obdii diagnostic articles motor magazine

When diagnosing anyMIL-on driveabilitycomplaint, be sureto read and recordfreeze frame databefore turning out

the Check Engine light. Once the codesare cleared, it’s too late; the freeze framedata will be lost. Although the informa-tion contained in the mandatory freezeframe data is a small subset of whatyou’d see in current live data, there areoften important clues that can helpguide and speed your diagnosis of theunderlying cause of a fault. Let’s look ata real-world example.

The screen capture on page 44 showsfreeze frame data from a HondaOdyssey with insufficient EGR flow.You can see that the freeze frame wascaptured during closed-throttle deceler-ation from freeway speeds. EGR is nor-mally commanded on only under mod-erate load to reduce combustion cham-ber temperatures that might otherwiseresult in excessive NOX formation.

This example might strike you as a bit

tricky, since there doesn’t seem to be anygood reason to activate the EGR underthese circumstances. But appearances canbe deceiving. In fact, these are preciselythe conditions under which the EGRmonitor runs its self-test. By command-ing full EGR on closed-throttle decel, thePCM can observe the change in manifoldabsolute pressure (MAP) and compare itto the expected value. If the change is toosmall, or too slow, the test fails. In thisparticular instance, the freeze frame dataalways shows essentially the same high-speed, closed-throttle, light-load condi-tions, because these are the only condi-tions under which the self-test occurs.

Perhaps in this instance it helps usmost by reminding us of the self-testconditions under which the repair willbe evaluated. You can clean the pas-sages as much as you like and drivearound all day, but you won’t get theEGR monitor to run unless and untilyou provide the necessary conditions.Only then will your repair be judged.

Experience suggests that the under-lying cause for this code in the Odyssey

42 July 2008

DATASTREAM PRELIMINARY ANALYSIS

BY SAM BELLDon’t be in a hurry to blow out the candle

of the Check Engine light. Dim though it maybe, you can still use it to avoid stumbling

around in the dark. Part one of two parts.P

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is carbon-blocked EGR passages, not anEGR valve problem, which brings upanother valuable lesson. The DTCpoints out a problem area but does notdefine its exact cause, or provide a pre-cise solution. For example, replacingthe EGR valve or squirting a carbon sol-vent into the EGR hole won’t do the jobmost of the time. Removing the plenumand the throttle body will be required.In some instances, it may be necessaryto drill out one of the passages and in-stall a countermeasure EGR flow tube.Consult your service information data-base for related TSBs and details.

Meanwhile, the screen capture onpage 46 provides current datastream in-formation and a pending DTC P0118(Engine Coolant Temperature CircuitHigh Input) for a fairly common failedECT sensor on a Volkswagen. I wasable to catch this data set after havingcleared the code previously, and waslucky enough to have the sensor dropout while I was monitoring it on my lap-top. The very low temperature value isthe same one you’d see with the sensorunplugged. It could be the result of an

open circuit, anywhere between thePCM’s 5-volt reference signal to thesensor and the sensor’s ground returnterminal. In this instance, the fault wasinternal to the sensor. But the importantquestion is, How can you tell?

There may be more than one way toanalyze the situation. If you found a plau-sible value for the ECT in current data,a loose connection could be the culprit,but you can easily eliminate the possibil-ity by checking the ECT PID, KOEO,while wiggling the harness and connec-tor plugs. If the ECT report is stable butthe freeze frame shows �40°F, you’vejust nailed down the diagnosis.

Double-check your work by observ-ing the current ECT PID while you dis-connect the sensor. It should immedi-ately drop to �40°F, and the sensor’sPCM feed wire should show full refer-ence voltage. Now short the two PCMECT leads to one another, paying care-ful attention to the wire colors, becausethis sensor features four wires—two forthe gauge and two for the PCM! Yourscanner display should show a very hightemperature, well in excess of 300°F.

These steps verify that the correct refer-ence voltage is being sent from thePCM, and that the sensor ground is in-tact. I’ll be coming back to more KOEOand KOER diagnosis throughout thisarticle and its conclusion next month.

Code-Setting CriteriaWhen analyzing a MIL-on driveabilitycomplaint, it’s critical that you deter-mine the exact code-setting criteria forthe particular vehicle you’re workingon. This is true equally whether thecode is generic or manufacturer-specif-ic. Here’s why:

So-called generic codes, which usual-ly take the form P0xxx, have a commongeneral label or descriptor, so that, forexample, a P0401 code indicates an In-sufficient EGR Flow condition whetherthe vehicle in question is a Chevy or aLexus. It’s the common general, orgeneric, descriptor that leads us to labelit a generic code. But, in fact, the codebecomes, at least in some sense, specificas soon as its code-setting criteria areestablished by the manufacturer.

Consider these examples of a code

44 July 2008


OBD II requires that freeze frame data accompany at least one code, even in the gener-ic, or global, OBD II interface. This screen capture of a DTC P0401 taken from a HondaOdyssey illustrates the required PIDs. What can we learn about the conditions underwhich this EGR flow code set? VSS, load and TPS all converge to point toward closed-throttle highway deceleration when the code was set. In fact, this is exactly the sce-nario under which the monitor actually runs. It won’t set under city driving conditions.

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ent code-setting criteria to differentmodels or even to different engineswithin the same model line, yet alwaysunder the same assigned generic codenumber. For the VW I tapped for thisexample, P0118 is a two-trip code, re-quiring a recurrent fault before thecode actually sets.

In any event, the exact details of thecode-setting criteria are critical to de-veloping and carrying out effective test-ing and troubleshooting procedures inthe field. Look for these criteria in thecode-setting definitions in your serviceinformation database. When they’renot spelled out in sufficient detail, youmay have to uncover them from specif-ic testing values embedded within fac-tory flowcharts or trouble trees (see the“Flowchart Diagnostics” sidebar onpage 48).

Freeze frame data sets and pendingDTCs are not the only things lost whenyou extinguish an illuminated MIL.Mode 5 and 6 results are erased as well.While not available on all models, bothMode 5 (Oxygen Sensor Test Results)

and Mode 6 (Non-Continuous MonitorTest Results) may contain diagnosticallypertinent data. Again, if you don’t readand record this data early on, you maylose a valuable resource. Refer to MarkWarren’s Driveability Corner column inthe June 2002 issue of MOTOR (www.motormagazine.com) for an introduc-tion to the topic of Mode 6 diagnosis.

Mode 5 and Mode 6 test results arenot exactly the same as watching “live”datastream. First of all, they’re general-ly—with at least some important excep-tions—not current data values; they’retest results. That does not mean they’renecessarily static values. New resultswill overwrite existing data as the on-board monitors repeatedly run varioustests to completion. In some instances,the results will update as you drive, butin many cases, they do not update tooverwrite existing data until the key isturned off. The data you find in Mode 5or Mode 6 is simply the most recentlyrecorded test results—provided no onehas cleared the codes yet!

Unfortunately, neither Mode 5 nor,

46 July 2008


In this screen capture of a Volkswagen DTC P0118, the low ECTvalue corresponds with the pending high ECT voltage code.

P0118, Engine Coolant TemperatureSensor Circuit High. One manufac-turer applies the following criterionto one of its models:

KOEO, the PCM detected an ECTsignal of more than 4.96 volts for 3seconds or more.

Here’s how another OEM appliesP0118:

When the PCM detects an ECT signalof more than 4.6 volts (�46°F),KOEO or KOER.

So here we see the same genericcode being applied under two verysimilar, but slightly differently de-fined, conditions. A third manufactur-er confuses the issue a bit more byapplying the same code only when:

The PCM detects an ECT voltage inexcess of 4.85 volts, the VSS indicatesgreater than 5 mph and the enginerun time is greater than 4 seconds.

To further muddy the waters, somemanufacturers may even apply differ-

especially, Mode 6 results have to con-form to any specified format definitionbeyond defined communications proto-cols. Thus, for example, a 1998 CadillacCatera’s report of “hexadecimal testidentification 5, hexadecimal compo-nent identification 13” results (abbrevi-ated as TID$ 05, CID$ 13) refers to the“fuel tank pressure sensor check re-sponse time” measured in seconds,while the same parameters (TID$ 05,CID$ 13) reported on a 1998 ChevyLumina (using the J1850/Class 2 datalink) would indicate the “B1S1 low sen-sor voltage for half-period time calcula-tion” measured in millivolts. The sameTID$s and CID$s will have entirely dif-ferent meanings on different vehicles.

As you can see, you’ll need access tothe particular manufacturer’s “secretdecoder ring” to get the most benefitfrom this data. But don’t despair; all theinformation is available, no matter howcryptic it appears so far. Visitwww.nastf.org for an up-to-date listingof all the OEMs’ websites.

Unfortunately, you’ll lose all of thisdata as soon as the codes are cleared. So

what does that mean about any data youfind in Mode 6 after a MIL reset? Youmay have stumbled onto a cache of un-trustworthy gibberish. Some manufac-turers are kind enough to restrict avail-ability of Mode 5 or Mode 6 data to theresults only for monitors that are cur-rently complete. Others, I’m sorry to say,may populate this data report with po-tentially misleading place-holding valuesthat in no way reflect actual test results.This is yet another example of a situationwhere checking and recording data fromknown-good vehicles is indispensable.Only then will you know if you’re lookingat the wheat or the chaff.

Light at the EndOf the TunnelOnce you have valid Mode 5 or Mode 6data, however, you may be able to put itto good use. Here’s an example: You’relooking at a 1999 Taurus showing aP0420 code (low catalyst efficiency, bankone). The raw data is contained in theMode 6 results for TID$10, CID$ 11.With the help of the handy decoder ring(available at www.motorcraftservice.com

/vdirs/retail/default.asp?pageid=diag_theory_retail&gutsid=diagsheet &menuIndex1=17 or members.iatn.net/ tech/ford/99obdov), you know that thethreshold for this conventionally fueledvehicle is a 75% rear-to-front O2 sensorswitch ratio. The recorded value readsout as $0031, which, translated fromhexadecimal to decimal, equates to 49.The secret decoder ring-supplied con-version factor instructs us to multiply by.0156 to get a value from 0 to 1.0. Wecan now calculate 49 � 0.0156 � .7644,or 76.44%. This tells you that the rear O2

sensor switches 77 times for every 100switches of the front sensor. That’s just abit too much activity in the rear sensor ifthe converter is actually doing its job.

The monitor entry conditions tell usthat the monitor won’t run until at least330 seconds after start-up, an ECT of170° to 230°F, IAT between 20° and180°F, at least 10% engine load, 30 sec-onds or more since entering closed-loop, VSS from 5 to 70 mph, an inferredcatalyst midbed temperature of at least900°F, steady MAF between 1.0 and5.0 lbs./min., fuel level of at least 15%

48 July 2008


Students sometimes ask me why Idon’t just follow the factory trouble

tree flowcharts. First of all, I side withAlbert Einstein, who famously said,“The significant problems we have can-not be solved at the same level ofthinking with which we created them.”Without meaning to offend the engi-neers who write these flowcharts, Ihave simply encountered too manyproblems that just aren’t accounted foranywhere in their flowcharts. You prob-ably have, too.

Second, flowcharts often call for a lotof tedious tests to rule out failures thatnever seem to occur in the real world.I’m not saying these potential problemscouldn’t happen, I’m just saying theirlikelihood is vanishingly small. I’ll inves-tigate them eventually if I haven’tfound another solution. I need my testprocedures to home in on the problemquickly, efficiently and accurately. That’s

how I make my living. Devising my owntests is usually far more efficient thanfollowing a flowchart. But flowchartscan be helpful in this regard, becausethey often contain specific pass/fail cri-teria, voltage range specifications, etc.

Third, as an independent repair facili-ty tech, I don’t have the luxury of fol-lowing the “substitute a known-goodunit” instruction. If I want to try that,it’s pretty likely I’m going to have tobuy the part first, so I’d better be surethat’s the solution.

Finally, while blindly following aflowchart’s path might eventually yielda solution, it’s not usually much of alearning experience. If you’re like mostof MOTOR’s readers, you want to knowwhy even more than you want toknow what. Which, after all, may beexactly what Einstein had in mind in hisimplicit advice to change our level ofconsciousness.

Flowchart Diagnostics

and EGR flow of 1% to 12%. Wow!Sounds pretty definitive, doesn’t it?

So, are you ready to condemn the cator not? I’m in no rush. I want to look atTID$ 53 and CID$s 01 to 06. Further-more, I want to look at these Mode 6PIDs while driving the vehicle under

the same conditions as those reportedin the accompanying freeze frame.Why? These PIDs (and I know this alsofrom the decoder ring) are the misfirecounters for each cylinder. They do notappear elsewhere in the datastream. IfI see a significant percentage of mis-

fires on any of the cylinders feeding thebank one converter, I’ll want to fix themisfire first, then retest the converter,because I know that a misfiring cylin-der will send a slug of unburned fuelmixture downstream, and that, in turn,will throw off both O2 switch rates forthat bank.

Whoa! Shouldn’t there be a misfirecode if that’s the issue? As helpful as thatwould be, the short answer is “Not al-ways.” It’s up to each manufacturer todetermine its own threshold criteria forflagging misfire codes. In Ford’s case,there seems to be a reluctance to setmisfire codes except when catalyst dam-age is likely. As an experiment, a localFord guru deliberately grounded theNo. 1 plug wire on his F-150. Althoughthe MIL would flash even on the high-way, indicating imminent catalyst dam-age, the PCM never set a code for thisfault under any circumstance, evenwhen he drove the truck in this condi-tion daily for over a week. In fact, scandata never even showed a pending code!

In general, I’d probably be willing tocondemn the cat at this point if I foundno misfire. There is, however, at leastone more circumstance that would causeme to hesitate. If freeze frame datashows a relatively low IAT value, I wouldwonder whether the inferred midbedtemperature was correct. In my part ofthe country (the midwest), winter snowscan pile up fast and deep. Accumulationsof slush and ice can prevent a catalyticconverter from reaching proper operat-ing temperature, sometimes for weekson end, especially when a driver justbulls his way into or out of his drivewaywithout plowing or shoveling first.

Inferred midbed catalyst tempera-ture is a calculated value based on ap-plication of an algorithm combining en-gine run time, ambient temperature atstart-up, ECT, elapsed mileage, loadand numerous other factors. Being acalculation, it’s subject to errors arisingfrom unforeseen circumstances. Theparticular calculated value does not ap-pear in the generic datastream, but mayappear as a data PID in certain en-hanced interfaces. I’ve seen inferredmidbed temperature PIDs of over350°F after an overnight cold-soak atKOEO, before engine start-up!

50 July 2008


Circle #29

KOEO DatastreamFor the final section of this installment,I’ll focus on a too-often-overlooked as-pect of so-called driver’s seat diagnos-tics—KOEO datastream.

Once again, my initial exhortation isnot to be in too much of a hurry. In this

instance, I want you to turn the key on,but to not actually start the vehicle yet.Use the generic or global scanner inter-face first. Note the status of all moni-tors, check and record all DTCs (in-cluding pending and history codes),read and record freeze frame and all

available Mode 5 and Mode 6 informa-tion and, finally, select current data.

What are we looking for? As KarlSeyfert, MOTOR’S Executive Editor,suggested in his article “OBD IIGeneric PID Diagnosis” (September2007), there’s a wealth of informationwaiting to be put to good use. Take alook, for example, at the BARO andMAP PIDs. They should be in totalagreement with one another before theengine starts. If the engine is cold, lookfor ECT and IAT to agree closely, usu-ally within 5° to 20°F, depending onsunload and car color. Watch O2 sensorvoltages as their heaters kick in. Incor-rect bias voltages or defective heatercircuits can cause a myriad of fuel con-trol problems.

Slowly depress and release thethrottle while watching the TPS read-ing. I have resolved several lack-of-power complaints by smoothingbunched-up floor mats that preventedproper throttle opening, as well as sev-eral high idle complaints where some-thing underfoot or underhood inter-fered with achieving closed throttle.

KOEO data also reveals things likeminimum calculated load or minimumcalculated airflow rate for the particu-lar vehicle you’re working on. Some ofthis data may be surprising. For exam-ple, some manufacturers routinelyshow a calculated load of 10% or even20% with the engine not running!

Unusual or incorrect data seenKOEO should be noted and recorded.If the customer just drove the car in,does the ECT reflect a likely value? Ifthe complaint involves an inability toreset a sufficient number of readinessmonitors to pass a state emissions in-spection, is there some PID that mayindicate a problem in attaining monitorentry criteria? (A common example in-volves a low IAT reading on a Chrysler.The low reading prevents several mon-itors from running.)

I’ll conclude this discussion ofdatastream analysis next month withan examination of KOER datastream.Stay tuned.

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Circle #30

This article can be found online atwww.motormagazine.com.

37August 2008

Last month’s installmenton datastream analysisfocused on the value offreeze frame data, Mode5 and Mode 6 data andKOEO (key on, engine

off) datastream. This month’s discussionpicks up where we left off, with KOER(key on, engine running) analysis. So goahead, start the engine!

I recommend that KOER data col-lection always start in the generic, orglobal OBD II interface. Why? Becausegeneric datastream PID values are nev-er substitutes for actual sensor readings.For example, you can disconnect theMAP sensor connector on a Chrysler

product and drive it around while moni-toring datastream in the enhanced(manufacturer-specific) interface. (Trythis yourself; don’t just take my word forit.) You’ll see the MAP PID changealong with the TPS sensor reading andrpm, showing a range of values that re-flect likely MAP readings for each con-dition, moment by moment. These aresubstituted values. If you looked at theMAP voltage PID, however, it wouldshow an unchanging reference voltage.In the enhanced interface, substitutionscan and do occur. But in the generic in-terface, substituted values are never al-lowed. You would see MAP shown at aconstant pressure equal to something a

DATASTREAM IN-DEPTH ANALYSIS

BY SAM BELL

We began this two-part article with a

discussion of preliminary OBD II datastream

analysis, conducted with the engine off.

We’re going deeper this time, to explain

the value of datastream information

collected with the engine running.

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bit higher than BARO. The generic in-terface allows calculated values, butnever substituted values.

So, what are we looking for, now thatwe’ve finally started the engine? Thespecific answer, of course, will dependlargely on the details of the customercomplaint and/or DTC(s) that arestored. We might, for example, be focus-ing on fuel trim numbers (and trends) ifour code suggests an underlying air/fuelmetering problem. We might be lookingmost closely at engine coolant tempera-ture, and time-until-warm measure-ments when that seems warranted. Per-haps our problem lies in the evap area,or involves EGR flow. But ultimately, itdoesn’t matter what the specific issue is;we’ll have to focus in on the systemic in-teractions that determine the overallcharacteristics of a particular data set.

