ETWEEN SCAN TOOL & SUCCESSFUL DIAGNOSIS FILLING IN THE GAPS 38 April 0 Y ERNIE THOMPSON A scan tool is an invaluable aid to vehicle diagnostics, but you may need to rely on other methods as well when vital information is either missing or incomplete. A solid understanding of vehicle electronics is indispensible in these situations.
Photoillustration: Harold A. Perry; images: Thinkstock The phrase no-code driveability can send shivering fear clear through a technician, and intermittent no-code driveability is even worse. A technician can hope the symptoms of the problem are indicative of a pattern failure, but what if they re not? Even with known symptoms and codes, it may be another problem that appears to be the same but is not. The question is: How does one troubleshoot either coded or no-code driveability problems? The diagnostic process is actually very similar for both types of problems. The first step is to connect a scan tool to check for both existing codes and pending codes. Once this is done, Parameter Identification Data (PIDs) must be collected during a test drive, if the vehicle is driveable, and then analyzed. An indepth understanding of the circuits and systems involved is needed to understand what the data is conveying. It will be necessary to check a wiring schematic in order to know which sensors and actuators are part of the system. Not all data is always shown on a scan tool; many times sensors and actuators that are part of the system being diagnosed are not listed as PIDs. This means that the possible cause of a problem may not be seen on the scan tool at all. Also, if the microprocessor is confused in any way, the data it gives to the scan tool will be incorrect. Yet another concern is the inability of the scan tool to provide the technician with live data, so there s always a delay in all data communications from the scan tool. Therefore, if the problem occurs quickly it may be missed, and the scan tool will not be able to provide this information to the technician at all. One of the keys to successful troubleshooting of vehicles is having an understanding of a scan tool s limitations. Unfortunately, many technicians believe that a scan tool always conveys the correct data. This could not be further from the truth, and is one of the reasons multiple parts may be replaced on a vehicle in an attempt to repair a problem. What s needed is a sound understanding of the tools, circuits and systems involved in successfully diagnosing a problem. Once a scan tool has acquired the data from a vehicle and the technician has analyzed it, the circuit or circuits in question must be checked. This can be accomplished with a digital volt-ohmmeter (DVOM), logic probe or oscilloscope. Due to the speed with which modern electronic systems operate, a DVOM or logic probe are very poor choices for this task. These tools are severely limited by their actuation speed and their inability to show the technician the actual changes that are occurring in the circuit. For troubleshooting electronic systems in modern vehicles, the tool of choice is a very clear one the oscilloscope. It s important to understand what the oscilloscope is displaying. Voltage changes over time represent what s occurring in the circuit or system under diagnosis and to which you re connected. To properly understand what the oscilloscope is displaying, you ll need to check and understand the wiring schematic for the system. In many wiring schematics, electronic devices may be shown as empty boxes and may even show some of the wiring without proper labels. This is where an understanding of basic electronics is vital to understanding what the circuit is designed to do. We ll start with a basic voltage divider circuit known as the engine coolant temperature (ECT) sensor circuit (Fig. on page 40). This device can be used for any basic temperature-sensing circuit. In diagram A, it cannot be determined how this device would function due to the incompleteness of the circuit as drawn. It will be necessary to complete the circuit (diagram ) so the function of this device can be understood. The ECT sensor is a variable resistor that changes with temperature. It can be constructed in several configurations. In some early fuel injection systems in use during the 60s and 70s, a high-temperature nickel or nichrome wire sealed in epoxy was used. As this nickel wire sensing element takes on heat from the engine coolant, the resistance of the nickel wire increases. This type of sensing element is referred to as a positive temperature coefficient thermistor, or PTC thermistor. As the resistance in the nickel wire increases, the voltage also increases. So a cold engine will have a low voltage reading and a hot April 0 39
