before determining a shorted coil.

of current, causing a gradual buildup. The high or low resistance of the winding or its circuit will alter the amount of current draw and the ramp or humps of the current waveform. What may be considered normal for one component or system may not be for another. Fuel injectors are an example. A peak and hold injector has a significantly lower resistance than a saturated port-style injector. Saturated port injectors are high resistance and low amperage. Peak and hold injectors flow more amperage, a result of their lower resistance. Basic current waveform analysis focuses on several key points: ■■

Maximum current levels: The level varies by component/circuit resistance. •• High current indicates low resistance, such as an internal component winding short or circuit fault. •• Low current indicates high resistance, an internal winding with an open, poor brush contact in a motor, etc.

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Chapter 12 Diagnosing with an Oscilloscope

■■

•• Use Ohm’s law to approximate the maximum current level. •• For example, the resistance of most saturated port injectors ranges from 12 to 14 ohms, so 1 amp of current should flow. •• By comparison, ignition coil current flow for a COP ranges from 6 to 9 amps. Time to build current (ramping): A good coil will resist current buildup, providing a gradual ramp up until the current reaches its maximum level. The ramp should be gradual. The amount of time it takes the current to reach its maximum level is based on voltage and circuit/coil resistance. •• Shorted windings decrease the time required to build current due to low resistance. •• Excessive voltage will also decrease the current ramp due to the stronger force available overcoming the coil’s normal resistance. •• Low voltage or high resistance internal to the coil windings or circuit will increase the time it takes for current to build. Angle of the current ramp: The rate at which the current ramp builds •• A normal current ramp should be gradual. •• A shorted coil winding will have a fast, more vertical current ramp increase when compared to a normally operating coil. •• The faster/steeper rise in the ramp is due to the lack of resistance. Transition integrity: How sharp the transition to another place happens. •• The waveform should show clean sharp edges when transitioning from low to high and high to low voltage. •• Check the integrity of the waveform: •• Check for rounding of the edges, gaps, sudden spikes, or drops in the current, because any of them can indicate a fault. •• Compare the waveform to a known-good signal to determine the quality of the signal.

▶▶Wrap-Up Ready for Review ▶▶ ▶▶ ▶▶ ▶▶ ▶▶

▶▶ ▶▶ ▶▶ ▶▶

▶▶

Analyzing the motor current allows the technician to evaluate the motor before it is removed. The average current that is higher than specification can cause the electrical system to fail. Using embedded information can increase the speed at which the technician can diagnose a failure. When analyzing a waveform, the technician must determine which part of the waveform has failed. Alternating current (AC) voltage is a sine wave format that goes from positive to negative based on how the voltage is produced. Permanent magnet (PM) sensors generate their own power based on magnetics, and they create an AC waveform. Knock sensors use frequency to indicate a knock in the engine usually referred to spark knock. Alternator ripple can be used to view a failing alternator. A potentiometer takes a mechanical position and changes it into an electrical signal that can be processed by the electronic control module (ECM). Hall-effect signals have an exact on position and an exact off osition, which is why they are preferred over a sine wave signal. Ignition coils and fuel injectors are coils that use power to induce either movement or power amplification.

▶▶ ▶▶

▶▶

Using the trigger mode for the oscilloscope allows the scope to start recording when an event happens. Using Peak Detect mode allows the oscilloscope to record the highest voltage from the component. Pressure transducers can convert mechanical pressure into an electrical waveform that can be compared to other waveforms. A technician can use an amp clamp to detect excessive amperage from one component.

Key Terms

amp clamp A tool used to measure amperage in a circuit. The tool is placed around the circuit conductor anywhere in the applicable circuit. The DMM will display a reading in mV, and this is multiplied by the factor selected on the tool. The result indicates the amount of amperage present in the circuit. duty cycle The percentage of one period of time in which the circuit is powered on. inductive probe A probe that creates a signal based on a magnetic pulse of which wire it is around. permanent magnet (PM) sensor A material with natural or artificially created constant magnetic properties, which are used to create a magnetic signal that is interpreted by the ECM. transducer An electrical device that creates an electrical signal based on a pressure input and displays it graphically on a lab scope.