Here’s a concrete example to illus-trate what I mean. The vehicle in ques-tion is a 1999 Chevy Venture minivanwith the 3.4L V6. There was a DTC

P0171 (Exhaust Too Lean, Bank 1) inmemory with an active MIL. The sumof Short Term and Long Term FuelTrims in freeze frame was in excess of

50%. Fuel pressure and volume hadbeen verified as within specification.

When evaluating a fuel trim troublecode, one of the first steps must alwaysbe to verify that the oxygen sensor (onwhich the DTC is based) is functioningcorrectly. During the test drive, I ob-served the O2 sensor switching rich, butnot as often as would be expected if thevery large fuel trim corrections shownwere actually effective. Indeed, on theface of it, datastream seemed to confirmthe DTC. Longtime readers, however,can probably anticipate what my nexttests were: I checked the actual lambdavalue of the exhaust gases. Then Ilooked for a dynamic response as I artifi-cially enriched the system with a blast ofpropane, then enleaned it by discon-necting a major vacuum hose. (See“What Goes In…Harnessing Lambda asa Diagnostic Tool” in the September2005 issue of MOTOR. Search the indexat www.motormagazine.com for all MO-TOR magazine articles mentioned.) Hav-

38 August 2008


Data collection and analysis might yield somehelpful information, if you can find the wheatwithin the chaff. This is only a small portion of alarger data set with 100 values per PID.

When evaluating afuel trim troublecode, one of thefirst steps must

always be to verifythat the oxygen

sensor (on which theDTC is based) is

functioning correctly.

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39August 2008

ing found the idle lambda at a ridicu-lously low value of .85 (indicating a mix-ture with 15% more fuel than needed), Iwas not surprised to see that the O2 sen-sor didn’t register a rich condition untilthe engine was very nearly flooded withpropane. When I removed the purgehose, engine rpm climbed and the en-gine smoothed out, while lambdamarched toward the stoichiometric idealvalue of 1.00. Once the faulty O2 sensorwas replaced, all aspects of driveabilityimproved, and the minivan returned toits previous fuel consumption levels.

Dynamic tests verify DTC accuracy.In some instances, we may be able toutilize bidirectional controls embeddedwithin our scan tool packages to actuatevarious components. In other cases, wemay need to improvise, using signalsimulators, power probes, jumpers,propane or just good, old-fashioned testdriving as required to initiate changewithin the system we’re working on.(I’m not saying that it will always be as

easy as it was with the Venture. You andI know there will be problems that don’tset DTCs, problems that do set DTCsthat have no apparent connection to the

actual root fault and, of course, prob-lems that set appropriate codes yet arestill really hard to diagnose.)

Floodlights and SpotlightsOne of the most powerful features ofmost scan tools is, as nearly as I cantell, one of the least used. This is theso-called flight recorder, data logger ormovie mode. By whatever name it’sknown, this is an analytical tool of con-siderable value.

Take a look at the portion of savedscan data portrayed in the chart on page38. As you see, any value in that infor-mation is well hidden. This might betermed a “floodlight” view, showing toomany values for too many parameters.But look at the “spotlight” view above,where I’ve selected and graphed a fewof the same PID values. This was a ve-hicle where there was no DTC stored inmemory. By including both upstreamO2 sensors, I have provided myself across-check, as there is less likelihood of

Graphical representations of scan data “movies” can speed analysis. As an added bonus,using your scanner’s flight recorder mode allows you to concentrate on your driving. Thedata set here clearly points to a lack of adequate fuel volume. This graphical representa-tion is derived from the exact same movie capture seen in the chart on the previous page.

One of the mostpowerful features

of most scan tools—the so-called flightrecorder—seems tobe one of the leastused. But it’s ananalytical tool of

considerable value.

both being bad. Similarly, MAF andrpm track nicely with one another, againproviding a good cross-check. The datavalues at the cursor (the vertical line atframe 2) are called out at the left side ofeach PID’s plot. The upstream O2 sen-sors are switching nicely at 2000 rpm (asshown at frame �51), but the graphicinterface reveals an obvious problem at

higher speeds as the O2 sensors flat-linelean. A new fuel pump restored themissing performance.

Slow Motion andHigh SpeedMoviemakers speed up or slow downthe action on the screen by shooting atdifferent numbers of frames per sec-

ond. When film shot at 20 frames persecond is played back at 60 frames persecond, the action seems to be occur-ring at three times the speed. Just as a56k dial-up modem is slower than aDSL Internet connection, scan datatransfer rates also vary according to theinterface used. Generic communica-tion modes often travel at a crawl, es-

40 August 2008


Most MOTOR readers have at least a passing fa-miliarity with the concept of OBD II monitor

completion status. Even so, a brief refresher may bein order. OBD II monitors are simply formalized setsof self-tests all related to a particular system orcomponent.

CCoonnttiinnuuoouuss mmoonniittoorrss.. With a few very rare ex-ceptions (mostly for 1998 and earlier models), theso-called continuous monitors always show up as“complete,” “done” or “ready.” Take this status re-port with a grain of salt. Unplug the IAT sensor, startthe engine and check that the “ComprehensiveComponent Monitor” readiness status shows com-plete. Is the MIL on? Are there any pending codes?How long would you have to let the vehicle idle be-fore it will trip the MIL and show a P0113 (IAT Sen-sor Circuit Voltage High) DTC?

As it turns out, depending on the specific make,model and powertrain package, there are severalspecific criteria that must be met before the codewill set. In one instance, the PCM must detect a VSSsignal of 35 mph or more and an ECT value of 140°For more, the calculated IAT must be less than �38°Fand all of these conditions must be met for at least180 seconds of continuous duration, during whichno other engine DTCs are set—all while MAF is lessthan 12 grams per second. (This particular example,incidentally, is a two-trip code. Some other manu-facturers may make this and other DTCs under thecomponent monitor’s jurisdiction into one- or two-trip codes, sometimes with even more complicatedentry criteria.)

Continuous monitors include the comprehensivecomponent monitor, the fuel monitor and the mis-fire monitor. Each monitor runs continuously whenconditions are appropriate, but not during all actu-al driving. For example, the misfire monitor is oftensuspended during 4WD operation, since feedbackthrough the axles over rough roads might causeuneven disruption of the CKP signals, which could

otherwise be misidentified as misfires. Similarly, ex-tremely low fuel tank levels may suspend both mis-fire and fuel system monitors to avoid setting aDTC for running out of gas.

NNoonnccoonnttiinnuuoouuss mmoonniittoorrss.. As I pointed out lastmonth, it’s important to note the readiness status ofthe other, noncontinuous monitors as well. These arethe monitors whose status will change to “incom-plete,” “not ready” or “not done” when the codesare cleared. If a vehicle arrives at your shop showingone or more incomplete monitors, it’s likely thatsomeone has already cleared the codes before it gotto you. (There are a few vehicles—for example, some1996 Subarus—which may reset monitor status to in-complete at every key-off, or other vehicles whichmay have certain monitors which cannot be made torun to completion in normal driving, such as the evapmonitor on some Toyota Paseos.) If a vehicle showsup with incomplete monitors, however, you shouldcertainly document that fact on your work order andbe sure to advise the customer that there’s a very realpossibility that one or more other codes may recur af-ter the current repair has been completed. For moreon this subject, see my article “How Not to Get MIL-Stoned” in the April 2004 issue of MOTOR.

More importantly, for our present purposes, theexistence of incomplete monitors means that youmay not be getting the whole picture as to whatails the vehicle you’re looking at. Keep an openmind, remembering that there may be other, as yetunknown issues hidden behind that incompletemonitor, and try not to rush your diagnosis. Asmentioned in last month’s installment, there maybe some valuable data accessible via Mode 6 evenif the monitor is not complete, but there is a veryreal possibility that Mode 6 data for any incom-plete monitor may turn out to be unreliable. And,of course, don’t overlook any pending DTCs. Re-member, these do not illuminate the MIL, so youmust seek them out on your own.

Monitors 1.01

42 August 2008


pecially in comparison to CAN speeds.If you’re stuck with a generic interface,you can often accomplish more bylooking at less.

The key here is PID selection.Choose the smallest number of PIDsthat will give you the information you

actually need. Three or four are usu-ally sufficient. This is your version ofthe filmmaker’s high-speed actiontrick, as you get more updates perunit time the fewer PIDs you select.With several hundred possible PIDsfrom which to choose, it’s just too

easy to miss an intermittent dataglitch, or to drown in a sea of toomuch information (see “Live Data vs.‘Live Data’” on page 44).

Most MOTOR readers are familiarwith the ways in which some of the ma-jor OEMs have organized data PIDsfor display in their enhanced scan toolinterfaces. Groupings such as Misfire,Driveability, Emissions, Accessoriesand the like are good examples of thetypes of data sets you may want to con-struct while analyzing different sorts ofproblems. Tracking down a nasty inter-mittent problem? Don’t hesitate topare down the OEM groupings evenfurther to speed data updates.

Code-Setting Criteria andOperating ConditionsIf we’re trying to resolve a MIL-oncomplaint, it’s critical that we first re-view both the exact code-setting crite-ria and the operating conditions as re-vealed in our previously recordedfreeze frame data. We’ll need to drivein such a way as to complete a good“trip” so the affected monitors canrun to completion. (For a more de-tailed discussion of OBD II trips andmonitors, see “Monitors 1.01” onpage 40.) If we fail to meet the condi-tions under which the self-test (moni-tor) will run, we cannot hope to makeprogress. Using the previously record-ed freeze frame parameters gives us agood general idea of the operatingconditions required. Merely duplicat-ing speed, load, temperature and oth-er basic characteristics may not beenough. This is why we need to re-view and understand the details of thecode-setting criteria and the monitor’sself-test strategy. For example, somemonitors cannot run until others havealready reached completion. A typicalexample would be a catalytic convert-er monitor that is suspended until theoxygen sensor monitors have run andpassed.

Some trouble codes, or even pendingcodes, suspend multiple monitors. Oth-er vehicle faults may then go undetect-ed until all monitors can run again. AP0500 (VSS Malfunction) in a Corolla,for example, will effectively suspendeven the misfire monitor.

It seems like a no-brainer: When you’re done with allyour diagnostic tests and you’ve made the necessary

repairs, you should turn off the MIL, right? That’swhat your customer probably expects, and as we allknow, meeting customer expectations is an importantpart of running a successful business.

But there are often times when you should leavethe MIL on. If your area uses an OBD II “plug & play”emissions test, the regulations usually require that nomore than one monitor can be incomplete as of thetime of testing for model year 2001 and newer vehi-cles, with no more than two incomplete monitors for1996 to 2000 models. In some areas, retest eligibilityrequires that the converter monitor must show “com-plete” before a retest is valid.

If an emissions test or retest is looming in your cus-tomer’s future, you and he must work out the prosand cons of clearing the codes and resetting the mon-itors to “incomplete.” If you clear the codes, the mon-itors will reset as well. This will require that someonewill have to drive a sufficient number of monitors tocompletion before a retest will be valid. If localweather conditions, for example, will prevent themonitors from running in a timely way, your customermight be better off if you leave the MIL on. Then yourcustomer would have to drive only those portions ofthe drive trace needed to run the monitor underwhich the current DTC set.

For example, if you’re in the frigid climes of an up-per Midwestern winter and a customer’s vehicle failedan emissions test because of a faulty O2 sensor heater,you’ll both be ahead if you don’t clear the code, lettingit expire naturally as the heater monitor runs success-fully to completion on the next two trips. This willavoid the necessity of rerunning all the rest of themonitors. Of course, if the vehicle failed the evap mon-itor, you’ll be better off clearing the code, because pro-longed subfreezing temperatures may make runningthat particular monitor successfully virtually impossiblefor weeks at a time.

Lights Out?

The net result is that we may have to clear the currentDTCs and extinguish the MIL before our test drive canbear fruit. (But again, please be sure to read and recordall the freeze frame data, the status of all monitors, the listof both current and pending DTCs and any availableMode 6 data before clearing the MIL (see “Lights Out?”on page 42).

We’ll need to drive long enough to let the monitors inquestion reach completion. In some cases, this may requirean extended period of time. Many Ford products, for exam-ple, normally require a minimum of a six-hour cold-soak be-fore the evap monitor can run, although there may be ways toforce this issue in some instances. Many Chrysler oxygen sen-sor monitors run only after engine shut-down (with key off),

so that no amount of driving will ever bring them to comple-tion. Certain monitors, and apparently even certain scan tools,may require a key-off sequence before the monitor status willupdate from incomplete to complete. MOTOR offers an excel-lent resource to help you understand these details—the OBDII Drive Cycle CD Version 7.0, available from your localMOTOR Distributor (1-800-4A-MOTOR).

In some cases, local weather conditions may make monitorcompletion seem impossible until a later date, usually be-cause of ambient temperature requirements, although some-times as a result of road conditions. In most cases, however, itwill still be possible to complete the monitor by running thevehicle on a lift or dynamometer. This option may occasional-ly result in setting, say, an ABS code, but most monitors canbe run to completion swiftly and successfully on a lift. Thisoption may also offer a safer, faster alternative to actual driv-ing, as trees and telephone poles are less likely to jump infront of a vehicle on a stationary lift.

44 August 2008

DATASTREAM ANALYSIS

Circle #22

Intermittent interruptions of sensor datacan cause tricky driveability problems.

Some glitches may set a DTC while othersmay not. While viewing datastream mayreveal an intermittent sensor problem, itshould not be relied upon to do so. The is-sue, once again, is in the data rate. Even amoderately fast interface, say the 41.6kbps (kilobytes per second) J-1850 PWMused on many Ford products, can easilymiss a several-millisecond dropout if it’snot that particular PID’s turn in the data-stream. Where symptoms or DTCs point to-ward an intermittent sensor glitch, you’reprobably better off breaking out yourscope or graphing multimeter.

Live Data vs. ‘Live Data’

ConclusionsProper in-depth datastream analysiscan often light the way toward correctdiagnosis of driveability concerns.Recording all available DTCs, pendingDTCs, freeze frame data and Mode 5and Mode 6 results before clearing anyDTCs is essential. Specific setting cri-teria for each DTC are manufacturer-determined, regardless of whether thecode assigned is generic or manufac-turer-specific. Freeze frame data setscan be used to recreate the operatingconditions under which a previous fail-ure occurred and can help illuminatethe conditions under which certainself-tests are conducted. Mode 5 andMode 6 test results can help in analyz-ing the type and extent of certain fail-ures. KOEO datastream analysis cansometimes reveal sensor faults or ration-ality concerns that might otherwise beoverlooked.

Looking at KOEO and KOER data-stream on a regular basis makes known-good values familiar. Once you know

the correct values, the conditions ac-companying problems identified byfreeze frame are easier to spot. KOERdata can highlight current problems, es-

pecially when used in conjunction withgraphical scanner interfaces. Genericdata PIDs cannot include substitutedvalues, and so may point up faults easilyoverlooked in more enhanced inter-faces. Careful selection of custom-grouped PIDs can provide faster scan-ner update rates.

Pick your tools wisely. To verify hardfaults, monitor datastream as you runactuator tests. Look for any mismatchbetween the command sent to a com-ponent and its actual response. For in-termittent problems, record and graphdata. In tough cases, test circuits withyour scope or meter to verify actualvoltage for comparison to specs.

Used properly, these techniqueswill help you arrive quickly and confi-dently at an accurate diagnosis of theroot cause of most driveability com-plaints.

45August 2008

Circle #23

When trying toresolve a MIL-oncomplaint, it’scritical to first

review the exactcode-setting

criteria and theoperating

conditions asrevealed in the

freeze frame data. This article can be found online at

www.motormagazine.com.

If you don’t have a good starting point, driveability diagnosticscan be a frustrating experience. One of the best places to startis with a scan tool. The question asked by many is, “Which

scan tool should I use?” In a perfect world with unlimited re-sources, the first choice would probably be the factory scan tool.

Unfortunately, most techniciansdon’t have extra-deep pockets. That’swhy my first choice is an OBD IIgeneric scan tool. I’ve found that ap-proximately 80% of the driveabilityproblems I diagnose can be narroweddown or solved using nothing morethan OBD II generic parameters. Andall of that information is available onan OBD II generic scan tool that canbe purchased for under $300.

The good news is the recent phase-in

of new parameters will make OBD IIgeneric data even more valuable. Fig. 1on page 54 was taken from a 2002 Nis-san Maxima and shows the typical para-meters available on most OBD II-equipped vehicles. As many as 36 para-meters were available under the originalOBD II specification. Most vehiclesfrom that era will support 13 to 20 para-meters. The California Air ResourcesBoard (CARB) revisions to OBD IICAN-equipped vehicles will increase

the number of potential generic para-meters to more than 100. Fig. 2 on page56 shows data from a CAN-equipped2005 Dodge Durango. As you can see,the quality and quantity of data has in-creased significantly. This article willidentify the parameters that provide thegreatest amount of useful informationand take a look at the new parametersthat are being phased in.

No matter what the driveability is-sue happens to be, the first parame-

53March 2005

INTERPRETING G E N E R I CSCAN DATA

BY BOB PATTENGALE

Readily available ‘generic’ scan data provides anexcellent foundation for OBD II diagnostics.

Recent enhancements have increased the value ofthis information when servicing newer vehicles.

ters to check are short-term fuel trim(STFT) and long-term fuel trim(LTFT). Fuel trim is a key diagnosticparameter and your window into whatthe computer is doing to control fueldelivery and how the adaptive strategyis operating. STFT and LTFT are ex-pressed as a percentage, with the idealrange being within �5%. Positive fueltrim percentages indicate that thepowertrain control module (PCM) isattempting to enrichen the fuel mix-ture to compensate for a perceivedlean condition. Negative fuel trimpercentages indicate that the PCM isattempting to enlean the fuel mixtureto compensate for a perceived richcondition. STFT will normally sweeprapidly between enrichment and en-leanment, while LTFT will remainmore stable. If STFT or LTFT ex-ceeds �10%, this should alert you toa potential problem.

The next step is to determine if thecondition exists in more than one op-

erating range. Fuel trim should bechecked at idle, at 1500 rpm and at2500 rpm. For example, if LTFT B1 is25% at idle but corrects to 4% at both1500 and 2500 rpm, your diagnosisshould focus on factors that can causea lean condition at idle, such as a vacu-um leak. If the condition exists in allrpm ranges, the cause is more likely tobe fuel supply-related, such as a badfuel pump, restricted injectors, etc.

