between scan tool and succesful diagnosis

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BETWEEN

SCAN TOOL &SUCCESSFUL

DIAGNOSIS—FILLING IN

THE GAPS

38 April 2011

BY BERNIE 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.



T

he phrase “no-code drive-ability” can send shiveringfear clear through a tech-nician, and intermittent no-code driveability iseven worse. A technician

can hope the symptoms of the problem

are indicative of a pattern failure, butwhat if they’re not? Even with knownsymptoms and codes, it may be anotherproblem that appears to be the same butis not. The question is: How does onetroubleshoot either coded or no-codedriveability problems?

The diagnostic process is actuallyvery similar for both types of problems.The first step is to connect a scan tool tocheck for both existing codes and pend-ing codes. Once this is done, ParameterIdentification Data (PIDs) must be col-

lected during a test drive, if the vehicleis driveable, and then analyzed. An in-depth understanding of the circuits andsystems involved is needed to under-stand what the data is conveying. It willbe necessary to check a wiring schemat-ic in order to know which sensors andactuators are part of the system.

Not all data is always shown on a scantool; many times sensors and actuatorsthat are part of the system being diag-nosed are not listed as PIDs. Thismeans that the possible cause of a prob-

lem may not be seen on the scan tool atall. Also, if the microprocessor is con-fused in any way, the data it gives to thescan tool will be incorrect.

Yet another concern is the inability of the scan tool to provide the technicianwith live data, so there’s always a delayin all data communications from thescan tool. Therefore, if the problem oc-curs quickly it may be missed, and thescan tool will not be able to provide thisinformation to the technician at all.

One of the keys to successful trou-

bleshooting of vehicles is having an un-derstanding of a scan tool’s limitations.Unfortunately, many technicians believethat a scan tool always conveys the cor-rect data. This could not be further fromthe truth, and is one of the reasons multi-ple parts may be replaced on a vehicle inan attempt to repair a problem. What’sneeded is a sound understanding of thetools, circuits and systems involved insuccessfully diagnosing a problem.

Once a scan tool has acquired the data

from a vehicle and the technician has an-alyzed it, the circuit or circuits in ques-tion must be checked. This can be ac-complished with a digital volt-ohmmeter(DVOM), logic probe or oscilloscope.Due to the speed with which modernelectronic systems operate, a DVOM or

logic probe are very poor choices for thistask. These tools are severely limited bytheir actuation speed and their inabilityto show the technician the actualchanges that are occurring in the circuit.For troubleshooting electronic systemsin modern vehicles, the tool of choice is avery clear one—the oscilloscope.

It’s important to understand what theoscilloscope is displaying. Voltagechanges over time represent what’s oc-curring in the circuit or system underdiagnosis and to which you’re connect-

ed. To properly understand what the os-cilloscope is displaying, you’ll need tocheck and understand the wiringschematic for the system.

In many wiring schematics, electron-ic devices may be shown as empty boxesand may even show some of the wiringwithout proper labels. This is where anunderstanding of basic electronics is vi-tal to understanding what the circuit isdesigned to do.

We’ll start with a basic voltage dividercircuit known as the engine coolant tem-

perature (ECT) sensor circuit (Fig. 1 onpage 40). This device can be used forany basic temperature-sensing circuit. Indiagram A, it cannot be determined howthis device would function due to the in-completeness of the circuit as drawn. Itwill be necessary to complete the circuit(diagram B) so the function of this de-vice can be understood.

The ECT sensor is a variable resistorthat changes with temperature. It canbe constructed in several configura-tions. In some early fuel injection sys-

tems in use during the ’60s and ’70s, ahigh-temperature nickel or nichromewire sealed in epoxy was used. As thisnickel wire sensing element takes onheat from the engine coolant, the resis-tance of the nickel wire increases. Thistype of sensing element is referred to asa positive temperature coefficient ther-mistor , or PTC thermistor . As the resis-tance in the nickel wire increases, thevoltage also increases. So a cold enginewill have a low voltage reading and a hot

39April 2011

P h o t o i l l u s t r a t i o n : H a r o l d A .