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Wrap-Up

Review Questions 1. When a technician uses an amp clamp, they intend to evaluate what about the electrical component? a. The performance of the component. b. The physical appearance of the component. c. The amperage draw of the component. d. The resistance of the component. 2. All of the following statements with respect to diagnosing with oscilloscopes are true except: a. Oscilloscopes can be used to determine how much amperage a component draws. b. A glitch in a waveform can be seen on an oscilloscope. c. Oscilloscopes can be used to determine whether engine fluids have broken down. d. Using an amp clamp with an oscilloscope can help to determine whether the component is drawing too much current. 3. When using an oscilloscope to diagnose an intermittent potentiometer, the technician should probe ____________. a. the signal wire, to view what the potentiometer is producing b. the ground wire, to determine whether it has a ground c. the PCM, to determine what output the PCM is providing d. the datalink connector (DLC), to determine whether a waveform can be pulled from the PCM 4. When diagnosing a waveform, what the technician should look for? a. A glitch or failure within the waveform. b. Nothing, because using the waveform will not help the technician. c. The charging of the coil on an injector. d. The discharging of the coil on an ignition coil. 5. When looking at the current draw of a component, the less current means _________. a. the more work that is being produced b. nothing, because current is not a measure of work c. the less work that is being produced d. the more voltage on the component 6. Using more than one channel on the oscilloscope allows the technician to _____________. a. compare the signal of the sensor to a known-good signal b. compare two different signals to see if there is a relationship c. do nothing more than they could by using only one channel d. create a waveform on different channels 7. Choose the correct statement. a. PM generators are powered sensors that produce a DC voltage. b. Amp clamps do not have a specific direction and can be used in either direction to get a positive voltage. c. AC voltage is not used in automotive sensor outputs. d. A Hall-effect sensor is a digital sensor that has an exact on and off. 8. How can the oscilloscope help with glitch capture? a. It can direct the technician to the cause of the failure. b. It can change the sample rate to the correct one.

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c. It can indicate what is failing and why. d. It can compress all of the frames recorded on one screen so that the technician can interpret what is happening. 9. A PM generator is used to generate what type of voltage? a. DC. b. BC. c. AC. d. DVAC. 10. Using an inductive trigger helps to do what on the oscilloscope? a. Minimize failures in an oscilloscope. b. Make the oscilloscope work better by including another reading. c. Capture the failure in real time, by starting recording when the trigger is tripped. d. Nothing, and it should not be used on an oscilloscope.

ASE Technician A/Technician Bâ&#x20AC;&#x201C;Style Questions 1. Technician A says an oscilloscope can be used to determine whether any component is drawing too much amperage. Technician B says that analyzing a waveform can help to determine whether the component has failed or is failing. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 2. Technician A says that looking at the alternator ripple allows the technician to determine whether the engine is operating at the correct rpm. Technician B says that changing the sample rate on an oscilloscope screen can dramatically change the picture that is produced. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 3. Technician A says that in Peak Detect mode, the oscilloscope can capture the highest and lowest values that happen on the screen. Technician B says a pressure transducer is used to change a pressure reading into an electrical reading that can be graphed. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 4. Technician A says that using the embedded information in the oscilloscope helps to direct the technician to the quickest possible route to completion. Technician B says an AC voltage signal has an exact on and off. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B

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Chapter 12 â&#x20AC;&#x201A; Diagnosing with an Oscilloscope

5. Technician A says that using an amp clamp can help to verify whether a motor is failing. Technician B says an oscilloscope can scan the vehicle to retrieve DTCs. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 6. Technician A says transducers allow the technician to graph the pressure change of what it is hooked up to. Technician B says that current probes allow the technician to graph pressure changes. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 7. Technician A says that technicians can use the oscilloscope to look up service information on the vehicle. Technician B says an ignition coil has charge time that can be graphed on an oscilloscope so that glitches can be viewed. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B

8. Technician A says an alternator ripple can be used to verify whether the PCM is requiring the alternator to charge. Technician B says that when the technician is monitoring an average current reading of the waveform, they can determine whether the component is working correctly. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 9. Technician A says that using an oscilloscope to investigate a failing electrical motor will allow for a preventive approach. Technician B says a knock sensor generates a frequency if an actual knock occurs. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 10. Technician A says that increasing the duty cycle of a component will keep the component on longer. Technician B says that when viewing a square wave, clean, crisp edges make for good component operation. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B

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MASTER AUTOMOTIVE TECHNICIAN SERIES

SAMPLE CHAPTER 12

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