Fuel trim can also be used to identi-fy which bank of cylinders is causing aproblem. This will work only on bank-to-bank fuel control engines. For ex-ample, if LTFT B1 is �20% and LTFTB2 is 3%, the source of the problem isassociated with B1 cylinders only, andyour diagnosis should focus on factorsrelated to B1 cylinders only.

The following parameters could af-fect fuel trim or provide additionaldiagnostic information. Also, even iffuel trim is not a concern, you mightfind an indication of another problem

when reviewing these parameters:Fuel System 1 Status and Fuel

System 2 Status should be in closed-loop (CL). If the PCM is not able toachieve CL, the fuel trim data may notbe accurate.

Engine Coolant Temperature(ECT) should reach operating temper-ature, preferably 190°F or higher. Ifthe ECT is too low, the PCM mayrichen the fuel mixture to compensatefor a (perceived) cold engine condition.

Intake Air Temperature (IAT)should read ambient temperature orclose to underhood temperature, de-pending on the location of the sensor.In the case of a cold engine check—Key On Engine Off (KOEO)—theECT and IAT should be within 5°F ofeach other.

The Mass Airflow (MAF) Sensor,if the system includes one, measuresthe amount of air flowing into the en-gine. The PCM uses this informationto calculate the amount of fuel that

54 March 2005

INTERPRETING GENERIC SCAN DATA

Fig. 1

should be delivered, to achieve thedesired air/fuel mixture. The MAFsensor should be checked for accura-cy in various rpm ranges, includingwide-open throttle (WOT), and com-pared with the manufacturer’s recom-mendations. Mark Warren’s Dec.2003 Driveability Corner column cov-ered volumetric efficiency, whichshould help you with MAF diagnos-tics. A copy of that article is availableat www.motor.com, and an updatedvolumetric efficiency chart is availableat www.pwrtraining.com.

When checking MAF sensor read-ings, be sure to identify the unit ofmeasurement. The scan tool may re-port the information in grams per sec-ond (gm/S) or pounds per minute(lb/min). For example, if the MAFsensor specification is 4 to 6 gm/S andyour scan tool is reporting .6 lb/min,change from English units to metricunits to obtain accurate readings.Some technicians replace the sensor,only to realize later that the scan toolwas not set correctly. The scan toolmanufacturer might display the para-

meter in both gm/S and lb/min to helpavoid this confusion.

The Manifold Absolute Pressure(MAP) Sensor, if available, measuresmanifold pressure, which is used bythe PCM to calculate engine load. Thereading in English units is normallydisplayed in inches of mercury(in./Hg). Don’t confuse the MAP sen-sor parameter with intake manifoldvacuum; they’re not the same. A sim-ple formula to use is: barometric pres-sure (BARO) � MAP � intake mani-fold vacuum. For example, BARO27.5 in./Hg � MAP 10.5 � intakemanifold vacuum of 17.0 in./Hg. Somevehicles are equipped with only aMAF sensor, some have only a MAPsensor and some are equipped withboth sensors.

Oxygen Sensor Output VoltageB1S1, B2S1, B1S2, etc., are used bythe PCM to control fuel mixture. An-other use for the oxygen sensors is todetect catalytic converter degradation.The scan tool can be used to check ba-sic sensor operation. Another way totest oxygen sensors is with a graphing

scan tool, but you can still use the datagrid if graphing is not available onyour scanner. Most scan tools on themarket now have some form of graph-ing capability.

The process for testing the sensorsis simple: The sensor needs to exceed.8 volt and drop below .2 volt, and thetransition from low to high and highto low should be quick. In most cases,a good snap throttle test will verifythe sensor’s ability to achieve the .8and .2 voltage limits. If this methoddoes not work, use a bottle ofpropane to manually richen the fuelmixture to check the oxygen sensor’smaximum output. To check the lowoxygen sensor range, simply create alean condition and check the voltage.Checking oxygen sensor speed iswhere a graphing scan tool helps. Fig.3 on page 57 and Fig. 4 on page 58show examples of oxygen sensor datagraphed, along with STFT, LTFT andrpm, taken from two different graph-ing scan tools.

Remember, your scan tool is not alab scope. You’re not measuring the

56 March 2005


Fig. 2

sensor in real time. The PCM re-ceives the data from the oxygen sen-sor, processes it, then reports it to thescan tool. Also, a fundamental OBDII generic limitation is the speed atwhich that data is delivered to thescan tool. In most cases, the fastestpossible data rate is approximately 10times a second with only one parame-ter selected. If you’re requestingand/or displaying 10 parameters, thisslows the data sample rate, and eachparameter is reported to the scan tooljust once per second. You can achievethe best results by graphing or dis-playing data from each oxygen sensorseparately. If the transition seemsslow, the sensor should be tested witha lab scope to verify the diagnosis be-fore you replace it.

Engine Speed (RPM) and Igni-tion Timing Advance can be usedto verify good idle control strategy.Again, these are best checked using agraphing scan tool.

The RPM, Vehicle Speed Sensor(VSS) and Throttle Position Sensor(TPS) should be checked for accuracy.

These parameters can also be used asreference points to duplicate symptomsand locate problems in recordings.

Calculated Load, MIL Status,Fuel Pressure and Auxiliary InputStatus (PTO) should also be consid-ered, if they are reported.

Additional OBD IIParametersNow, let’s take a look at the more re-cently introduced OBD II parameters.These parameters were added on 2004CAN-equipped vehicles, but may alsobe found on earlier models or non-CAN-equipped vehicles. For example,the air/fuel sensor parameters wereavailable on earlier Toyota OBD II ve-hicles. Fig. 2 was taken from a 2005Dodge Durango and shows many ofthe new parameters. Parameter de-scriptions from Fig. 2 are followed bythe general OBD II description:

FUEL STAT 1 � Fuel System 1Status: Fuel system status will displaymore than just Closed Loop (CL) orOpen Loop (OL). You might find one

of the following messages: OL-Drive,indicating an open-loop conditionduring power enrichment or decelera-tion enleanment; OL-Fault, indicatingthe PCM is commanding open-loopdue to a system fault; CL-Fault, indi-cating the PCM may be using a differ-ent fuel control strategy due to anoxygen sensor fault.

ENG RUN TIME � Time Since En-gine Start: This parameter may beuseful in determining when a particu-lar problem occurs during an enginerun cycle.

DIST MIL ON � Distance TraveledWhile MIL Is Activated: This para-meter can be very useful in determin-ing how long the customer has al-lowed a problem to exist.

COMMAND EGR � EGR_PCT:Commanded EGR is displayed as apercentage and is normalized for allEGR systems. EGR commandedOFF or Closed will display 0%, andEGR commanded to the fully open

57March 2005

Fig. 3

position will display 100%. Keep inmind this parameter does not reflectthe quantity of EGR flow—only whatthe PCM is commanding.

EGR ERROR � EGR_ERR: Thisparameter is displayed in percentageand represents EGR position errors.The EGR Error is also normalized forall types of EGR systems. The readingis based on a simple formula: (ActualEGR Position � Commanded EGR) �Commanded EGR � EGR Error. Forexample, if the EGR valve is command-ed open 10% and the EGR valve movesonly 5% (5% � 10%) � 10% � �50%error. If the scan tool displays EGR Er-ror at 99.2% and the EGR is command-ed OFF, this indicates that the PCM isreceiving information that the EGRvalve position is greater than 0%. Thismay be due to an EGR valve that isstuck partially open or a malfunctioningEGR position sensor.

EVAP PURGE � EVAP_PCT: Thisparameter is displayed as a percent-age and is normalized for all types ofpurge systems. EVAP Purge Control

commanded OFF will display 0% andEVAP Purge Control commandedfully open will display 100%. This isan important parameter to check ifthe vehicle is having fuel trim prob-lems. Fuel trim readings may be ab-normal, due to normal purge opera-tion. To eliminate EVAP Purge as apotential contributor to a fuel trimproblem, block the purge valve inletto the intake manifold, then recheckfuel trim.

FUEL LEVEL � FUEL_PCT: Fuellevel input is a very useful parameterwhen you’re attempting to completesystem monitors and diagnose specif-ic problems. For example, the misfiremonitor on a 1999 Ford F-150 re-quires the fuel tank level to begreater than 15%. If you’re attempt-ing to duplicate a misfire condition bymonitoring misfire counts and the fuellevel is under 15%, the misfire moni-tor may not run. This is also impor-tant for the evaporative emissionsmonitor, where many manufacturersrequire the fuel level to be above15% and below 85%.

WARM-UPS � WARM_UPS: Thisparameter will count the number ofwarm-ups since the DTCs were cleared.A warm-up is defined as the ECT risingat least 40°F from engine starting tem-perature, then reaching a minimumtemperature of 160°F. This parameterwill be useful in verifying warm-up cy-cles, if you’re attempting to duplicate aspecific code that requires at least twowarm-up cycles for completion.

BARO � BARO: This parameter isuseful for diagnosing issues withMAP and MAF sensors. Check thisparameter KOEO for accuracy relat-ed to your elevation.

C AT TMP B1S1 /B2S1 �CATEMP11, 21, etc.: Catalyst tem-perature displays the substrate temper-ature for a specific catalyst. The tem-perature value may be obtained directlyfrom a sensor or inferred using othersensor inputs. This parameter shouldhave significant value when checkingcatalyst operation or looking at reasonsfor premature catalyst failure, say, dueto overheating.

58 March 2005


Fig. 4

CTRL MOD (V) � VPWR: I wassurprised this parameter was not in-cluded in the original OBD II specifi-cation. Voltage supply to the PCM iscritical and is overlooked by manytechnicians. The voltage displayedshould be close to the voltage presentat the battery. This parameter can beused to look for low voltage supply is-sues. Keep in mind there are othervoltage supplies to the PCM. The igni-tion voltage supply is a common sourceof driveability issues, but can still bechecked only with an enhanced scantool or by direct measurement.

ABSOLUT LOAD � LOAD_ABS:This parameter is the normalized valueof air mass per intake stroke displayedas a percentage. Absolute load valueranges from 0% to approximately 95%for normally aspirated engines and 0%to 400% for boosted engines. The infor-mation is used to schedule spark andEGR rates, and to determine thepumping efficiency of the engine for di-agnostic purposes.

OL EQ RATIO � EQ_RAT: Com-manded equivalence ratio is used to de-termine the commanded air/fuel ratioof the engine. For conventional oxygensensor vehicles, the scan tool should dis-play 1.0 in closed-loop and the PCM-

commanded EQ ratio during open-loop. Wide-range and linear oxygensensors will display the PCM-com-manded EQ ratio in both open-loopand closed-loop. To calculate the actualA/F ratio being commanded, multiplythe stoichiometric A/F ratio by the EQratio. For example, stoichiometric is a14.64:1 ratio for gasoline. If the com-manded EQ ratio is .95, the command-ed A/F is 14.64 � 0.95 � 13.9 A/F.

TP-B ABS, APP-D, APP-E, COM-MAND TAC: These parameters relateto the throttle-by-wire system on the2005 Dodge Durango of Fig. 2 and willbe useful for diagnosing issues with thissystem. There are other throttle-by-wiregeneric parameters available for differ-ent types of systems on other vehicles.

There are other parameters of inter-est, but they’re not displayed or avail-able on this vehicle. Misfire data will beavailable for individual cylinders, similarto the information displayed on a GMenhanced scan tool. Also, if available,wide-range and linear air/fuel sensorsare reported per sensor in voltage ormilliamp (mA) measurements.

Fig. 5 above shows a screen capturefrom the Vetronix MTS 3100 Mas-tertech. The red circle highlights the“greater than” symbol (>), indicatingthat multiple ECU responses differ in

value for this parameter. The blue cir-cle highlights the equal sign (=), indi-cating that more than one ECU sup-ports this parameter and similar valueshave been received for this parameter.Another possible symbol is the excla-mation point (!), indicating that no re-sponses have been received for thisparameter, although it should be sup-ported. This information will be usefulin diagnosing problems with data onthe CAN bus.

As you can see, OBD II generic datahas come a long way, and the data canbe very useful in the diagnostic process.The important thing is to take time tocheck each parameter and determinehow they relate to one another.

If you haven’t already purchased anOBD II generic scan tool, look forone that can graph and record, if pos-sible. The benefits will immediatelypay off. The new parameters will takesome time to sort out, but the diag-nostic value will be significant. Keepin mind that the OBD II genericspecification is not always followed tothe letter, so it’s important to checkthe vehicle service information forvariations and specifications.

60 March 2005


Fig. 5

Visit www.motor.com to downloada free copy of this article.

Some scan tools call it theglobal OBD II mode, whileothers describe it as theOBD II generic mode. TheOBD II generic mode allowsa technician to attach his

scan tool to an OBD II-compliant vehi-cle and begin collecting data withoutentering any VIN information into thescan tool. You may need to specificallyselect “OBD II Generic” from the scantool menu. Some scan tools may need asoftware module or personality key be-fore they’ll work in generic OBD II testmode.

The original list of generic data pa-rameters mandated by OBD II and de-scribed in SAE J1979 was short and de-signed to provide critical system dataonly. The useful types of data we can re-trieve from OBD II generic include

short-term and long-term fuel trim val-ues, oxygen sensor voltages, engine andintake air temperatures, MAF or MAPvalues, rpm, calculated load, spark tim-ing and diagnostic trouble code (DTC)count. Freeze frame data and readinessstatus also are available in OBD IIgeneric mode. A generic scan tool alsoshould be able to erase trouble codesand freeze frame data when command-ed to do so.

Data coming to the scan tool throughthe mandated OBD II generic interfacemay not arrive as fast as data sent overone of the dedicated data link connec-tor (DLC) terminals. The vehicle man-ufacturer has the option of using afaster data transfer speed on other DLCpins. Data on the generic interface alsomay not be as complete as the informa-tion you’ll get on many manufacturer-

52 September 2007

OBDII GENERIC PIDDIAGNOSIS

BY KARL SEYFERT

A wealth of diagnostic information is

available on late-model OBD II-compliant

vehicles, even when ‘enhanced’ or

‘manufacturer-specific’ PIDs are not

accessible. It doesn’t take much to use

this information to its best advantage.

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specific or enhanced interfaces. For ex-ample, you may see an engine coolanttemperature (ECT) value in degrees onthe OBD II generic parameter identi-fication (PID) list. A manufacturer-specific data list may display ECT statusin Fahrenheit or Celsius and add a sep-arate PID for the ECT signal voltage.In spite of these and other limitations,OBD II generic mode still containsmany of the trouble codes, freeze framedata and basic datastream informationneeded to solve many emissions-relatedissues.

There are nine modes of operationdescribed in the original J1979 OBD IIstandard. They are:

Mode 1: Show current dataMode 2: Show freeze frame dataMode 3: Show stored trouble codesMode 4: Clear trouble codes and storedvaluesMode 5: Test results, oxygen sensorsMode 6: Test results, noncontinuouslymonitoredMode 7: Show pending trouble codesMode 8: Special control modeMode 9: Request vehicle information

Modes 1 and 2 are basically identical.Mode 1 provides current information,Mode 2 a snapshot of the same datataken at the point when the last diag-nostic trouble code was set. The excep-tions are PID 01, which is available only

in Mode 1, and PID 02, available onlyin Mode 2. If Mode 2 PID 02 returnszero, then there’s no snapshot and allother Mode 2 data is meaningless. Vehi-cle manufacturers are not required tosupport all modes. Each manufacturermay define additional modes aboveMode 9 for other information.

Most vehicles from the J1979 era sup-ported 13 to 20 parameters. The recentphase-in of new parameters will makeOBD II generic data even more valu-able. The California Air ResourcesBoard (CARB) revisions to OBD IICAN-equipped vehicles have increasedthe number of potential generic param-eters to more than a hundred. Not allvehicles will support all PIDs, and thereare many manufacturer-defined PIDsthat are not included in the OBD IIstandard. Even so, the quality andquantity of data have increased signifi-cantly. For more information on the newPIDs that were added to 2004 and laterCAN-equipped vehicles, refer to BobPattengale’s article “Interpreting Gener-ic Scan Data” in the March 2005 issue ofMOTOR. A PDF copy of the article canbe downloaded at www.motor.com.

Establish a BaselineIf you’re repairing a vehicle that hasstored one or more DTCs, make sureyou collect the freeze frame data beforeerasing the stored codes. This data can

54 September 2007

OBD II GENERIC PID DIAGNOSIS

Here’s a basic scanner display showing OBD II genericPIDs. Slow-changing PIDs like IAT and ECT can be fol-lowed fairly easily in this format, but it’s difficult to spotglitches in faster moving PIDs like Spark Advance.

This scan tool also allows the user to graph some PIDs,while continuing to display the others in conventionalnumeric format. Due to OBD II’s refresh capabilities onsome vehicles, it’s best to limit your PID choices tothose directly related to your diagnostic approach.

This photo illustrates how far PID data collection and display have come. Severalhundred thousand techs are still using the original Snap-on “brick” (on the left),which displays a limited amount of PID data on its screen. Scrolling up or downrevealed more PIDs. The color version on the right brought graphing capability tothe brick, and extended the product’s life span by several years.

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be used for comparison afteryour repairs. The “before”freeze frame shot and its PIDdata establish the baseline.

As you begin your diagno-sis, correct basic problemsfirst—loose belts, weak bat-teries, corroded cables, lowcoolant levels and the like.The battery and charging sys-tem are especially important,due to their effect on vehicleelectronics. A good battery, aproperly functioning alterna-tor and good connections atpower and ground circuitsare essential. You can’t as-sume that OBD II will detecta voltage supply problem thatcan affect the entire system.If you have an intermittentproblem that comes andgoes, or random problemsthat don’t follow a logical pattern, checkthe grounds for the PCM and any othercontroller in the vehicle.

If the basics check out, focus your di-agnosis on critical engine parametersand sensors first. Write down what youfind; there’s too much information tokeep it all in your head. Add any infor-mation collected from the vehicle own-er regarding vehicle performance. Jot

down the battery voltage and the resultsof any simple tests, such as fuel pressureor engine vacuum. Look at the Readi-ness Status display to see if there areany monitors that aren’t running tocompletion.

Datastream AnalysisTake your time when you begin lookingat the live OBD II datastream. If you se-lect too many items at one time, the scantool update will slow. The more PIDsyou select, the slower the update ratewill be. Look carefully at the PIDs andtheir values. Is there one line of data thatseems wrong? Compare data items toone another.