P e r r y ; i m a

g e s : T h i n k s t o c k



engine will have a high voltage reading.In later fuel injection systems, the

newer thermistors are made of a semi-conductor material. This usually ceram-ic- or polymer-based material is dopedwith a sintered mixture of metal oxides,

so when the material takes on heat fromthe liquid coolant, the resistance of thethermistor decreases. This type of sens-ing element is referred to as a negativetemperature coefficient thermistor , or

NTC thermistor . For example, the ther-mistor’s resistance at -40°F will read ap-proximately 100,700 ohms, and as thethermistor is heated to approximately70°F it will have a resistance of approxi-mately 3400 ohms. As the thermistorcontinues to take on heat to approxi-mately 210°F, the resistance will be ap-

proximately 185 ohms. Note: This resis-tance varies among manufacturers.

In the ECT sensor circuit, the ther-mistor is put into a series circuit that’ssupplied voltage from a voltage regulatorlocated inside the engine control mod-ule. This supplied voltage can be 5, 8 or9 volts, but is most commonly 5 volts.The regulator supplies voltage to oneend of a fixed resistor located within theECM. The other end of the fixed resistoris connected to the thermistor. The ther-mistor can be grounded through the

ECT housing (one-wire sensor) or wiredwith a redundant ground back to theECM (two-wire sensor).

In earlier applications a single-wireECT is common. However, if theECM’s ground becomes affected by re-sistance, an analog-to-digital converterwill read not only the voltage drop

across the thermistor but the voltagedrop from the ECM’s ground as well. Inlater model injection systems, a redun-dant ground is wired back to the ECM.If the ECM’s grounds experience a volt-age drop, the voltage drop of the ther-mistor that’s read by the A/D converterwill not be affected. Thus, this two-wireconfiguration is more robust.

The ECT sensor circuit configurationis referred to as a voltage divider . As the

thermistor takes on more heat from theliquid coolant, its resistance decreases,causing the voltage between the fixedresistor and the thermistor to change. Ina voltage divider circuit, two resistors arewired in series. The larger resistor willconsume more voltage and the smallerresistor will consume less voltage.

When the thermistor is cold, it has aresistance of about 100,000 ohms, andthe fixed resistor within the ECM has aresistance of 1300 to 3500 ohms, depend-ing on the manufacturer. In this condi-tion, the thermistor, having a large resis-tance, will consume most of the voltage.Therefore, the voltage reading betweenthe resistors will be about 4.8 volts.

When the thermistor is hot, its resis-tance decreases to about 200 ohms, and

the fixed resistor within the ECM has aresistance of about 2200 ohms. In thiscondition, the fixed resistor within theECM being a large resistance will con-sume most of the voltage. Therefore,the voltage reading between the resis-tors will be about .42 volt.

The output of this circuit (Fig. 2) can

BETWEEN SCAN TOOL & SUCCESSFUL DIAGNOSIS

Fig 1. Vehicle circuit diagrams may provide an incomplete or partial view of what’s going on inside various electronic modules.

Troubleshooting electronic circuits in modern vehicles can be accomplishedwith (l. to r.) a logic probe, DVOM or oscilloscope. Because a logic probe and

DVOM have speed limitations, the oscilloscope is the best choice.

40 April 2011

P h o t o ,

i l l u s t r a t i o n s & w a v e f o r m s : B e

r n i e T h o m p s o n

A

ECM ECT

B

ECM ECT

5V Reg.

A/ DConverter

Microprocessor

1

2

1

2



be predicted when an oscilloscope is in-stalled. Voltage divider circuits are usedthroughout the vehicle’s electronic cir-cuits, so it becomes very important tounderstand this type of circuit.

Now let’s look at a basic pull-downcircuit that can be used as a camshaft(CMP) or crankshaft (CKP) sensor cir-cuit (Fig. 3). In diagram A, it cannot bedetermined how this device wouldfunction because it’s incomplete as

drawn. It will be necessary to completethe circuit (diagram B) so the functionof this device can be understood.

The Hall effect element puts out ananalog voltage of about 30 microvoltswhen a magnetic field penetrates it.With this very low-voltage output, theHall effect signal must be amplified,

which is accomplished by an operationalamplifier. The op amp works with twoinputs that are connected to the nonin-verting () and inverting () terminals.When the two input potentials are equalto one another, there is no output fromthe amplifier. As the positive and nega-tive inputs start to have a potential dif-ference between them, there will be anoutput produced by the amplifier. Thisoutput polarity will be based on the in-

verting () input signal; the polarity of asignal applied to the inverting input isreversed at the output of the op amp.The greater the difference between thepositive and negative inputs, the greaterthe gain, or output, from the amplifier.