ETWEEN SCAN TOOL & SUCCESSFUL DIAGNOSIS engine will have a high voltage reading. In later fuel injection systems, the newer thermistors are made of a semiconductor material. This usually ceramic- or polymer-based material is doped with a sintered mixture of metal oxides, so when the material takes on heat from the liquid coolant, the resistance of the thermistor decreases. This type of sensing element is referred to as a negative temperature coefficient thermistor, or NTC thermistor. For example, the thermistor s resistance at -40 F will read approximately 00,700 ohms, and as the thermistor is heated to approximately 70 F it will have a resistance of approximately 3400 ohms. As the thermistor continues to take on heat to approximately 0 F, the resistance will be approximately 85 ohms. Note: This resistance varies among manufacturers. In the ECT sensor circuit, the thermistor is put into a series circuit that s supplied voltage from a voltage regulator located inside the engine control module. This supplied voltage can be 5, 8 or 9 volts, but is most commonly 5 volts. The regulator supplies voltage to one end of a fixed resistor located within the ECM. The other end of the fixed resistor is connected to the thermistor. The thermistor can be grounded through the ECT housing (one-wire sensor) or wired with a redundant ground back to the ECM (two-wire sensor). In earlier applications a single-wire ECT is common. However, if the ECM s ground becomes affected by resistance, an analog-to-digital converter will read not only the voltage drop Troubleshooting electronic circuits in modern vehicles can be accomplished with (l. to r.) a logic probe, DVOM or oscilloscope. ecause a logic probe and DVOM have speed limitations, the oscilloscope is the best choice. across the thermistor but the voltage drop from the ECM s ground as well. In later model injection systems, a redundant ground is wired back to the ECM. If the ECM s grounds experience a voltage drop, the voltage drop of the thermistor that s read by the A/D converter will not be affected. Thus, this two-wire configuration is more robust. The ECT sensor circuit configuration is referred to as a voltage divider. As the thermistor takes on more heat from the liquid coolant, its resistance decreases, causing the voltage between the fixed resistor and the thermistor to change. In a voltage divider circuit, two resistors are wired in series. The larger resistor will consume more voltage and the smaller resistor will consume less voltage. When the thermistor is cold, it has a resistance of about 00,000 ohms, and the fixed resistor within the ECM has a resistance of 300 to 3500 ohms, depending on the manufacturer. In this condition, the thermistor, having a large resistance, will consume most of the voltage. Therefore, the voltage reading between the resistors will be about 4.8 volts. When the thermistor is hot, its resistance decreases to about 00 ohms, and the fixed resistor within the ECM has a resistance of about 00 ohms. In this condition, the fixed resistor within the ECM being a large resistance will consume most of the voltage. Therefore, the voltage reading between the resistors will be about.4 volt. The output of this circuit (Fig. ) can Photo, illustrations & waveforms: ernie Thompson ECM ECT ECM ECT 5V Reg. A/D Converter Microprocessor A Fig. Vehicle circuit diagrams may provide an incomplete or partial view of what s going on inside various electronic modules. 40 April 0
ETWEEN SCAN TOOL & SUCCESSFUL DIAGNOSIS 45 F Start 45 F Start 00 F 00 F Pull-Up Divider Circuit Fig. The engine coolant temperature (ECT) sensor waveforms above were taken from two different types of ECT voltage divider circuits. The waveform on the left shows a basic ECT divider circuit, the other an ECT pull-up divider circuit. In a pull-up circuit, once the engine has obtained a set coolant temperature at warm-up, the resistance is switched inside the PCM. This allows for higher resolution in the ECT signal during the time the engine goes from warm to hot. be predicted when an oscilloscope is installed. Voltage divider circuits are used throughout the vehicle s electronic circuits, so it becomes very important to understand this type of circuit. Now let s look at a basic pull-down circuit that can be used as a camshaft (CMP) or crankshaft (CKP) sensor circuit (Fig. 3). In diagram A, it cannot be determined how this device would function because it s incomplete as drawn. It will be necessary to complete the circuit (diagram ) so the function of this device can be understood. The Hall effect element puts out an analog voltage of about 30 microvolts when a magnetic field penetrates it. With this very low-voltage output, the Hall effect signal must be amplified, which is accomplished by an operational amplifier. The op amp works with two inputs that are connected to the noninverting () and inverting () terminals. When the two input potentials are equal to one another, there is no output from the amplifier. As the positive and negative inputs start to have a potential difference between them, there will be an output produced by