Do MAP and BARO agree key on,engine off (KOEO)? Are IAT and ECTthe same when the engine is coldKOEO? The ECT and IAT should bewithin 5°F of each other. ECT shouldreach operating temperature, preferably190°F or higher. If the ECT is too low,the PCM may richen the fuel mixture tocompensate for a (perceived) cold-engine condition. IAT should read ambi-ent temperature or close to underhoodtemperature, depending on the locationof the sensor.

Is the battery voltage good KOEO?Is the charging voltage adequate whenthe engine starts? Do the MAP andBARO readings seem logical? Do the

IAC counts look too high ortoo low? Compare data itemsto known-good values you’dexpect to see for similar op-erating conditions on similarvehicles.

Check short-term fueltrim (STFT) and long-termfuel trim (LTFT). Fuel trimis a key diagnostic parameterand tells you what the com-puter is doing to control fueldelivery and how the adap-tive strategy is operating.STFT and LTFT are ex-pressed as a percentage, withthe ideal range being within±5%. Positive fuel trim per-centages indicate that thepowertrain control module(PCM) is attempting to en-richen the fuel mixture tocompensate for a perceived

lean condition. Negative fuel trim per-centages indicate that the PCM is at-tempting to enlean the fuel mixture tocompensate for a perceived rich condi-tion. STFT will normally sweep rapidlybetween enrichment and enleanment,while LTFT will remain more stable. Ifeither STFT or LTFT exceeds ±10%,this should alert you to a potentialproblem.

56 September 2007


The Snap-on MODIS is a combination scanner, lab/igni-tion scope, DVOM and Troubleshooter. In scanner mode,MODIS can graph several parameters simultaneously,as seen in this screen capture. Remember, althoughthese may look like scope patterns, the reporting ratefor PID data on a scanner isn’t nearly as fast.

When scan tool screen real estate islimited, porting the scan tool into alaptop or desktop PC allows you tograph more PIDs simultaneously.The PC’s much larger memory ca-pacity also makes it possible to col-lect PID data in movie format forlater playback and analysis.

An on-screen description of the PIDdisplayed below the graphing datamay help you to understand whatyou’re looking at, and avoid misunder-standings with measurement units.

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Determine if the condition exists inmore than one operating range. Checkfuel trim at idle, at 1500 rpm and at2500 rpm. If LTFT B1 is 20% at idle butcorrects to 5% at both 1500 and 2500rpm, focus your diagnosis on factors thatcan cause a lean condition at idle, suchas a vacuum leak. If the condition existsin all rpm ranges, the cause is more like-ly to be fuel-related, such as a bad fuelpump, restricted injectors, etc.

Fuel trim can also be used to identifywhich bank of cylinders is causing aproblem on bank-to-bank fuel controlengines. For example, if LTFT B1 is�25% and LTFT B2 is 5%, the sourceof the problem is associated with B1cylinders only, and your diagnosisshould focus on factors related to B1cylinders only.

The following parameters could af-fect fuel trim or provide additional diag-nostic information. Also, even if fueltrim is not a concern, you might find anindication of another problem when re-viewing these parameters:

Fuel System 1 Status and Fuel Sys-tem 2 Status should be in closed-loop(CL). If the PCM is not able toachieve CL, the fuel trim data may notbe accurate.

If the system includes one, the massairflow (MAF) sensor measures theamount of air flowing into the engine.

The PCM uses this information to cal-culate the amount of fuel that should bedelivered to achieve the desired air/fuelmixture. Check the MAF sensor for ac-curacy in various rpm ranges, includingwide-open throttle (WOT), and com-pare it with the manufacturer’s recom-mendations.

When checking MAF sensor read-

ings, be sure to identify the unit of mea-surement. The scan tool may report theinformation in grams per second (gm/S)or pounds per minute (lb/min). Sometechnicians replace the sensor, only torealize later that the scan tool was notset correctly. Some scan tools let youchange the units of measurement fordifferent PIDs so the scan tool matchesthe specification in your reference man-ual. Most scan tools let you switch easilybetween Fahrenheit and Celsius tem-perature scales, for example. But MAFspecs can be confusing when the scantool shows lb/min and we have a specfor gm/S. Here are a few common con-version formulas, in case your scan tooldoesn’t support all of these units ofmeasurement:

Degrees Fahrenheit �� 32 �� 5/9 �� Degrees Celsius Degrees Celsius �� 9/5 + 32 �� Degrees Fahrenheit lb/min �� 7.5 �� gm/Sgm/S �� 1.32 �� lb/min

The Manifold Absolute Pressure(MAP) Sensor PID, if available, indi-cates manifold pressure, which is usedby the PCM to calculate engine load.The reading is normally displayed ininches of mercury (in./Hg). Don’t con-fuse the MAP sensor parameter with in-take manifold vacuum; they’re not thesame. Use this formula: barometric

58 September 2007


Graphs aren’t the only way to display PID data. Oncetransferred to the PC with its greater screen real es-tate, PID data can be converted to formats that relateto the data. A red thermometer scale is much easier tofollow than changing numbers on a scan tool.

PC-based scan tools excel at capturing and displayinglarge amounts of PID data for later analysis. Graphingthe data, then analyzing it on-screen, may allow you tospot inconsistencies and provides an easy method foroverlaying similar or related PID data.

Here’s a peek at some of the addition-al PID data that’s available on late-model vehicles. This screen capturewas taken from a CAN-enabled 2005vehicle, and includes PIDs for EVAPPURGE, FUEL LEVEL and WARM-UPS, aswell as familiar PIDs like BARO. Thismuch PID data in generic mode shouldaid in diagnosis when manufacturer-specific PID data is not available.

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pressure (BARO) � MAP � intakemanifold vacuum. For example, BARO(27.5 in./Hg) � MAP (10.5) � intakemanifold vacuum (17.0 in./Hg). Somevehicles are equipped with only a MAFsensor, some have only a MAP sensorand some are equipped with both.

The PIDs for Oxygen Sensor OutputVoltage B1S1, B2S1, B1S2, etc., areused by the PCM to control fuel mix-ture and to detect catalytic converterdegradation. The scan tool can be usedto check basic sensor operation. Thesensor must exceed .8 volt and drop be-low .2 volt, and the transition from lowto high and high to low should be quick.A good snap throttle test will verify thesensor’s ability to achieve the .8 and .2voltage limits. If this method doesn’twork, use a bottle of propane to manu-ally richen the fuel mixture to check theoxygen sensor’s maximum voltage out-put. To check the sensor’s low voltagerange, simply create a lean conditionand check the voltage.

Remember, your scan tool is not a labscope. You’re not measuring the sensorin real time. The PCM receives the datafrom the oxygen sensor, processes it,then reports it to the scan tool. Also, afundamental OBD II generic limitationis the speed at which that data is deliv-ered to the scan tool. In most cases, thefastest possible data rate is approximate-ly 10 times a second, with only one pa-rameter selected. If you’re requestingand/or displaying 10 parameters, thisslows the data sample rate, and each pa-rameter is reported to the scan tool justonce per second. You can achieve thebest results by graphing or displayingdata from each oxygen sensor separately.If the transition seems slow, the sensorshould be tested with a lab scope to veri-fy the diagnosis before you replace it.

The Engine Speed (RPM) and Igni-tion Timing Advance PIDs can be usedto verify good idle control strategy.Again, these are best checked using agraphing scan tool. Check the RPM,

Vehicle Speed Sensor (VSS) and Throt-tle Position Sensor (TPS) PIDs for ac-curacy. These parameters can also beused as reference points to duplicatesymptoms and locate problems inrecordings.

Most PID values can be verified bya voltage, frequency, temperature,vacuum or pressure test. Enginecoolant temperature, for example, canbe verified with a noncontact temper-ature tester, while intake manifold vac-uum can be verified with an accuratevacuum gauge. Electrical values alsoshould be tested with a DVOM. If theelectrical value exists at the sensor butnot at the appropriate PCM terminal,then the component might be experi-encing a circuit fault.

Calculated ValuesCalculated scan tool values can cause alot of confusion. The PCM may detect afailed ECT sensor or circuit and store aDTC. Without the ECT sensor input,

59September 2007

Circle #31

the PCM has no idea what the coolanttemperature really is, so it may “plug in”a temperature it thinks will work tokeep the engine running long enough toget it to a repair shop. When it doesthis, your scanner will display the fail-safe value. You might think it’s a live val-ue from a working sensor, when it isn’t.

Also be aware that when a compo-nent such as an oxygen sensor is discon-nected, the PCM may substitute a de-fault value into the datastream displayedon the scan tool. If a PID is static anddoesn’t track with engine operating con-ditions, it may be a default value thatmerits further investigation.

Graphing DataIf you’ve ever found it difficult to com-pare several parameters at once on asmall scan tool screen, graphing PIDs isan appealing proposition. Graphingmultiple parameters at the same timecan help you compare data and look forindividual signals that don’t match up toactual operating conditions.

Although scan tool graphing isn’tequivalent in quality and accuracy to alab scope reading, it can provide a com-parative analysis of the activity in thetwo, three, four or six oxygen sensorsfound in most OBD II systems.

Many scan tools are capable of stor-ing a multiple-frame movie of selectedPIDs. The scan tool can be programmedto record a movie after a specific DTCis stored in the PCM. Alternatively, thescan tool movie might be triggeredmanually when a driveability symptomoccurs. In either case, you can observethe data or download it and print it lat-er. Several software programs let youdownload a movie, then plot the valuesin a graphical display on your computermonitor.

Make the Most ofWhat You’ve GotTake the time to learn what your scantool will do when connected to a spe-cific make or model. Do your best togather all relevant information aboutthe vehicle system being tested. Thatway you can get the most out of whatthe scan tool and PCM have to offer.The OBD II system won’t store a DTCunless it sees (or thinks it sees) a prob-lem that can result in increased emis-sions. The only way to know what thePCM sees (or thinks it sees) is to lookthrough the window provided by thescan tool interface.

You have a DTC and its definition.You have freeze frame data that mayhelp you zero in on the affected compo-nent or subsystem. PIDs have alreadyprovided you with additional cluesabout the operation of critical sensors.Keep your diagnosis simple as long asyou can. Now fix the car.

60 September 2007



Circle #32

Circle #33

Circle #34Circle #35

would reach the top of Mount Everest.On May 29, 1953, Sir Edmund Hillaryand Tenzing Norgay had reached the

highest point on earth, some 29,029 ft.above sea level. At this elevation thereis very little oxygen in the atmosphere,

Aflash of sunlight reflected off the ice hammer’shead as it swung forward, breaking through theage-old ice. Normally this would have made aloud noise, but the only noise that Edmund heard

was his own heart racing. The lack of oxygen had every musclein his body aching in agony as he took his next step up themountain. He thought to himself, Just a few more steps and I’llbe standing on top of the world. He had dreamed of the day he

29July 2008

FUEL INJECTION DIAGNOSIS: IT’S ALL ABOUT THE AIR

BY BERNIE THOMPSON

An engine can’t run without fuel and air.

But how much fuel and how much air

are needed to make it run efficiently?

If one quantity can be established,

the fuel injection system has a better

chance of figuring out how much of

the other should be required.

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and this climb had been accomplishedwithout the aid of bottled oxygen. Theamount of oxygen that’s contained inthe atmosphere is important becauseit’s this oxygen that’s taken in by ourbodies and converted chemically togive us the energy we need to do work.A similar release of energy is what pow-ers the internal combustion engine.

In order for this combustion energy

to be released, a chemical reaction musttake place between oxygen and the hy-drocarbons in the fuel. This chemicalreaction is based on the weight mass ofthe two elements—oxygen and hydro-carbons—that react together. In thespark ignition internal combustion en-gine, this weight ratio can change from11:1 under a power demand to 17:1 in alean cruise condition. At both weight ra-

tio extremes, the tailpipe emissions lev-els will rise considerably.

Many years ago tailpipe emissions lev-els were not regulated. As the concernabout air quality around large citiesgrew, government regulations were im-posed on vehicle manufacturers. For themanufacturers to meet these emissionsregulations, a new technology emerged.This technology employs a method ofweighing precisely the air entering theengine and then delivering the correctweight of hydrocarbons or fuel for anengine’s running condition. This tech-nology is referred to as fuel injection.

Fuel injection can be either mechani-cal or electronic, or a combination ofboth. This discussion will center on elec-tronic fuel injection for the spark igni-tion internal combustion engine. Thereare two basic methods of fuel injectioncurrently used in vehicles—speed densi-ty and airflow. It’s important to knowwhich system you’re working on. For ex-ample, an exhaust gas recirculation(EGR) valve stuck open on a speed den-sity system produces a lower vacuumreading, which would normally indicatethat the engine was under load. Underthis condition additional fuel would beadded, which would overfuel the en-gine, so the fuel trim correction wouldgo negative and take fuel away. (Moreon fuel trim later.) On an airflow systemwith the same EGR valve problem, theairflow would be read correctly, so nofuel trim correction would be needed.

With the speed density method, anindirect calculation of air weight ismade by measuring the intake pressurechanges using the manifold absolutepressure (MAP) sensor. This sensordoes not directly measure the intakemanifold pressure; instead, it measuresthe displacement of a diaphragm that’sdeflected by intake manifold pressure.This intake pressure change is convert-ed by the MAP sensor to an outputmeasurement of pressure in kilopascals(kPa). The change in intake manifoldpressure can be used to calculate theload placed on an engine. The MAPsensor accomplishes this by monitoringthe intake pressure; as the throttle bladeis opened, it allows more air to enter the

30 July 2008

FUEL INJECTION DIAGNOSIS

Fig. 1

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engine and, thus, changes the pressurefrom a negative state (vacuum) to onethat’s slightly under atmospheric pres-sure at wide-open throttle.

The MAP sensor is desirable to usebecause the absolute engine workingpressure (vacuum) at idle and light loadis unchanged by elevation. At sea level,the barometric pressure is 101 kPa, anda good engine idle pressure is 27 kPa.Therefore, the engine vacuum is 101kPa � 27 kPa � 74 kPa, or 29.9 in./Hg� 10 in./Hg � 19.9 in./Hg. At 5500 ft.of elevation, the barometric pressure is84 kPa and a good engine idle pressureis 27 kPa. Therefore, the engine vacu-um is 84 kPa � 27 kPa � 57 kPa, or24.9 in./Hg � 10 in./Hg � 14.9 in./Hg.

An equation is needed to calculatethe airflow into the engine, and certainvalues must be known: the size of theengine in liters; the intake manifold ab-solute pressure, as determined by theMAP sensor; and the revolutions perminute (rpm), because in a four-strokeengine, only one stroke produces in-coming air. The rpm is determined bythe crankshaft position sensor. This willbecome a factor since the air mass iswhat we’re trying to measure. The airtemperature will also become a factor

because a change in temperature willcause a change in the density of the air.This is read by the intake air tempera-ture (IAT) sensor. At an air temperatureof �40°F, the air weighs 1.51 grams perliter (g/L); at an air temperature of

104°F, the air weighs 1.12 g/L—a 35%difference. The speed density base airequation is made with only three sen-sors and is as follows:RPM x Liters x MAP x IAT Air Density = Mass Air in g/sec60 2

With the airflow method, a calcula-tion of air weight can be made that isan indirect measurement or a direct

measurement of the air entering theintake manifold, depending on whichtype of sensor is used. This airflow ismeasured with a device called a massairflow (MAF) sensor. There are sever-al styles of these devices, the most pop-ular of which is the heated-elementtype. It’s based on the hot-wireanemometer weather forecasters usefor measuring wind velocity. A wire orelement is electrically heated to a settemperature above the temperature ofthe inlet air. As the throttle blade isopened, the velocity of air increases,which transfers the heat from the ele-ment into the air. An electronic circuitis designed to keep the element at a settemperature so that as its temperaturedecreases, the current flow across it in-creases. By monitoring the current, theairflow will be known. The PCM con-verts this signal into air weight, whichis read in g/sec.

In either of these methods—speeddensity or airflow—the air is the un-known quantity; therefore, the airflowing into the engine is what must bedetermined. Fuel injection is based onairflow, not fuel flow. The fuel deliveryweight is a known factor.

One example can be seen by using a25-lbs./hr. fuel injector. This number isbased on the engine’s brake-specificfuel consumption (BSFC), which indi-cates the engine’s fuel consumption ef-ficiency. The BSFC is usually mea-sured in pounds of fuel used per hourfor each unit of horsepower. This rela-tionship means that horsepower multi-plied by BSFC equals pounds of fuelconsumed per hour.

A 25-lbs./hr. fuel injector’s fuel deliv-ery is based on a constant fuel pressureand volume equal to 255cc/min., or.00425cc/millisecond. One cubic cen-timeter is equal to .162 gram of gaso-line. This fuel weight is a known quan-tity that will be delivered by the fuelinjector to the engine.

Now that both the air weight enter-ing the engine and the fuel weight be-ing delivered can be determined, anequation can be derived that will set aprecise air/fuel ratio for the engine.The equation is used to determine the

32 July 2008


Fuel injection isbased on airflow,not fuel flow. Theair flowing into

the engine is whatmust be determined.

Fig. 2

air mass weight contained in eachcylinder so the fuel weight can be de-livered properly. If there were anyproblems, such as the sensors misread-ing or incorrect fuel delivery, the baseair equation would need to be changedso that the correct air/fuel weightwould be maintained. This is donewith a multiplier to the base air equa-tion that’s called fuel trim (see Fig. 1on page 30). The purpose of fuel trimis to monitor the ratio of air to fuelweight and to keep it at a predeter-mined target value.

An oxygen sensor is located in theexhaust system to continually measurethe air/fuel ratio. It’s set up in a feed-back loop so it can report the air/fuelratio to the microprocessor, which will

use this information to adjust the fueltrim multiplier to keep the air/fuel ra-tio at the target value. This method ofcontrol is referred to as a closed-looplimit-cycle control system. One exam-ple of this type of control system is anoven. When the temperature is set to,say, 350°F, the electrical elementcomes on to heat the oven. The ovenstays on until it reaches a temperatureof 355°F, then shuts off. This tempera-ture is sensed by a sensor in the oven.The oven then cools down until itreaches 345°F. At this point the heat-ing element turns on, heating the ovento 355°F again. This cycle continues,to keep the oven close to the targettemperature of 350°F.