This amplification is determined by afeedback resistor that feeds a portion of

the amplified signal from the output tothe inverting input (Fig. 4 on page 44).This will reduce the amplitude of theoutput signal and thus the gain. Thesmaller the resistor, the lower the gainproduced from the op amp. If there’sno resistor installed in the feedback cir-cuit, the op amp will work as a follower.

The op amp gain is independent fromthe supply voltage and can be from sev-eral times to many thousands of times

greater than the supply. When the opamp does not have a feedback circuit, it’ssaid to be in open-loop mode. When inopen-loop mode, a very small potentialdifference between positive and negativewill create an output of maximum gain.Therefore, open-loop mode is not practi-cal for linear amplification, but is used as


Fig 2. The engine coolant temperature (ECT) sensor waveforms above were taken from two different types of ECT volt-age 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 insidethe PCM. This allows for higher resolution in the ECT signal during the time the engine goes from warm to hot.

Fig 3. Diagram A on the left is all you may have to go on when consulting a circuit diagram. Diagram B on the right pro-vides a clearer explanation of the relationships among the components and their internal components.

42 April 2011

45°F Start

200°F

45°F Start 200°F

Pull-UpDividerCircuit

A B

ECM Crankshaft Posit ion Sensor ECM Crankshaft Posit ion Sensor

1

2

3

1

2

3

5V

Reg.

Microprocessor

NPN

Schmitt Trigg erOp

Amp

Hall Effect Sensor



a comparator since it compares one in-put voltage to the other input voltage.Many electronic circuits on the vehicleare amplified by an op amp and can beconfigured as a noninverting amp, an in-verting amp, a differential amp, a com-

parator and follower or a buffer.In the configuration shown in dia-

gram B in Fig. 3, the operational ampli-fier is in open-loop mode. Therefore, avery small potential difference betweenpositive and negative will allow the volt-age to swing from full off to full on. Itwill be necessary to feed this voltage toa circuit that will make sure a retriggerwill not occur within the circuit. This isaccomplished by the Schmitt trigger. Asthe voltage from the op amp increases,it reaches a turn-on threshold, or oper-

ating point. At this operating point, theSchmitt trigger changes states, allowinga digital voltage signal to be sent out.This signal is applied to the base of atransistor.

A bipolar transistor is a three-termi-nal semiconductor device. Its three ter-minals are referred to as the base, col-lector and emitter. A small commandcurrent at the base can control a muchlarger current flowing between the col-lector and emitter. This allows the tran-sistor to work as an amplifier or switch.

If the base voltage is modulated, thetransistor works as an amplifier. If thebase voltage is fully saturated, the tran-sistor works as a switch. The type of transistor used will determine how thecircuit will operate.

There are two basic classifications of the bipolar transistor, based on thesemiconductor doping within the tran-sistor itself—positive-negative-positive(PNP), and negative-positive-negative(NPN), as illustrated in Fig. 5 on page44. The P-type semiconductor junction

is made with pure silicone that’s dopedwith b oron which does not have afourth valence electron, creating “bro-ken bonds,” or holes. The N-type semi-conductor junction is made with puresilicone that’s doped with phosphorus,which adds valence electrons.

A PNP transistor consists of a layer of N-doped semiconductor material sand-wiched between two layers of P-dopedsemiconductor material. A small currentleaving the base is amplified in the col-

lector output; this type of transistor isturned on when its base is pulled low rel-ative to the emitter. This transistor isused to control a power output, say, 12volts. In Fig. 5, notice the arrow in dia-gram A. It shows the direction of current

for the conventional theory of current

flow. The transistor leg with the arrow isthe emitter and is always connected tothe higher potential in the circuit. For aneasy way to remember which transistorthis is, think arrow “pointing in”—PNP.

An NPN transistor consists of a layer

of P-doped semiconductor between two

43April 2011

Circle #24



layers of N-doped semiconductor mate-rial. A small current entering the base isamplified in the collector output; thistype of transistor is turned on when itsbase is pulled high relative to the emit-ter. This transistor is used to control aground or sync power. In diagram B inFig. 5, notice the direction of the arrow,which shows the direction of current forthe conventional theory of current flow.The transistor leg with the arrow is theemitter and is always connected to thelower potential in the circuit. An easy

way to remember which transistor thisis, think arrow “not pointing in”—NPN.