the amplifier. This output polarity will be based on the inverting () input signal; the polarity of a signal applied to the inverting input is reversed at the output of the op amp. The greater the difference between the positive and negative inputs, the greater the gain, or output, from the amplifier. This amplification is determined by a feedback resistor that feeds a portion of the amplified signal from the output to the inverting input (Fig. 4 on page 44). This will reduce the amplitude of the output signal and thus the gain. The smaller the resistor, the lower the gain produced from the op amp. If there s no resistor installed in the feedback circuit, the op amp will work as a follower. The op amp gain is independent from the supply voltage and can be from several times to many thousands of times greater than the supply. When the op amp does not have a feedback circuit, it s said to be in open-loop mode. When in open-loop mode, a very small potential difference between positive and negative will create an output of maximum gain. Therefore, open-loop mode is not practical for linear amplification, but is used as ECM Crankshaft Position Sensor ECM Crankshaft Position Sensor 5V Reg. Schmitt Trigger Op Amp 3 3 NPN A Microprocessor Hall Effect Sensor Fig 3. Diagram A on the left is all you may have to go on when consulting a circuit diagram. Diagram on the right provides a clearer explanation of the relationships among the components and their internal components. 4 April 0
a comparator since it compares one input voltage to the other input voltage. Many electronic circuits on the vehicle are amplified by an op amp and can be configured as a noninverting amp, an inverting amp, a differential amp, a comparator and follower or a buffer. In the configuration shown in diagram in Fig. 3, the operational amplifier is in open-loop mode. Therefore, a very small potential difference between positive and negative will allow the voltage to swing from full off to full on. It will be necessary to feed this voltage to a circuit that will make sure a retrigger will not occur within the circuit. This is accomplished by the Schmitt trigger. As the voltage from the op amp increases, it reaches a turn-on threshold, or operating point. At this operating point, the Schmitt trigger changes states, allowing a digital voltage signal to be sent out. This signal is applied to the base of a transistor. A bipolar transistor is a three-terminal semiconductor device. Its three terminals are referred to as the base, collector and emitter. A small command current at the base can control a much larger current flowing between the collector and emitter. This allows the transistor to work as an amplifier or switch. If the base voltage is modulated, the transistor works as an amplifier. If the base voltage is fully saturated, the transistor works as a switch. The type of transistor used will determine how the circuit will operate. There are two basic classifications of the bipolar transistor, based on the semiconductor doping within the transistor itself positive-negative-positive (PNP), and negative-positive-negative (NPN), as illustrated in Fig. 5 on page 44. The P-type semiconductor junction is made with pure silicone that s doped with boron which does not have a fourth valence electron, creating broken bonds, or holes. The N-type semiconductor junction is made with pure silicone that s doped with phosphorus, which adds valence electrons. A PNP transistor consists of a layer of N-doped semiconductor material sandwiched between two layers of P-doped semiconductor material. A small current leaving the base is amplified in the collector output; this type of transistor is turned on when its base is pulled low relative to the emitter. This transistor is used to control a power output, say, volts. In Fig. 5, notice the arrow in diagram A. It shows the direction of current for the conventional theory of current flow. The transistor leg with the arrow is the emitter and is always connected to the higher potential in the circuit. For an easy way to remember which transistor this is, think arrow pointing in PNP. An NPN transistor consists of a layer of P-doped semiconductor between two Circle #4 April 0 43