This type of control system canmaintain an average value very close to

the command input. On an internalcombustion engine, this system worksin much the same way. The fuel trimworks like the oven’s heating element,driving the system rich or lean. Theoxygen sensor works like the heat sen-sor in the oven, only it reports the

air/fuel changes. The oxygen sensor re-porting limits are set between .1 and .8volt. The oxygen sensor in this range isstoichiometric. For this sensor to berich it must be above .8 volt; to be leanit must be below .1 volt.

A vehicle’s fuel control system under

34 July 2008


Fig. 4

Fig. 3The purpose of fueltrim is to monitorthe ratio of air tofuel weight and tokeep it at a pre-determined value.

most conditions will cycle the oxygensensor in this .1- to .8-volt range. This isusually accomplished with Short TermFuel Trim (STFT). Since STFT drivesthe oxygen sensor, if the oxygen sensorresponse is slow, the STFT peak-to-peak value will increase. If the STFTvalue exceeds 8% peak-to-peak, the O2

sensor will have to be replaced (Fig. 2on page 32). The cycling oxygen sensorwill maintain the air/fuel ratio at 14.66lbs. of dry air to 1 lb. of gasoline. This isreferred to as stoichiometry, which isthe ideal mixture of air and fuel that,when ignited, will completely burn allof the hydrocarbons and leave only car-bon dioxide and water.

In a running engine, the air/fuelmixture will never completely burn,due in part to unvaporized fuel and hy-

drocarbons packing into the piston ringlands and the valve pocket areas. Thisair/fuel ratio is desirable for the catalyt-ic converter to work correctly, therebylowering the levels of tailpipe emis-sions.

Now that we have an understandingof the fuel injection fuel control system,let’s put it to work in repairing vehicles.Since the fuel injection system is allabout the air, it will be necessary to cal-culate the volumetric efficiency (VE) ofthe engine (see Mark Warren’s June2003 Driveability Corner for a conciseexplanation of VE). A Toyota 4Runnerwith a 3.0L engine was brought in be-cause of low power. The Check Enginelight came on and the driver com-plained of low power. The following di-

agnostic trouble codes (DTCs) werepulled: P0171 (system too lean), P0325(knock sensor 1 circuit) and P0330(knock sensor 2 circuit).

To find a diagnostic direction quicklyit’s necessary to calculate the VE. Thiscan be done by collecting the parame-

ter identifications (PIDs) that will beneeded while test driving the vehicle.Once you have the information, justrun the VE calculation to see whetherthe air going into the engine is correct.In this example (Fig. 3 on page 34), thescan tool automatically calculated the

36 July 2008


Fig. 6

Fig. 5A vehicle’s fuel

control system usesShort Term FuelTrim to cycle the

oxygen sensor in a.1- to .8-volt range.

volumetric efficiency of the enginewhile the vehicle was being driven. Theyellow trace is the MAF sensor signalthat the scan tool reads as grams persecond (g/sec) and the red trace is theVE reading, or theoretical airflow.

When these two measurements of theair flowing into the engine are com-pared, it’s easy to see whether a prob-lem exists. In this case, the actual air-flow reported from the MAF sensor(yellow trace) is much lower than the

VE calculation (red trace). This lowMAF reading shows that a problem ispresent in the airflow to the engine.

A low airflow reading could be asso-ciated with many problems, such as arestricted exhaust or intake, an air leakbetween MAF sensor and throttle, anincorrect MAF sensor calibration, in-correct camshaft timing, engine me-chanical faults, etc. To identify whatthe problem is, it’s necessary to checkthe fuel trim. When doing this, youmust check the trim values over arange of engine load and rpm. In thisfuel trim test (Fig. 4), total fuel trimreadings are taken. Total fuel trim isLong Term Fuel Trim added to ShortTerm Fuel Trim. When checking thefuel trim chart, look at the way the fueltrims change over the load of the en-gine. In this example at idle, the fueltrim is taking away �29% from thebase air equation. As the load and rpmchange, the fuel trim starts to add+23% to the base air equation. As theload steadily increases, the fuel trimstarts to add up to +49% to the base airequation. This indicates that the MAFsensor is dirty. The MAF sensor uses aheated element to measure the incom-ing air to the engine. When this ele-ment becomes dirty it overreads theincoming airflow at idle, so the fueltrim has to modify the base air equa-tion to compensate.

At hot unloaded idle, the MAF sen-sor reading in g/sec should be veryclose to the liter size of the engine, soon this 3.0L Toyota, at hot idle theMAF sensor should read about 3 to 3.2g/sec. This is a good way to seewhether the MAF sensor is readingcorrectly at idle. If the MAF sensorreading in g/sec is higher or lower thanthe liter size of the engine at idle,check the fuel trim. If the fuel trim isgood (�10%), then the MAF sensor isreading the airflow correctly.

If the fuel trim is greater than this,it’s an indication of a problem. As theengine load changes, the dirty MAF el-ement cannot give up its heat to the airflowing over it, thus it underreads theairflow. The fuel trim has to correct thisairflow reading from the MAF sensor.

38 July 2008


Circle #21

Circle #22

Circle #23

Circle #24

It does this by multiplying the base airequation by the trim value needed.

Another example of a MAF sensorreading incorrectly is if the MAF sen-sor’s Wheatstone bridge is out ofrange, the actual g/sec reading wouldalso be out of range. Since the MAFsensor reading sets the fuel deliveryweight, the fuel trim would correct theairflow. This would create out-of-rangefuel trims as well. However, there’s adifference in the way the fuel trimsload on the chart; rather than goingfrom a negative to a positive value, thefuel trims stay linear. In other words,they stay very close to the same per-centages from the bottom of the chartto the top. In this case, the MAF sen-sor would need to be replaced.

In another example, if the enginehas a fuel delivery problem, the MAFsensor reading would be correct butthe fuel trims would read out-of-range.Whether the fuel trims are positive or

negative tells you which direction togo. When they’re negative (taking awayfuel), there’s too much fuel getting tothe engine. When they’re positive(adding fuel), there’s not enough fuelgetting to the engine. If the engine hasa misfire with low fuel trim values, theproblem could be the ignition systemor engine mechanical. If the engine

has a misfire with high fuel trim values,look at a possible problem with the in-jectors.

Now back to our 3.0L Toyota’s MAFsensor problem. The sensor was re-moved and cleaned, repairing not onlythe P0171, but the P0325 and P0330(Figs. 5 and 6 on page 36). Recheckingthe work you’ve done is important, as itverifies that the repair has correctedthe problem. This entire diagnosis wasmade while on a test drive.

So the next time you go for a testdrive, take your scan tool with you. Itmay save you hours of diagnostic timelater. Also, remember the lesson ofMount Everest: In an internal combus-tion engine, just as in our bodies, theamount of available air determines theamount of work that can be done.

39July 2008


Circle #25

Fuel trim correctsthe MAF sensorreading by multi-plying the base airequation by the

trim value needed.

Are you using the fullpower of your scantool? My experiencehas shown that manytechnicians use a scantool only to retrieve di-

agnostic trouble codes (DTCs) andlook at parameter data, while overlook-ing other powerful features. One ofthese overlooked features is bidirec-tional control, and most enhanced scantools have this capability.

Bidirectional control is a genericterm used to describe sending and re-ceiving information between one deviceand another. The vehicle engineers re-sponsible for designing computer con-trol systems programmed them so ascan tool could request information orcommand a module to perform specifictests and functions. Some manufactur-ers refer to bidirectional controls asfunctional tests, actuator tests, inspec-tion tests, system tests or the like. Reini-tialization and reprogramming also canbe included in the list of bidirectionalcontrols.

This article will explain the benefitsand limitations of bidirectional controlsand demonstrate how they can be usedin the diagnostic process. Specific scantools are mentioned for illustrative pur-poses only. No attempt was made toevaluate every available tool.

The scan tool is the primary bidirec-tional control device and could actuallybe called a bidirectional tool, becauseit sends information to, and receivesinformation from, vehicle control mod-ules. For example, in the case of OBDII generic information Mode 1 (whichrelates to data parameters), the scantool user initiates a request for infor-mation from the powertrain controlmodule (PCM), and the PCM re-sponds by sending the informationback to the scan tool for display. Mostenhanced scan tools also have the abili-ty to actuate relays, injectors and coils,perform system tests, etc.

Fig. 1 on page 40 shows severalscreen captures taken from a 2004Honda Civic, using the Teradyne Pock-et Tester. The bidirectional controls forthis vehicle are listed under the Inspec-tion Menu and, as you can see, manyuseful tests are available. The techni-cian can turn the fuel pump on and off,

cycle the a/c clutch on and off and per-form an evaporative emissions leak test.The options programmed into both thevehicle and the scan tool will deter-mine the range of options available.

This brings up a key question: Arethere differences among scan tools?Everyone knows the answer is yes. De-signing and building scan tools is a diffi-cult process for vehicle manufacturersand aftermarket scan tool manufactur-ers. Automakers spend lots of time andmoney designing the best possible diag-nostic tool for their product lines. Costis a consideration, of course, but theydon’t have the luxury of deciding whatto leave out. The diagnostic platformmust communicate with and diagnoseall possible systems. Aftermarket scan

tool manufacturers, on the other hand,have a slight advantage in knowing whatthe factory scan tool is capable of doing.But these companies have several issuesto consider that don’t concern theirOEM counterparts. Here are a couplethat relate to bidirectional controls:

•Is the information available fromthe vehicle manufacturer to build an af-termarket scan tool? If the vehicle man-ufacturer makes the design informationfor scan tool bidirectional control avail-able, in most cases building an after-market version is simplified. But thisdoes not mean it will be easy or cost-ef-fective. Bidirectional controls are themost difficult feature to implement.Two issues are liability and safety. Forexample, it would not be wise for a

38 April 2005

BIDIRECTIONAL SCANNER CONTROLS: THE 2-WAY DIAGNOSTIC HIGHWAYBY BOB PATTENGALE

Maintaining an up-to-datearray of diagnosticequipment will cost a prettypenny. But it’s a downrightwaste of money if you aren’texploiting the full capabilities ofthe equipment you already have.

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technician to command an increase inengine speed if the transmission were ingear and the brake not applied. Toavoid damage to the vehicle and/or per-sonal injury, some type of protectionmust be implemented. In some cases,this protection was programmed intothe vehicle module; in others, the pro-tection was designed into the scan tool.An aftermarket scan tool manufacturerneeds to know this information.

•What are shop owners and techni-cians willing to pay for an aftermarketscan tool? Each scan tool maker mustevaluate the cost involved in building ascan tool and determine the optionsmost important to technicians. In thepast, most of the computer technologywas designed into engine control sys-

tems, so the primary focus was on build-ing a good engine diagnostic tool. Evenwith this focus, many aftermarket scantools do not include all the parametersand tests available in the factory scantool. Many shop owners and technicianshave made the decision to purchase thefactory scan tools to ensure all the infor-mation is available. This is becomingmore of an issue when dealing with non-engine control systems—e.g., antilockbrakes (ABS), supplemental restraintsystems (SRS), climate control systems,electronically controlled transmissions,body control systems and the like. After-market scan tool companies need to in-crease the level of coverage built intotheir equipment to stay competitive.

Aftermarket scan tool makers have

done a good job of working throughthe issues. But don’t be surprised orupset if you pick up a factory scan tooland find it has features or tests notavailable on your aftermarket tool.Each is built for a different market andtherefore must satisfy different objec-tives and needs.

Getting Down to DiagnosticsWhen properly utilized, bidirectionalcomponent and system tests can sig-nificantly reduce your diagnostic time.Consider a no-start situation. You’veconnected a fuel pressure gauge, cy-cled the key and cranked the engine,but no fuel pressure was measured. Atthis point you don’t know if the prob-lem is a component, a circuit or com-

39April 2005

mand-related. The scan tool you’re us-ing has a fuel pump command avail-able. Using the scan tool, commandthe fuel pump on. If the fuel pressuregauge now shows proper fuel pres-sure, you know the fuel pump circuitand pump are functioning properly.

Your diagnostic focus should moveto the command side of the circuit.The PCM may not be receiving thenecessary enabling criteria to activethe fuel pump circuit. This may becaused by a faulty input sensor, low oilpressure or some other factor. Theprocess described above took less thanfive minutes. If you had to manuallytest the fuel pump circuit, it wouldlikely take three times as long.

As another example, a Honda Civicrecently rolled into my shop with themalfunction indicator light (MIL) illumi-nated. I retrieved the DTCs and found a

P1404 (EGR Stuck Closed). At this pointI had to determine if the problem cur-rently existed or if it was an intermittent.

Fig. 2 shows screen captures fromthe Teradyne Pocket Tester. Box 1shows the EGR Test under the Inspec-tion Menu. To the right of the EGRTest is a box circled in red. This is aninformation box; when selected, it willprovide details about the test you’reabout to perform. Box 2 provides criti-cal instructions related to the test. Box3 shows the start of the test process.Box 4 instructs you to increase the en-gine speed to between 2500 and 3000rpm. Box 5 shows a sample of the databeing considered during the test. Thebar graph in the center is for the EGRLift Sensor. Based on the reading, itlooks like the EGR Valve is functioningat this time. Box 6 shows the results ofthe EGR Test. The system is normal at

this time. If you suspect the EGR valvemay be sticking intermittently, youmight want to run this test severaltimes. If the EGR valve test failed asubsequent test, you could proceedwith checking and/or replacing theEGR valve.

I also recently serviced a 1995Dodge Stratus with an illuminatedMIL. The DTC was a P0443 (EvapPurge Solenoid Circuit). The diagnos-tic information shows the DRB IIIfactory scan tool is capable of com-manding the purge solenoid open andclosed. The Vetronix Mastertech andSnap-on Scanner also can perform thistest.

Fig. 3 on page 42 shows screen cap-tures from the Vetronix Mastertech3100. Box 1 shows the F6: Purge Testoption. Box 2 provides the option ofblocking or permitting purge flow. Inthis case, we need to select F1: Flow.Box 3 provides a suggestion: An inspec-tion of vacuum lines and hoses may re-veal a problem. This is an importantnote; many problems can be discoveredquickly with a visual inspection. Box 4explains that the up and down arrowson the scan tool control the purge sole-noid. Box 5 displays the initial LongTerm Adaptive Fuel Trim values forBanks 1 and 2.

Once the purge valve is commandedopen, we expect the fuel trim values tochange based on what’s present in thegas tank. If there’s a high concentrationof fuel vapors present, the fuel trim willdecrease to compensate for the richair/fuel mixture condition. If there’s ahigh concentration of oxygen, the fueltrim will increase to compensate for thelean air/fuel mixture condition. In Box6, the purge valve was commanded on.The upper box shows the initial test: nochange in fuel trim. At this point, I sus-pected a sticking purge solenoid. I cy-cled the purge valve on and off severaltimes and on the third try, the fuel trimlevels increased, indicating the solenoidopened. Based on the results, the purgesolenoid should be replaced.

The next vehicle is a 2004 ToyotaCamry with the SRS and PassengerAir Bag lights illuminated. This vehiclewas sent to us by a body shop, follow-ing accident repairs. A Mastertech

40 April 2005

BIDIRECTIONAL SCANNER CONTROLS: THE 2-WAY DIAGNOSTIC HIGHWAY

Fig. 1

Fig. 2

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with Toyota software isneeded to access this system.For most shops, diagnosingand repairing this problemwould not be an option, un-less they have the necessaryequipment. Using the Mas-tertech, I retrieved DTCB1782 (Occupant Classifica-tion Sensor Rear LH CircuitMalfunction). After consult-ing the diagnostic informa-tion, a visual inspection re-vealed the Rear LH sensorconnector was not fully con-nected.

Figs. 4-6 are screen cap-tures taken from the Mastertech. InFig. 4, Box 2 shows the controller op-tions for this vehicle. After properlyconnecting the sensor, we should checkthe sensor data (Box 3). In Box 4, all ofthe sensors are reading, but the sensorweights range from �5.50 to 6.60 lbs.The diagnostic chart recommendedperforming a Zero Calibration after seatreplacement.

Fig. 5 shows the screen captures re-lated to Zero Calibration. Box 1 ZeroCalibration is option 5. Box 2 and Box 3provide specific instructions to preparefor the test. Box 4 indicates that ZeroCalibration is complete and recom-

mends performing a Sensitivity Check.Fig. 6 shows the screen captures re-

lated to the Sensitivity Check. Box 1Sensitivity Check is option 6. Box 2 pro-vides instructions to begin the test. Box3 is the first sensor reading measure-ment. The sensor reading is 0.00 lbs.,which passes the �7 to �7 lbs. test.Box 4 indicates that 66 lbs. of weightneed to be placed on the seat. Box 5shows three weights—50 lbs., 10 lbs.and 5 lbs.—for a total of 65 lbs., placedon the seat. Box 6 shows the sensorreading with the weights applied. Thereading is 66.00 lbs., which agrees withthe weight we applied and falls within

the 59 to 73 lbs. recommend-ed. As you can see, this repairwould not have been possiblewithout the proper equip-ment. In addition to an en-hanced scan tool with bidirec-tional controls and OEM soft-ware, a weight set is neededto properly perform this test.

When BidirectionalControl Won’t WorkThese examples show just afew of the ways that bidirec-tional controls can be used.The scan tools used provid-ed information on how to

prepare for the tests and explained thetest actuation procedures. However,you may run into a situation where abidirectional control will not actuateor stops working without your control.Fig. 7 on page 46 is a combinationscreen capture from EASE Diagnos-tics and Vetronix scan tools that illus-trates this possibility.

The EASE Chrysler Enhanced soft-ware displays information related tobidirectional controls. The Test De-scription box in the upper left-hand cor-ner indicates the purge solenoid will cy-cle on and off approximately every 1.5seconds. The Notes box in the upper

42 April 2005


Fig. 4

Fig. 3

Fig. 5

right-hand corner indicates the solenoidwill timeout after seven minutes.

The white box with blue border inthe lower right corner shows Mas-tertech screen captures from a 1995Dodge Stratus. In red are two boxes la-beled Time On and Time Off. At3:52:59 p.m., I commanded the purgesolenoid on. Approximately 30 secondslater, at 3:53:32 p.m., the solenoidturned off. This automatic-off proce-dure is designed to protect the sole-noid. Depending on the component ortest, the solenoid may turn on again af-ter a specific period of time.

This example demonstrates that theinformation provided to the scan toolmanufacturers is not always accurate. Isthis a significant issue that will preventme from diagnosing this vehicle? No. Ihad a basic understanding of what thetest was going to do and the test per-formed the necessary function: openingand closing the purge solenoid. It wouldbe nice if the information were accu-rate, but in many instances, the infor-mation is released to the scan tool de-signers before the first production vehi-cles have even left the assembly plant.Vehicle engineers can and do makechanges after this point that are not re-flected in the original scan tool designspecification. Also, once vehicles are on

the road, PCM reprogramming canchange bidirectional control specifica-tions and operation in ways that are notreflected in the scan tool information.