In diagram B in Fig. 3, the transistoris an NPN type, which will ground thecircuit. A voltage regulator in the ECMsupplies voltage to one side of a fixedresistor; the other side of the fixed resis-tor is connected to the NPN transistorin the CKP sensor. When the NPNtransistor base voltage goes high it’sturned on, thus pulling the circuit toground. When the NPN transistor basevoltage goes low it’s turned off, openingthe circuit so source voltage is present.

This turning on and off produces a

high-low/high-low digital voltage signaloutput 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 andangular velocity. Now that we have aproper understanding of the circuit,when the oscilloscope is attached thenormal output (Fig. 6) can be anticipat-ed. This type of pull-down circuit is byfar the most common used in vehicleelectronic systems.

However, if a PNP transistor wasused instead of an NPN transistor, theCKP sensor would send the voltage out

of the sensor (pull-up circuit). TheECM signal circuit would also change;


44 April 2011

Fig 4. Operational amplifier circuit design. Fig 5. Basic transistor circuit designs.

Fig 6. In the waveforms above, a crankshaft position sensor (CKP, shown in red) and a camshaft position sensor (CMP,green) from a 2001 Dodge 2.7L engine are shown. This is a pull-down circuit that uses an NPN transistor. This means that thesensor is holding its signal and then releasing it, so we know that source voltage from the ECM is present. The right-handwaveform shows the green signal failed to return to ground. At this point you know the control unit and the circuit are goodbecause voltage is present at the sensor signal wire. The problem is in the sensor, sensor ground or trigger wheel. In thiscase, the trigger wheel was bent, causing an intermittent problem, where the signal had one pulse missing every 30 minutes.

Feedback Circuit

V In

Op Amp

V

V

V Out

12V

E

C

B

C

E

B

12V

PNP NPN

A B

E B CE B C

PNP NPN

Camshaft Signal Good Camshaft Signal Failing



it would no longer supply voltage to thesensor signal circuit, but instead wouldreceive the digital output voltage fromthe CKP sensor and count the edgesfrom this signal. When diagnosing thesesensors, it’s very important to realize

which sensor type is used—a pull-downcircuit or a pull-up circuit.

If a signal is being produced, a quick way to determine which circuit is beingused is to unplug the sensor and check the harness-side signal voltage. If thesignal wire has voltage, it’s a groundingcircuit; if the signal wire does not havevoltage, it’s an output circuit.

If a signal is not being produced andthere’s no signal voltage present, severalpossibilities exist: The ECM is not sup-plying the signal voltage, the signal cir-

cuit is open, the signal circuit is shortedto ground, the sensor is pulling the sig-nal to ground, the sensor is an outputtype that’s not producing a signal or thetrigger wheel is not turning. If the cir-cuit type is unknown and there’s novoltage on the signal wire, unplug the

sensor from the circuit. If voltage is nowpresent, check the sensor’s power andground. If these are good, check thetrigger wheel condition and make sureit’s turning, then replace the sensor. If there’s no voltage on the signal wire

when the sensor is unplugged, take asignal generator with an analog voltageand connect it to the unplugged signalwire. If voltage is now present on thesignal wire, the signal wire is notgrounded. If no voltage is present onthe signal wire, unplug the ECM fromthe circuit. If voltage is now present, theECM is pulling the signal circuit down.

Make sure all the power supplies andgrounds are good at the ECM before re-placing the module, because a power orground problem can cause the ECM to

pull this circuit down. If the voltagefrom the sensor simulator is still not present with the ECM unplugged, the signalwire is grounded. If the sensor is un-plugged and the signal wire has the sen-sor simulator voltage on it, plug the sen-sor back in and crank the engine over.

Now watch the oscilloscope to see if the voltage changed to make a digitalsignal. If the signal is now present,check the sensor signal wire at theECM to check for an open wire be-tween the sensor and ECM. If the sig-

nal did not change, this could be an out-put-style sensor that has no output;check the power and ground at the sen-sor. If they’re good, check the triggerwheel and replace the sensor. If multi-ple sensors have been installed but keepfailing, suspect a short circuit betweenthe sensor signal and a power circuit.

It’s hard to imagine life without ascan tool. But when that trusted friendcan’t give you the answers you need,possessing an understanding of vehicleelectronics, typical circuits and compo-

nents, coupled with the ability to usethe appropriate diagnostic equipment,can prove equally valuable.

45April 2011

This article can be found online at

www.motormagazine.com .

Circle #25

between scan tool and succesful diagnosis

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