ETWEEN SCAN TOOL & SUCCESSFUL DIAGNOSIS V In Feedback Circuit V Op Amp V Out PNP E C PNP V E C NPN E C NPN V C E V A Fig 4. Operational amplifier circuit design. Fig 5. asic transistor circuit designs. layers of N-doped semiconductor material. A small current entering the base is amplified in the collector output; this type of transistor is turned on when its base is pulled high relative to the emitter. This transistor is used to control a ground or sync power. In diagram in Fig. 5, notice the direction of the arrow, which shows the direction of current for the conventional theory of current flow. The transistor leg with the arrow is the emitter and is always connected to the lower potential in the circuit. An easy way to remember which transistor this is, think arrow not pointing in NPN. In diagram in Fig. 3, the transistor is an NPN type, which will ground the circuit. A voltage regulator in the ECM supplies voltage to one side of a fixed resistor; the other side of the fixed resistor is connected to the NPN transistor in the CKP sensor. When the NPN transistor base voltage goes high it s turned on, thus pulling the circuit to ground. When the NPN transistor base voltage goes low it s turned off, opening the circuit so source voltage is present. This turning on and off produces a high-low/high-low digital voltage signal output from the CKP sensor. The ECM now counts the edges from the sensor, and compares this count against a clock to determine CKP angular position and angular velocity. Now that we have a proper understanding of the circuit, when the oscilloscope is attached the normal output (Fig. 6) can be anticipated. This type of pull-down circuit is by far the most common used in vehicle electronic systems. However, if a PNP transistor was used instead of an NPN transistor, the CKP sensor would send the voltage out of the sensor (pull-up circuit). The ECM signal circuit would also change; Camshaft Signal Good Camshaft Signal Failing Fig 6. In the waveforms above, a crankshaft position sensor (CKP, shown in red) and a camshaft position sensor (CMP, green) from a 00 Dodge.7L engine are shown. This is a pull-down circuit that uses an NPN transistor. This means that the sensor is holding its signal and then releasing it, so we know that source voltage from the ECM is present. The right-hand waveform shows the green signal failed to return to ground. At this point you know the control unit and the circuit are good because voltage is present at the sensor signal wire. The problem is in the sensor, sensor ground or trigger wheel. In this case, the trigger wheel was bent, causing an intermittent problem, where the signal had one pulse missing every 30 minutes. 44 April 0
it would no longer supply voltage to the sensor signal circuit, but instead would receive the digital output voltage from the CKP sensor and count the edges from this signal. When diagnosing these sensors, it s very important to realize which sensor type is used a pull-down circuit or a pull-up circuit. If a signal is being produced, a quick way to determine which circuit is being used is to unplug the sensor and check the harness-side signal voltage. If the signal wire has voltage, it s a grounding circuit; if the signal wire does not have voltage, it s an output circuit. If a signal is not being produced and there s no signal voltage present, several possibilities exist: The ECM is not supplying the signal voltage, the signal circuit is open, the signal circuit is shorted to ground, the sensor is pulling the signal to ground, the sensor is an output type that s not producing a signal or the trigger wheel is not turning. If the circuit type is unknown and there s no voltage on the signal wire, unplug the sensor from the circuit. If voltage is now present, check the sensor s power and ground. If these are good, check the trigger wheel condition and make sure it s turning, then replace the sensor. If there s no voltage on the signal wire when the sensor is unplugged, take a signal generator with an analog voltage and connect it to the unplugged signal wire. If voltage is now present on the signal wire, the signal wire is not grounded. If no voltage is present on the signal wire, unplug the ECM from the circuit. If voltage is now present, the ECM is pulling the signal circuit down. Make sure all the power supplies and grounds are good at the ECM before replacing the module, because a power or ground problem can cause the ECM to pull this circuit down. If the voltage from the sensor simulator is still not pres - ent with the ECM unplugged, the signal wire is grounded. If the sensor is unplugged and the signal wire has the sensor simulator voltage on it, plug the sensor back in and crank the engine over. Now watch the oscilloscope to see if the voltage changed to make a digital signal. If the signal is now present, check the sensor signal wire at the ECM to check for an open wire between the sensor and ECM. If the signal did not change, this could be an output-style sensor that has no output; check the power and ground at the sensor. If they re good, check the trigger wheel and replace the sensor. If multiple sensors have been installed but keep failing, suspect a short circuit between the sensor signal and a power circuit. It s hard to imagine life without a scan tool. ut when that trusted friend can t give you the answers you need, possessing an understanding of vehicle electronics, typical circuits and components, coupled with the ability to use the appropriate diagnostic equipment, can prove equally valuable. This article can be found online at www.motormagazine.com. Circle #5 April 0 45