There’s a lot more that could be writ-ten about bidirectional controls andtheir place in your diagnostic routine.But how much more? Until recently, Iwasn’t even sure how many bidirection-al functions and tests were available foreach of the current scan tools. To get ageneral idea, I called Bob Augustine atVetronix and asked about the capabili-ties of the Tech 2. As it turns out, GMhas a set of documents called the Tech2 Pathing Tables. These are basicallythree separate documents: Body, Pow-ertrain and Chassis. All together, they’re14 pages long and list more than 1300tests in alphabetical order—1300 tests!If you own a Tech 2, you need thesedocuments, which can be purchasedfrom ACDelco at acdelcotds.com/store.All three sections are sold as a set for$25 under Part No. ROM00190. Simplyinsert the part number into the ItemSearch box and select Search. The doc-ument is displayed with a descriptionand cost. GM also offers a handy pocketreference card, Part No. ROM00164.

Here’s an example of how thePathing Tables can help: A GM vehi-cle equipped with electric mirrors is

43April 2005

Fig. 6

not working properly. You’d like totest the operation of the Driver’sElectric Mirror using the Tech 2, butdon’t know where to find the specific

tests. Using the Body Pathing Table,simply look up Driver’s Electric Mir-ror in the alphabetical chart. Driver’selectric mirror functions are located in:

Body-Memory Mirror Module-SpecialFunctions-Output Controls. Fromhere you can command the mirrorDown/Left/Right/Up.

Among the other function tests Ifound interesting are IncandescentDimming, Microphone Test, Military orStandard Time, Phone Call Test/OnStarand Theater Dimming. This is just onevehicle manufacturer and one factoryscan tool. Multiply this by 22 vehiclemanufacturers and you can see thatthe scan tool’s potential power is in-credible.

Can you afford to work only on old-er vehicles and limit yourself to justengine control systems? Maybe, butfor how much longer? My advice is toget ready to invest a lot more money inscan tools. The benefits will far out-weigh the cost in the long run.

46 April 2005



Fig. 7

Let’s start with a quick explanation ofwhat a scope is. Basically, a scope is adevice that presents an image showingvoltage levels (represented by verticalposition) over a period of time (hori-zontal position.) You could think of it asalmost being like a movie of a very fast

voltmeter. Digital storage scopes—andhere I’ll include GMMs, or graphingmultimeters, as well—not only displaywaveforms, but can store them, too.Most analog scopes, by contrast, cancapture but not store such events.

A friend of mine—I’ll call him Bob—

27July 2009 26 July 2009

UPSCOPE!

BY SAM BELL

When you need extra help navigating

the rough seas of an unfamiliar

diagnostic ocean, it’s tough to beat a

scope. You may have a tough time

reaching safe harbor without it.

I’ve been using a digital storage oscilloscope tohelp me with complex driveability and electricaltroubleshooting for years, so it came as somethingof a surprise to learn that one of my most respect-

ed competitors doesn’t even have a scope. Wow! Ithought to myself. I wonder how he does it. Or is it thelack of a scope that makes him call me with a problem somuch more often than I call him? I decided to pursuethe question in a more general form for this article.

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uses an array of DMMs (digital multi-meters), test lights and a wide selectionof “known-good parts” he’s accumulatedover the years to arrive at his diagnoses.His view, as he expressed it to me, is

“You’ve got to start somewhere.” WhenI asked him exactly what he meant, hereplied that he would do whatever test-ing he could to identify “the most likelysuspect,” and he’d then replace it. If it

took care of the issue, he was done. Ifnot, he’d look for another likely suspect.

Bob’s a good technician with awealth of knowledge and experience,and, just as important, good diagnosticinstincts. So he’s usually right. Now, ifyou were a major league batter, you’dbe doing really well to hit over .300. Tobat over .500 would be unheard of. Butin our business, anyone who bats lessthan .950 has way too many come-backs. After all, batting .950 wouldmean that one in every 20 jobs goesawry. Given the serious consequencesof failure, that sounds like one toomany. Imagine being in rush-hourfreeway traffic and knowing that oneout of every 20 cars in front of you islikely to stall!

In this particular case, Bob wasworking on a Jeep no-start. He had de-termined that the PCM wasn’t energiz-ing the fuel pump relay. When hemanually provided the missing ground,the car started and ran. He ordered upa replacement PCM from a well-known and respected remanufacturer,installed it and now had fuel pressure,

30 July 2009

UP SCOPE!

The screen capture on the left shows the CKP pattern from a ’94 Jeep Grand Cherokee with the 5.2L V8. The scope is setfor 100 milliseconds (mS) per division on the horizontal scale. The truck runs extremely rough. It has a major hesitation,

backfires, bucks and jerks. The screen capture on the right shows the same sensor viewed at 50mS/div. What’s the deal with the extra-wide pulse? Is this some kind of synch signal?

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Here we’vezoomed in on the

pattern by set-ting the scope for

20mS/div. Is thispulse some kind

of reference sig-nal, like maybe

“TDC is coming in60°”? And if this isa “zoom in” com-pared to the pre-vious image, whyis the wide pulsenarrower? Hint:It’s a function of

engine speed. Theengine is goingmuch faster in

this picture. Thusthere are fewermilliseconds of

duration for eachsignal pulse.

Fig. 1

Fig. 3

Fig. 2

but no spark. A high-impedance testlight connected between the batterypositive and the ignition coil negativeterminal flashed brightly. Could thecoil have coincidentally died just then?Stranger things have happened to usall. Bob rummaged through his collec-tion of known-good coils, found a likelycandidate and hooked it up. Oops! Stillno spark. A second known-good coil alsofailed to respond.

This job was going south in a hurry.When Bob swapped the old PCM backin and manually grounded the fuelpump relay, the car started right up. Sohe knew the old PCM could fire thecoil, but apparently the new one couldnot. That evening I stopped by on myway home, bringing my scope with meat his request. I was able to verify thatthe replacement PCM was providing asignal to the coil, but that the signalvoltage never quite reached ground.Instead, it hit a minimum of 1.42 volts,not quite enough to cause the coil toproduce a sufficient spark. (The coilprimary voltage spike was a mere 30volts, not the expected 300+ volts nec-essary to produce a fat blue 30kV sparkin the secondary circuit.)

Of course, Bob had already contact-ed the supplier to arrange for anotherreplacement PCM. Did he really need

a scope to know what was going on?Not really, I suppose. But it sure madeit easier to understand.

34 July 2009

UP SCOPE!

A final scope check confirms the fix. Irregularities in the trace have been elimi-nated, along with the driveability problems for which they were responsible.

Here’s the original reluctor ring mounted on a wheel balancer as it’s being checked for an out-of-round condition. The positionof the dial indicator in each of the two photos illustrates just how small a deviation can cause this major driveability symptom.

Fig. 4

he removed the old one for closer ex-amination. He mounted it to his tirebalancer and set up a dial indicator tomeasure concentricity (see the pho-tos on page 34). Sure enough, in thearea between the two windows hisscope-and-razor blade technique hadidentified as the boundaries, therewas a small but measurable dip in thering—about .7mm.

Replacement eliminated the extrapulse in the CKP signal, in turn elimi-nating the extra ignition pulse the PCMhad kept inserting. The final waveformis shown in Fig. 4.

Would your shop have been able tosolve this problem? Without a scope,how long would it have taken? Re-member that the dip in the reluctor

ring was very slight—so small that itrequired careful measurement to de-tect it. How would you have evenknown where to look?

It’s true, as my friend Bob says,that you’ve got to start somewhere.But if, like him, you’re still workingwithout a scope, that “somewhere”could be a long way down the roadfrom where it should be.

AlternativesGood scopes carry a significant pricetag, so you’ll want to make sure you geta unit that does what you need it to do,and that’s easy enough to use that you’llactually break it out and use it. I dis-cussed several factors that might influ-ence your scope choice in the last sec-

tion of “Current Clamp On-Ramp” inthe December 2007 issue of MOTOR.

One of the most valuable features ofa good scope is its ability to use onesignal as the trigger for another. Thisallows us, for example, to trigger theCKP signal from the CMP signal sothat we can analyze their exact phaserelationships. This is particularly help-ful in setting up the CMP after a cata-strophic failure on a Ford, for example.

There are some cheaper alternativesto a scope but, in general, they cannotmatch the scope’s speed, precision andanalytical ability. Among such tools arelogic probes and dedicated sensortesters. One of the most interesting anduseful logic probes is the TerminatorProbe, which features three three-colorLEDs, each indicating a discrete volt-age level, as well as a built-in DMMwith voltage, frequency and duty-cyclereadouts. There’s an excellent animation

of this tool at work analyzing a powertransistor at www.aeswave.com/images/Products/T-7205animPOWER%20TR.swf.

There are also a variety of stand-alone dedicated testers that cover, forexample, oxygen sensors or alternators.While these are undoubtedly usefultools that may provide quick and reli-able determinations, they lack the in-herent flexibility of a good scope.

Ultimately, of course, the real ques-tion is how else are you going to figureout what’s actually going on when it’ssomething out of the ordinary?

37July 2009

On to the Tough StuffThis next example, coincidentally, fea-tures another Jeep. This one’s a ’94 withthe 5.2L V8. The vehicle was driven toa different friend’s shop with a com-plaint of lack of power, backfiring, er-ratic idle and general lack of driveabili-ty. All of these problems surfaced im-mediately after yet a different shop hadinstalled a crate motor from a reputableremanufacturer. The original motorhad died from oil starvation after an oilchange that had apparently not includ-ed the removal of the original oil filter’sgasket. Ouch!

My friend Mark easily confirmedthe symptoms. “It drove almost likethe firing order was off,” he told melater. “You had to keep your foot into itpretty hard just to keep it running, andit couldn’t get out of its own way.”

He began by checking the basics:first the firing order (twice), then crank-

ing vacuum (since “idle” wasn’t really inthe truck’s vocabulary), compression,firing order (because it still ran like ithad a couple of wires crossed) and igni-tion timing (although thanks to the idleissue again, this was an approximation atbest). There were no red flags. Hechecked fuel pressure and volume. Hechecked for codes, and found none.Nothing in scan data was helpful.

By now Mark was suspecting deeperproblems. He scoped the cam sensorsignal and found nothing remarkable.Finally, scoping the crankshaft positionsensor signal, he found the waveformshown in Fig. 1 on page 30. Homing inon the signal, he found the wider-than-expected pulse seen in Fig. 2.

Not having captured a waveform

from a known-good signal for one ofthese Jeeps in the past, he searchedthe iATN Waveform Library. Unfortu-nately, none of the waveforms storedthere matched his. Could it be thatthe extra-width pulse was some sort oftrigger, like the missing tooth signa-ture from a Ford or Toyota CKP?(See Fig. 3.)

The iATN Fix Database yielded nosimilar situations. More research, andsome iATN Forum discussions, con-vinced him that the signal he was seeingwas the root of his problems. He triedswapping in a new CKP sensor, but thewaveform remained unchanged.

With the starter removed, he couldsee the reluctor ring through the bell-housing. He marked it with a crayon

and watched carefully as his helpermanually rotated the engine. Every-thing appeared to be fine; even whenhe bolted a spare piece of a bracket inplace to act as a reference pointer, hecouldn’t see any noticeable problem.

Mark hooked his scope back up andrepeated the procedure, this timewatching the scope to see where theunwanted signal originated. He eventaped a couple of razor blades over thetwo slots of the reluctor ring adjacentto the extra pulse’s location so that hecould pin it down with certainty.

Although there was no flaw visibleto the naked eye, Mark was sure heknew what the cause of the problemwas, so he ordered up a new reluctorring. When it arrived a few days later,

36 July 2009

UP SCOPE!


Ascope is just a tool. If youwant to get the most out ofit, you’ll have to learn how to

use it. And then, perhaps even moreimportantly, you’ll have to get inthe habit of using it regularly.

In my view, technicians whobreak out their scopes only whenfaced with problem cars are usually

setting themselves up for trouble.I’ve asked it before, and I’ll ask itagain: If the only cars you evercheck have problems, how will yourecognize “good” when you see it?

For example, is the waveformshown below good or bad? Canyou tell? Here are two hints:

•It’s a capture from the MAP sen-sor on a Corolla.

•The scope is AC-coupled.

Still can’t tell?Don’t worry. I’llcome back to it. Butfirst, I want to reem-phasize my pointthat having the toolis not enough; youhave to use the toolon a regular basis.

My best advice tobeginning scopeusers is to checkknown-good cars ona regular basis. Ifyou can—and mostscopes now comewith interface cablesto make it easy—capture and storeknown-good wave-forms, building a li-brary of waveforms

as you go. Excellent software pack-ages are available as well to help youorganize, categorize, store and re-trieve specimens from your library. Ifyou checked just two waveforms aweek, at the end of a year you’dhave a library of a hundred wave-forms. Then you’d know that theMAP waveform on the facing page is,in fact, good. You’d also recognizethat it’s just a small piece of a larger,richer and more informative wave-form, shown here,that, in this case, in-dicates a mechanical-ly sound engine.

I mentioned theWaveform Library atiATN in the main ar-ticle. I’ve found it tobe an invaluable re-source for checkingpreviously unfamiliarwaveforms whenev-er I’ve encounteredsomething new. Youmust be a sponsor toaccess the WaveformLibrary, but thewealth of informa-tion available witha sponsorship atiATN (access to theFix Database, ForumArchives and more)easily pays for itselfevery month.

MOTOR/ALLDATA includes a num-ber of helpful waveforms under theheading “Description and Opera-tion” for various systems and com-ponents. There are several othersources as well, and some scopes andGMMs may contain built-in databas-es showing examples of known-good patterns. In some instances,these waveforms may be containedin the user’s manual or on a datadisc supplied with the scope.

Making It Work for You

Is this waveform good, bad or indifferent? The answermay depend on the questions you ask, and your famil-iarity with the general characteristics you should ex-pect from the particular signal being sampled.

The longer sample shows a mechanically sound en-gine at about 2200 rpm. This trace is made with thescope AC-coupled to the MAP signal wire. Highermanifold pressure is toward the top of the screen,higher manifold vacuum is toward the bottom.

Good scopes

are expensive, so

get one that’s easy

to use so you’ll

actually use it.

A valuable scope

feature is its

ability to use one

signal as a trigger

for another.

Generations of elec-tronics students havelearned that voltageleads current. Andwhile there is nodoubt of the truth of

that statement, it’s sometimes moreprofitable to look first at current.

Take a look at the current waveformin Fig. 1 on page 46. Is there any com-pelling reason to look at voltage in thisexample? This scope trace showsstarter current draw during a relativecompression test. With a peak of over850 amps of current at cursor 2 whenthe starter begins to turn, and with anaverage current of about 150 amps, it’sclear that voltage supply must be ade-quate. The general uniformity of thewaveform once cranking is well un-derway indicates an engine with rela-tively even compression. Cursor 1highlights the beginning of the sole-noid’s “pintle hump.”

Now let’s take a look at Fig. 2.Channel A (red trace) displays thevoltage signal measured on the PCMside of this conventional fuel injector.The supply voltage through the injec-tor windings appears steady at an ap-propriate level (14.4 volts) prior toturn-on at 740�S into the picture. Thecircuit is then grounded by the PCM,to be released 2.3mS later. Note thatcurrent, shown on channel B (yellow

trace), begins to rise immediately afterturn-on, reaching a maximum value of1 amp after 2.3mS. This is a good ex-ample of voltage leading current. Thiscapture was made at idle on a known-good vehicle with a low-amp probe setat 10mV/amp.

Preliminary MattersWhile we’re still at the beginning, I’dlike to bring up three important pointsthat seem to cause a lot of confusionamong beginning scope users. The firstrelates to scope lead placement andtrace interpretation. Where were thevoltage leads placed to make the scopetrace in Fig. 2?

Channel A’s red lead was connectedto the PCM side of the injector plug,while its black lead was connected toground. So at the left side of Fig. 2,the voltage trace shows 14.4 volts, rep-resenting the supply voltage from thecharging system to the injector, andthen through the injector windings.This voltage level appears when theinjector is off.

The left cursor is positioned at thesharp vertical drop about 1-1⁄2 divisionsfrom the left edge and indicates thepoint at which the PCM applies groundto the injector, turning it on. The entireperiod of time the injector is on, thevoltage is zero. The sharp vertical riseabout six divisions from the left (where

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44 December 2007

CURRENT CLAMPON-RAMP

BY SAM BELLThere’s no reason to limit your oscilloscopeusage to voltage measurements alone. A wealthof additional diagnostic information can bederived from the study of current waveforms.

the right cursor is placed) indicates thepoint at which the PCM released theground, and the injector turned off.

The sharp vertical rise (to 113.6 volts)represents the inductive kick, a voltageinduced into the injector’s windings bythe rapid collapse of the magnetic fieldas the injector was turned off. This is thevery same inductive phenomenon har-nessed in ignition coil primary circuits.

Of course, the low-amp inductivepickup for channel B was clampedaround either one (but not both) of the

injector leads. Since current is the sameat any point throughout a simple circuit,make the hookup where it’s most conve-nient. If you’re bothered by a picturethat comes out upside down, reverseyour probe, or simply invert the picturewith your scope controls.

The second issue is, if anything, evenmore important. If you don’t hook upyour scope to known-good vehicles, butonly to those with a problem, you’ll nev-er know what good looks like. Thatmakes it awfully hard to figure out

whether what you’re looking at on aproblem vehicle is good or bad. After all,if you didn’t know it was normal for thistype of vehicle, would you really haveexpected an inductive spike in excess of100 volts? Some vehicles use a voltage-limiting diode to cut the spike off at aslow as 36 volts. If this was such a vehicle,and it had this reading, it would have aproblem. See what I mean?

My final preliminary caution con-cerns time. Specifically, the choice of in-appropriate time bases for viewing andanalyzing the waveforms you capturecan easily mislead you into overlookingreal problems or into mistakenly con-demning good components. The wave-forms in Figs. 3 and 4 on page 48 arefrom the same injector. In Fig. 3, thetime base is so long that the current risetime looks nearly vertical, the hallmarkof a shorted injector. Fig. 4 shows theinjector viewed with a shorter timebase, revealing a healthy current ramp.

Fig. 5 portrays the current to an indi-vidual plug-mounted ignition coil. Thecapture was made at idle and clearlyshows a flat top 2.6mS after turn-on, acharacteristic of current limiting goinginto effect. The current level wascapped at 1.07 amps. For comparison,Fig. 6 shows the same coil under aWOT snap-throttle condition. This time,coil saturation is achieved in the same2.6mS time frame, but current limitingnever comes into play because the coilfires as soon as it’s saturated. If the coilreaches full charge each time, do weneed to look at supply voltage? If thefalling edge of the coil current ramp isstraight vertical, do we need to examinecoil turn-off? Be careful. This is a trickquestion! We’ll come back to it later.

Remember the current limited “flattops” seen in Figs. 4 and 5? Fig. 7 (page50) presents a similar and related phe-nomenon. In this trace, a power win-dow was commanded down from a par-tially raised position. As it reached thelimit of its travel, current peaked at 17amps and remained at that “stall level”until the internal PTC circuit protectorcut off the voltage supply.

What about the waveform in Fig. 8?It shows a recurring slow ramp cap-tured during a bidirectional evap sole-noid test. The pintle hump is clearly vis-

46 December 2007

CURRENT CLAMP ON-RAMP

Fig. 2 Channel A (red trace) displays the voltage signal measured on the PCMside of this conventional fuel injector. The supply voltage through the injectorwindings appears steady at an appropriate level (14.4 volts) prior to turn-on at740�S into the picture. The circuit is then grounded by the PCM, to be released2.3mS later. Note that current, shown on channel B (yellow trace), begins to riseimmediately after turn-on, reaching a maximum value of 1 amp after 2.3mS. Thisis a good example of voltage leading current. This capture was made at idle on aknown-good car (the same Scion) with a low-amp probe set at 10mV/amp.

Fig.1 This scope traceshows starter currentdraw for a relative com-pression test on a 2005Scion xB 1.5L. With apeak of over 850 amps ofcurrent when thestarter begins to turn,at cursor 2, and with anaverage current ofabout 150 amps, it’sclear that voltage supplymust be adequate. Thegeneral uniformity of thewaveform once crankingis well underway indi-cates an engine with rel-atively even compres-sion. Cursor 1 highlightsthe beginning of the so-lenoid’s “pintle hump.”

ible at about 14mS (almost three divi-sions) after the beginning of the trace.From this we can be certain that the so-lenoid is opening as commanded, al-though a clogged or disconnected vacu-um hose wouldn’t be detected this way.What else can we learn from this wave-form? Look at the slow decay after thepeak current is reached.

Fig. 9, which also includes the voltagetrace, illustrates that current continuesto flow in the circuit after turn-off. Howis this possible if voltage leads current?As surprising as it may seem, this is nor-mal and occurs because the energy ofthe magnetic field is allowed to flow

slowly to ground through a spike-sup-pression resistor built into the solenoid.The slight “hash” on the voltage traceimmediately after turn-off representsthe dissipating oscillations of the in-duced voltage from the collapsing field.The battery and the rest of the vehicle’swiring function as a large capacitor toquickly absorb the induced energy be-fore it can raise overall system voltageappreciably. Without the resistor provid-ing an alternate path to ground, this so-lenoid would also generate an inductivevoltage kick. It’s this induced voltage thatleads current flow after turn-off occurs.

Fig. 10 shows a nearly vertical rise in

current. This, as we said before, is usu-ally a sign of a shorted component, ahigh current consumer like a motorstarting from a standstill or, as in thiscase, a heating element. With 12 ampsof draw, it looks like the heated backglass grid will defrost well come winter.

Fig. 11 shows the current draw whenall five power door locks are command-ed. Again, the nearly vertical rise (or inthis case, fall) indicates a fast, high-pow-ered motor moving from a standstill.This capture was made with a high-cur-rent probe set to 1mV/amp. The initialsurge was 75.2 amps, or an average ofabout 15 amps per lock motor, although

48 December 2007


In the screen capture on the left (from a 2001 Buick Regal 3800 Series), the time base is too long. In the screen capture on the right, the time base is correct.

Fig. 3 Fig. 4

Note the “flat top” in the screen capture on the left as current limiting kicks in. This capture was taken on a 2005 Scion xB at idle. The flat top is gone in the screen shot on the right.

The COP coil fires as soon as it’s fully saturated under this snap-throttle capture.

Fig. 5 Fig. 6

it tapered off quickly from there. Thiswaveform is from a known-good vehicle.

Fig. 12 shows a blower fan beingturned up step-by-step. Notice theslight downward slope after each stepis reached. This is a normal result ofcounterelectromagnetic force (EMF)generated as the speed increases, anddoes not indicate any problem. Wouldhaving a voltage trace here help withyour diagnosis?

AC or DC Coupling?Fig. 13 shows charging current in a nor-mal vehicle at idle with all electrical ac-cessory loads applied. Channel A, in yel-low, is DC-coupled at 200mV/div, while

50 December 2007


Fig. 7 Is the current limited to protect the circuit or wasthis waveform created by a window motor at stall limit?

Fig. 8 This is a screen capture of an evap purge solenoidon a 2005 Scion xB cycling open and closed. The left cur-sor is positioned over the pintle hump.

Fig. 10 A sharp vertical rise in a current waveform ischaracteristic of a shorted component, a high currentconsumer like a motor starting up or, in this case, arear-defrost heater grid that’s been switched on.

Fig. 11 In this example, the current ramp is shown as anegative draw. To invert it, either reverse your inductivecurrent clamp or use your scope controls. This screencapture shows all five power door locks in action.

Fig. 9 Does voltage lead current? Note in this screen capture from theScion xB that current flow continues after the solenoid is turned off.

channel B, in red, is AC-coupled at50mV/div. We use channel A to see thetotal charging current—in this case, anaverage of about 75 amps. Channel B,the AC-coupled trace, allows us to ex-amine and magnify the detailed shapeof the trace in channel A. For more onAC scope coupling, refer to Mark War-ren’s Driveability Corner in the October2007 issue of MOTOR, or download aPDF copy at www.motor.com.

Now let’s look at some other exam-ples where AC or DC coupling choicesmight help speed our diagnostics. I’llstart with a straightforward DC-coupledtrace of the voltage supply to a fuelpump. In Fig. 14 below, the pump hasbeen hot-wired to run from a batteryjump-box. The trace shows some hash,but is it meaningful? The same pump’sAC-coupled voltage trace, taken a fewminutes later, is shown in Fig. 15. Thespikes first seen in the DC-coupled im-age are now more prominent with ACcoupling in use, but what does it allmean? Figs. 16 and 17 on page 54 put itall together. Fig. 16 shows the DC cur-rent trace along with the DC voltagetrace. Fig. 17 shows the two AC-cou-pled traces. Of course, this is a very badpump; a good one is shown in Fig. 18.

FlyspecksCurrent ramps may contain significantamounts of diagnostic informationmasquerading as, well, flyspecks. Fig.19 on page 54 is the trace of a distribu-

51December 2007

Fig. 12 Noticethe downward

slope after eachpeak in this

screen capture.This is due tocounter-EMF

produced as themotor speed

increases.

The screen capture on the left is of fuel pump DC voltage. The pump had beenhot-wired for off-vehicle testing. The screen shot on the right is of the

same hot-wired pump, but this time we’re looking at AC voltage.

Fig. 14 Fig. 15

Fig.13 Changing scope coupling and voltage bases allows zooming in on currentflow details. Learn to adjust your equipment to produce meaningful waveforms.

tor-mounted ignition coil from a Hon-da Accord. At this time scale(.5mS/div), everything looks quite nor-mal, and, indeed, the car runs well.Note the small flyspeck just at the bot-

tom of the falling edge. Fig. 20 at1mS/div shows a time-expanded viewof the beginning of the current rampas a GM DIS coil pack is switched on.The oscillations are a normal phenom-

enon in most, but not all, coils, and in-dicate the time at which, as my friendMac VandenBrink puts it, the coil’smagnetic field goes from being noth-ing to being something.

Fig. 21 shows the Honda coil again,under an even more rapid time base—so rapid, in fact, that only the fallingedge of the ramp is shown in the firstthree divisions. Each division is only20 nanoseconds in duration; that’s 20billionths of a second. This is wherethe trick question I mentioned beforecomes into play. By zooming in on thefalling edge of the trace, we changed itfrom “vertical” to taking up over threetime divisions. Where before we hadonly a flyspeck, here we can see thepronounced coil current oscillations asthe spark energy is released throughthe secondary ignition circuit.

Although well beyond the scope ofthis article, there is a wealth of diag-nostic information about the overallcondition of the secondary ignitioncircuit contained in these expandedimages. Refer to Dan Marinucci’sForeign Service columns in the Sep-tember and October 2002 issues ofMOTOR (www.motor.com). Jim Lin-der, of Linder Technical Services, alsogave an excellent technical presenta-tion on this subject several years ago.Go to www.lindertech.com/docs/ignstd.pdf to download a copy of Jim’spresentation. You may also wish toconsult the iATN Technical Forumarchives (www.iatn.net) for more in-formation on this subject.

There’s a considerable amount ofcontroversy within the repair industryas to the practical value of ignition pri-mary current ramp analysis. Those, likeJim Linder and myself, who feel there’svaluable information to be gatheredare frequently forced to admit thatequipment selection and probe place-ment issues can and do cause misdiag-noses in many more cases than wewould like to see. As I said before,there’s no substitute for checkingknown-good vehicles. It’s vital that youlearn the strengths and especially theweaknesses of your scope and currentprobe combinations before recom-mending potentially expensive repairson the basis of a single test. Indeed, it’s

52 December 2007


Fig. 16

Fig. 17

Fig. 18 A goodfuel pump at last!

Is there anyreason to look at

voltage? Thecurrent clamp is

set for 1 amp/div.

The upper trace in the screen shot above is voltage,the lower is current on a 2005 Toyota Corolla. Both areDC-coupled. In the screen capture below we see bothvoltage and current as AC-coupled. What a bad pump!

usually possible to verify your diagnos-tic conclusions via other means, andyou should do so routinely until you’vedeveloped the necessary experienceand equipment familiarity to regularlyreach sound diagnostic conclusions,with near 100% accuracy.

If you experience difficulty in inter-preting low-amp waveforms, you maywish to shield your probe with a pieceof aluminum foil and a ground jumperto see if the scope pattern changes. Tryto avoid placing the probe too near thehigh-strength magnetic fields of an al-ternator or an engaged a/c clutch. Insome cases, the 120Hz noise fromoverhead fluorescent lights or other

electrically powered shop equipmentmay show up in your waveforms. Ifturning off the lights changes the scopepattern, you’ll have to devise appropri-ate countermeasures. These may entailusing different probe and scope com-binations, applying conductive probesurface finishes and connecting themto the probe’s existing shield wires orconducting your tests in another loca-tion less subject to electrical noise.

Taking the On-RampIf you don’t own a scope and a low-amp probe, or are considering replac-ing ones you currently own, insist on ademonstration in your own shop envi-

ronment on a variety of circuits. Makesure the equipment you select will al-low you to view, analyze and storemeaningful waveforms, and that youcan manipulate the time base to1mS/div or faster, and the voltage to5mV/div or smaller. Also, make sureyou can capture and download wave-forms to your computer.

Since many of the most vexing diag-nostic problems involve intermittentfailures that may occur only while driv-ing, make sure the equipment youchoose is readily road-portable. But re-member that trees, utility poles, othervehicles and pedestrians tend to jumpin front of moving vehicles while youreyes are fixed on the scope screen, soalways have an assistant drive.

Both voltage and current ramp wave-forms provide valuable diagnostic infor-mation. Checking known-good vehiclesand saving the waveforms for future re-view and comparison is crucial. Choos-ing the appropriate time base, voltagelevel and AC or DC coupling options al-lows the scope user to home in on prob-lems accurately and efficiently. Experi-ence and knowing the strengths and thelimitations of your equipment lead todiagnostic expertise.

54 December 2007


Fig. 19 This coil current ramp is from a ’98 Honda Ac-cord. The probe is set for 10mV/amp, or 2 amps/div. Isthis a good waveform? Here’s a hint: The car runs well,but the coil has 165,000 miles to its credit.

Fig. 21 Here’sthat Honda coilagain, this timeshown with thetail underextrememagnification.The time base is20nS/div. That’s20 billionths of asecond. Still, thecar runs well. Isthis waveformgood, bad ormerelyacceptablynormal?

Fig. 20 Note the coil turn-on oscillations in this screenshot from a 2001 Buick Regal 3800 Series. The probe isset to 100mV/amp, or 2 amps/div.


From its humble begin-nings, the internal com-bustion engine has beentransformed many timesover to produce morepower and to be more ef-

ficient. Today’s internal combustion en-gine comes in two forms: compressionignition (diesel) and spark ignition. Wewill analyze the spark ignition (SI) sys-tem here. At this point, it’s still the dom-inant system in use in this country.

It’s important to understand how en-ergy is released in the SI engine. In aninternal combustion engine, the air/fuelmixture is drawn into the cylinder,where it’s compressed. As the air/fuelmixture is compressed, the moleculesare forced into a smaller space. Thiscauses them to run into each other,which creates friction and heat.

It takes energy to hold together thedifferent atoms that form the molecularchain of the fuel molecules. In order for

the fuel to release this energy, the fuelmolecules must separate, or breakapart, then reform into a different mol-ecular structure with a lower energystate. Once the fuel molecules are bro-ken apart, the energy used to holdeverything together is no longer need-ed. This freed energy is what powersthe internal combustion engine.

In an SI engine, cylinder compres-sion alone does not provide enough en-ergy to separate the fuel molecules. Theheat that’s transferred into the fuel mol-ecules makes it unstable, but moreforce must be applied to separate theatoms contained in the fuel molecules.It would not be easy to separate twowrestlers locked together in combat. Toseparate them you’d have to apply moreforce than they’re using to hold on toeach other.

A stun gun that applied a spark of100,000 volts would do the job. The po-tential energy supplied by the stun gun P

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28 May 2005

IN THE

HOLEFIREUnderstanding Ignition Waveforms

BY BERNIE C. THOMPSON

The ignition waveform is a window that allows

you to see what’s occurring in the combustion

chamber. Join us for a closer look.

is greater than the energy the wrestlersare using to hold on to each other, sothey would let go and separate. Eventhough the cylinder compression cre-ates heat energy, more force is neededto separate the fuel’s molecular struc-ture and release its energy. That force issupplied by a high-energy spark froman ignition system.

Many different types of ignition sys-tems have been used to supply the high-energy spark necessary to ignite theair/fuel mixture. The most popular sys-tem in use today is the step-up trans-former, which uses a low-voltage, high-current pole to create a high-voltage,low-current pole. This is accomplishedwith two different coils, or windings, ofwire. The first coil is the primary and thesecond coil is the secondary (Fig. 1). Theprimary is wound around a core for mag-netic amplification. In newer transform-ers, this core is composed of many platesof a ferrous metal (usually soft iron), lay-ered or laminated together. This givesbetter amplification than a solid core.

The primary winding uses larger di-ameter wire with fewer windings. Thisallows the primary to have a very low re-sistance value. The secondary usessmaller diameter wire with many morewindings to produce a higher resistancevalue. The automotive coil is usuallywound at a ratio of approximately 1:100.In other words, for each turn of the pri-mary winding, the secondary has 100winding turns. The primary winding re-sistance is normally in the range of 1 to 4ohms, while the secondary winding usu-ally has a resistance of 8000 to 16,000ohms.

The primary and secondary windingsare insulated from each other via trans-former oil or epoxy. Transformer oil canhold off a breakdown voltage of only20kV to 25kV, so in newer high-energytransformers, vacuum-sealed epoxy thatcan hold off a breakdown voltage of50kV is used instead. The primary andsecondary are electromagnetically cou-pled, so anything that affects one wind-ing is mirrored in the other.

The step-up transformer uses elec-tromagnetic induction to produce thenecessary spark energy. To understandhow the transformer works, let’s look atthe waveform produced by this device,beginning with waveform segment A inFig. 2 below. (We’ll keep referring tothis waveform.) This is the open-circuitvoltage, or source voltage, because thecircuit has not been completed. There’sno current flowing through the primarycircuit at this point. The voltage thendrops abruptly when the module driveris turned on, thus completing the pri-mary circuit to ground (waveform seg-ment B). This voltage drop will comevery close to ground.

The initial voltage drop depends onwhether the driver used to control thecurrent is a transistor or a MOSFET. Ifa transistor is used, the voltage drop willbe .7 to 1 volt. This is due to the resis-tance across the transistor’s gate. AMOSFET has less resistance across itsgate, causing a lower voltage drop ofabout .1 volt to .3 volt. The initial volt-age drop is the voltage that remains inthe circuit to push the current acrossthe resistance of the module driver orgate (waveform segment C).

Once the module closes the driver,current starts to flow through the pri-mary winding circuit. When currentflows through a coil winding, all of thecurrent is used to create a magneticfield around the winding (Fig. 3). Thismagnetic field buildup is called induc-tance. The magnetic field is proportion-al to the inductance and the current. Inother words, the larger the current, thelarger the magnetic inductance.

30 May 2005

Fig. 2

Fig. 1 Fig. 3

UNDERSTANDING IGNITION WAVEFORMS

As the magnetic field builds, it movesacross the primary and secondary wind-ings, inducing voltage in both. However,the effect of this induction is differentwithin the two windings. As the mag-netic field builds and moves across thesecondary winding, it induces electro-motive force (emf) and frees electrons.This can be seen in the secondary wave-form when the module driver closes.There are voltage oscillations when thecircuit is first completed (Fig. 4). This iscaused by the magnetic field movingacross and inducing voltage in different

windings contained within the sec-ondary coil winding.

Capacitance exists between the coilwindings. It occurs when two conduc-tors are separated by space and currentis flowing through them. Electrical po-tential builds between the two conduc-tors. The size of the conductors and thedistance between them determines theamount of capacitance.

Ringing occurs in the circuit as ener-gy changes between electrical and mag-netic energy. These ringing oscillationsdiminish into a steady curve that flat-

tens out when the coil has become satu-rated. The saturation point will vary de-pending on the amount of current flow-ing through the primary, the amount ofresistance and the number of turnswithin the winding.

As the magnetic field builds andmoves across the primary winding, thevoltage that’s induced into the primarywinding frees electrons. However, sincecurrent is flowing through the primarywinding, these free electrons impedethe current flow. In my previous articleon fuel injectors (January 2005), I gavethe example of a school hallway packedshoulder-to-shoulder with children to il-lustrate this problem. The example alsoworks for ignition coils.

Imagine the children running downthe hallway as fast as they can run. Nowimagine more children entering the hall-way from classrooms located along thishallway. The children leaving the class-rooms can’t change the flow of childrenalready running down the hallway with-out increasing the pressure. Just like thechildren entering the hallway, the in-duced voltage (pressure) in the primarywinding creates resistance to the changein current flowing through the primarycircuit. This resistance is called counterelectromotive force, or counter voltage.

Whenever there’s inductance in a cir-cuit, a counter emf will be produced bya change in current in a way that resiststhe change in current. And wheneverthere’s resistance in a circuit, there willbe a voltage drop proportional to the re-sistance. This voltage drop can be seenas the slight rise at the bottom of theprimary waveform. If the oscilloscopevoltage setting is lowered to magnify thebottom of the ignition primary coilwaveform, the voltage drop can be seenmore clearly (waveform segment D inthe upper pane of Fig. 5 and magnifiedin the lower pane).

Since the current flowing throughthe winding makes the resistance forthe voltage drop, it mirrors the primaryignition coil waveform made with an in-ductive amperage clamp (lower pane ofFig. 5). Once the current rises to thepoint of full coil saturation (magneticfield not in movement), the magneticfield completely surrounds the sec-ondary windings. The ignition coil’s sat-

32 May 2005


Fig. 4

Fig. 5

uration point is based on the currentflowing through it. The larger the cur-rent, the larger the magnetic lines offorce. Likewise, the smaller the current,the smaller the magnetic lines of force.

The circuit then limits the currentflowing through the primary winding(waveform segment E in Fig. 2). How-ever, the magnetic field still remains atfull strength. Notice that when currentlimiting is switched on, the voltage isstill below the open-circuit voltage(waveform segment F). To accomplishthis, a resistor is switched into the cir-cuit to limit the current flowing throughit. If the primary circuit has unwantedresistance, the time for the current limitto switch on will be increased. If the coilis shorted or has lower-than-normal re-sistance, the time for the current limit tooccur will be reduced. If the designcharacteristics of the system are known,variations in the expected time to limitcurrent will be an indicator of a problem.

As engine rpm is increased, the timebetween cylinder firing becomes short-er, the time to saturate the coil decreas-es and current limiting will cease. (Notall ignition systems have current limit-ing.) The PCM then commands themodule driver off. This ends the currentflowing through the primary winding.The magnetic field then begins to fallacross the secondary winding.

When a magnetic field moves across a

wire or winding, voltage is induced intothat wire or winding. This inductionmakes electromotive force, which freeselectrons and pushes them through thecircuit until they return to the secondarywinding where they were produced.The amount of induction is proportionalto the size of the magnetic field and thespeed with which the magnetic field fallsacross the secondary winding.

A condenser or capacitor is used topromote a faster collapse of the mag-netic field. Neither component will al-low direct current to pass through it toground; however, alternating current isable to pass through. A direct currentthat pulses very fast becomes alternat-ing current and can pass through thecondenser or capacitor. This allows thecurrent in the primary coil circuit topass through either of these compo-nents to ground.

The condenser is connected to theprimary winding (Fig. 6). Once the cur-rent stops, the magnetic field falls backinto the primary winding to stabilize thecurrent within the winding. The fasterthe current in the primary winding dis-sipates through the condenser, thefaster the magnetic field will collapse.The rapid movement of the magneticfield increases the induction within thesecondary winding and the current, be-ing pushed by a high voltage of up to50kV, will look for a pathway or circuit.

The ignition coil’s secondary is con-nected to the spark plug. The electronsmove to the spark plug gap; however,this is an open circuit. When high volt-age is trying to push electrons across anopen circuit, it will first form a corona,or a low-energy field, across the sparkplug electrodes (Fig. 7A on page 38).

Once the corona has formed, ioniza-

34 May 2005


Fig. 6

tion will begin. A very high voltage is re-quired to start ionization. The electricalpotential will apply enough force on theatoms between the spark plug elec-trodes to rip electrons free (Fig. 7B).Atoms having an electron ripped fromthem become positive ions. (An ion is apositively or negatively charged atomand is the result of the atom having lostor gained one or more electrons.) Thisis the breakdown voltage, or theamount of voltage that was required topush the electrons across the resistance.

In this case, the resistance is thespark plug gap (waveform segment G inFig. 2). The wider the spark plug gap orthe greater the resistance between the

spark plug electrodes, the higher thebreakdown voltage will be. This break-down voltage is read as kV and is theamount of energy required to overcomethe total resistance within the circuit.Once the electrons have bridged thespark plug gap, ionization is complete.

Notice the oscillations that occur asthe electron flow starts after the break-down voltage (waveform segment H inFig. 2). This ringing, or oscillation, iscreated by the induction occurringacross the windings and the capacitancebetween the turns. The transformermakes it very easy for the energy tochange between electrical energy andmagnetic energy. The breakdown volt-

age that starts the arc is very fast (about2 nanoseconds). This fast energy spikestarts the energy change between elec-trical and magnetic. The harder thespike to start the arc, the more oscilla-tions that will follow.

These oscillations are analogous to achild on a swing. The child begins in astationary position on the swing. Astrong push causes the swing to move.The harder the push, the higher theswing will go. The swing will then oscil-late back and forth until the energy hasdissipated. The ignition coil changeselectrical energy into magnetic energyand vice versa in much the same way.The swing, being a mechanical device,needs a “push” or energy in order tomove, just like the coil’s discharge, or“push,” causes an energy spike. Oncethe electrons establish flow, the voltageis stabilized and the oscillations will di-minish into an even voltage (waveformsegment I in Fig. 2).

Once ionization occurs, the free elec-trons and the positive ions form a path-way across the spark plug electrodes.This occurs at a point where the num-ber of electrons flowing equals thenumber of positive ions and the sparkplug gap “plasmas” (waveform segmentH in Fig. 8). Plasma is a hot ionized gasthat enshrouds the electrons flowingthrough it, thus lowering the resistanceacross the spark plug electrodes (Fig.7C). The resistance of the plasma is af-fected by the gas and the pressure thatcomprise it. The plasma will decreasethe voltage required to maintain the

38 May 2005


Fig. 7A Fig. 7B Fig. 7C

Fig. 8

electron flow across the spark plug gap.The voltage level at which the ioniza-

tion turns to plasma is a very importantpoint to analyze. Since the breakdownvoltage is not stable, but moves up anddown on various discharge cycles, it’snecessary to check the voltage level ofthe plasma. This plasma voltage is morestable than breakdown voltage and willshow resistance values that cannot beseen in breakdown kV. The point atwhich the ionization turns to plasmawill be affected only by resistance in thecircuit.

In Fig. 9 below, the yellow trace has a20k resistor placed in the ignition wire.The red trace is the companion cylinderand the point of plasma is normal. Thepoint of plasma on the yellow trace is2.3kV higher than normal, indicating re-sistance in the circuit.

In Fig. 10, the yellow tracehas a .20-in. gap between theignition wire and the sparkplug. The red trace is thecompanion cylinder and thepoint of plasma is normal. Onthe yellow trace, the point ofplasma is 1.2kV higher thannormal, indicating resistancein the circuit.

In Fig. 11, the injector isunplugged, allowing no fueldelivery to the cylinder. Notethe point that the ionizationchanged to plasma did not dif-fer between the yellow andred traces, indicating normalresistance in the circuit. How-ever, the plasma waveformhas more resistance due to thelack of hydrocarbons in theplasma gas. This creates thevery steep voltage rise in theburn time that exceeds 10kV.

Once the electron flow isestablished across the pluggap, it will continue until thesecondary energy is depleted.As the transformer runs outof energy near the end of theburn time, there’s a slight risein voltage as the spark burnsout (waveform segment J inFig. 2). This is caused by theplasma breaking down. Theelectrons from the trans-

former start to decrease in number,causing an imbalance between the posi-tive ions and the electrons, allowing theplasma to break down. Since the plasmacreates an electrical pathway that hasless resistance, this plasma breakdownallows the resistance to increase, caus-ing the voltage rise at the end of theburn time.

The induction that put electrical en-ergy into the secondary coil winding islimited. An ignition coil that’s fully satu-rated is like a water bucket that’s totallyfilled. If a water pump were used topump the water out of the bucket un-der pressure through a fixed orifice,then the higher the pressure, the quick-er the water would be emptied. Oncethe water is gone, the pressure wouldalso be depleted. In the secondary igni-tion coil, the greater the voltage or pres-

sure the coil needs to push the electronsacross the resistance in the circuit, thequicker the electrons are used up.

The period when the electronsbridge the spark plug gap is called burntime (waveform segments G-J in Fig. 2).Burn time will change according to thepressure it took to start the electronsflowing through the circuit. If the pres-sure is low, the burn time will be longer;if the pressure is high, the burn timewill be shorter.

Let’s use a piece of rope to demon-strate this principle. Assume the rope isa set length, and is positioned to repre-sent the pattern that the breakdownvoltage and burn time make (Fig. 12 onpage 44). If the rope used to make thevertical line is longer, the horizontal linewill become shorter. Conversely, if thehorizontal line becomes longer, the ver-

tical line will become shorter.If the entire length of rope isshorter, just like when the ig-nition coil’s magnetic field isnot fully saturated, the verti-cal and horizontal sectionswill also be affected, due tothe reduced amount of storedenergy available.

The breakdown voltage andburn time are influenced bythe pressure or compressionand the content of the gasthat’s in the cylinder. Undernormal conditions, the cylin-der is filled with a gas com-prised of ambient air (approxi-mately 21% oxygen and 79%nitrogen) and C4H8 hydrocar-bons (gasoline) in a ratio of14.7 parts air to one part hy-drocarbons. The gas mixturein the cylinder is composed ofatoms that will ionize or allowthe spark to jump across thespark plug electrodes.

We know these atoms willionize. But if conditionschange, their ability to ionizewill change. The amount ofpressure or compression willchange the density of the mix-ture, which will have an effecton ionization. The turbulencewithin the cylinder will alsochange the characteristics of

43May 2005


Fig. 9

Fig. 10

Fig. 11

the ignition waveform. If any of these variables changes—compression or pressure, turbulence, gas content, fuel or wa-ter vapor—then the ionization that forms the plasma willchange. This, in turn, affects the spark waveform.

Spark stops when the electrical energy is not strongenough to keep the electrons flowing across the spark pluggap (waveform segment J in Fig. 2). Whatever energy is leftwithin the coil must be absorbed by the windings. The ab-sorbed energy is dissipated by changing between electricaland magnetic energy. This is what causes the oscillations inthe waveform at the end of the spark duration (waveform seg-ment K). This ringing can be used to see how much energywas used or not used during the ignition coil discharge. Alarge voltage change and a large number of ringing oscilla-tions at the end of the waveform indicate the amount of ener-gy left in the ignition coil. If there are no oscillations, the igni-tion coil’s energy has been totally dissipated.

The ignition waveform is a window that allows the techni-cian to see what’s occurring in the combustion chamber.Once you learn how to view the waveform during the break-down voltage and burn time, you’ll see how the waveformreflects what’s occurring within the cylinder. Examples ofconditions that can be identified via the ignition waveforminclude lean air/fuel ratio, rich air/fuel ratio, preignition, tur-bulence caused by cam timing or valves, turbulence caused

by exhaust backpressure, EGR, water vapor caused by anengine coolant leak, worn spark plugs, carbon tracking, re-sistance within the circuit, etc. There’s more informationwithin the ignition coil’s waveform than in any other wave-form produced on the vehicle.

44 May 2005


Visit www.motor.com to download a free copy of this article.

Fig. 12

Language is the key tounderstanding what’shappening around you.Imagine being in a for-eign country and notbeing able to speak the

language. Now, imagine being giventhe task of gathering information andmaking an informed decision basedon that information. This is whatmany of us do every day when we at-tempt to diagnose vehicle malfunc-tions, lacking a clear understanding ofthe language spoken by the vehicleswe’re attempting to service.

The basic unit of the electrical lan-guage of the automobile is impulses, orchanges in voltage (amplitude) overtime. In a written language, letters jointogether to convey useful information.In this electrical language, changes involtage convey the information. Thesechanges are the letters or symbols ofthe electrical language. Individually,the letters can’t tell you very much.But when they’re linked together, theyhave a story to tell.

The oscilloscope is a powerful toolthat can help you interpret and under-stand the electrical language. It dis-

plays voltage amplitude over time,thus creating a visual display or graphthat’s commonly referred to as a wave-form. These electrical waveforms carryinformation that’s needed to diagnosea vehicle. Each of these waveformscontains unique information about theelectrical circuits that affect the opera-tion of the vehicle’s systems.

Our purpose here is not to teachyou how to use a specific scope mod-el; that’s what instruction manualsand training videos are for. Rather,we’ll explain how certain vehicle elec-trical components work, show youwhat their waveforms typically looklike and explain how to diagnosecomponent faults based on the infor-mation those waveforms provide.

The first waveform we’ll examinebelongs to the very common satura-tion-style fuel injector. The cutawaydrawing in Fig. 1 on page 36 illustratesthe internal components of this type ofinjector. At first glance, the waveformproduced by this injector looks like avery simple signal. However, an in-depth analysis is required to under-stand what’s actually occurring.

Each part of the waveform (a

record of voltage over time) has a sto-ry to tell. Let’s look at waveform seg-ment A in Fig. 2. This is the open cir-cuit or source voltage. It’s referred toas open circuit voltage because the in-jector circuit has not been completedand there’s no current flowing at thispoint. The voltage drops abruptly inwaveform segment B when the pow-ertrain control module (PCM) driverturns on, thus completing the injectorcircuit to ground. A magnified view ofthe waveform can be seen in the low-er screen of Fig. 2.

The voltage should come very closeto ground at this point. The initial volt-age drop will depend on whether theelectrical device used is a transistor ora MOSFET. If a transistor is used, thedrop will be .7 to 1 volt. This is due tothe resistance across the transistor’soutput. MOSFETs have less resistanceacross their output, causing a lowervoltage drop of .2 to .3 volt.

The voltage drop is the voltage thatremains in the circuit to push the cur-rent across the resistance of the PCMdriver or gate (waveform segment C).Once the PCM commands the injec-tor driver closed, current starts to

34 January 2005

FUEL INJECTOR WAVEFORMSSLICED EXTRA THIN

BY BERNIE THOMPSON

We took a slice out of an injector to remind

you that injector waveforms are nothing

more than snapshots of thin slices of time.

flow through the injector coil circuit.When current flows through a coilwinding, all of the current is used tocreate a magnetic field around thewinding (Fig. 3, page 38).

The magnetic field is proportional tothe current and the number of turns inthe coil. In other words, the larger thecurrent, the larger the magnetic field.As the magnetic field is building, theinductance offers resistance to thechange in current flowing through theinjector circuit. As the field builds, itmoves across the coil winding and in-duces voltage into the coil winding.This induced voltage frees electrons,offering resistance to the change incurrent flowing through the coil.

Imagine a school hallway packedshoulder to shoulder with childrenrunning as fast as they can. Now imag-ine children entering the hallway fromclassrooms located along this hallway.The children leaving the classroomscan’t change the flow of children al-ready running down the hallway with-out increasing the pressure. Just likethe children entering the hallway, theinduced voltage (pressure) in the in-jector winding creates resistance tothe change in current flowing throughthe injector circuit. This resistance iscalled counter electromotive force (emffor short), or counter voltage.

Whenever there’s inductance in acircuit, a counter emf will be pro-duced by a change in current in sucha way as to resist the change in cur-rent. And whenever there’s resistancein a circuit, there will be a voltagedrop proportional to the resistance.This voltage drop can be seen as theslight rise at the bottom of the injec-tor waveform.

If the oscilloscope voltage setting islowered to magnify the bottom of theinjector waveform, the waveformvoltage drop can be seen more clearly(waveform segment A in the upperscreen of Fig. 4). Since the currentflowing through the winding pro-duces the resistance for the voltagedrop, it mirrors the injector wave-form made with an inductive currentclamp (waveform segment B in thelower screen of Fig. 4).

Once the PCM driver has closed,current starts to flow through the cir-

35January 2005

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cuit. The magnetic field buildsuntil it becomes strong enoughto overcome the mechanicalspring pressure holding the in-jector pintle in the seated posi-tion. At this point, the injectorpintle starts to move awayfrom the seat and through themagnetic field (Fig. 5).

When a ferrous metal movesthrough a magnetic field, itcauses the magnetic field toflex or change, thus inducingvoltage into the coil winding.This induction releases freeelectrons, which impede theflow of current in the injectorwinding circuit. In turn, thiscauses the current to diminishslightly and create the hump inthe waveform (waveform seg-ment C in the upper screen ofFig. 4). This hump shows whenthe injector pintle opens andallows fuel to flow through theinjector. The current then con-tinues to build until the wave-form reaches its maximum cur-rent. This is set by source volt-age and the resistance in thecircuit. Note: Not all injectordesigns will exhibit this charac-

teristic hump.The PCM has calculated the

proper injector ON time andturns the injector driver off,opening the circuit (waveformsegment F in Fig. 2 below).This creates an abrupt voltagerise, as the counter emf resiststhe change in current. Thevoltage level will continue torise, passing the open circuitvoltage, until it’s clipped at acritical point.

The voltage rise is caused bythe magnetic field that hasbeen built around the injectorcoil winding. Once the circuitis opened and current stopsflowing, the stored energy inthe magnetic field tries to sta-bilize the current in the injec-tor circuit. As this energy fallsback into the injector winding,it falls across the turns of thewinding, thus causing induc-tion at the injector coil (wave-form segment G in Fig. 2).The induced (flyback) voltageis clipped at a voltage levelspecified by the fuel injectionmanufacturer, based on theparticular injector design inthe circuit (waveform segment

G in Fig. 2). The flyback voltage clip-ping point is set based on the injector’selectromagnetic coupling and me-chanical spring rate.

The stored energy of the magneticfield around the injector winding isused to control the pintle closingspeed. If the pintle is allowed to closetoo fast, the pintle and seat will soonbecome pounded out and begin toleak fuel. A fast closing rate will alsocause the pintle to bounce, allowingextra fuel to be delivered to the en-gine. This extra fuel can’t be accu-rately controlled, so the engineermust adjust the energy held withinthe flyback voltage to control theclosing rate of the injector. This is ac-complished by placing a zener diodeacross the PCM driver (transistor orMOSFET), as illustrated in Fig. 6.Note: Some fuel injection systems donot clip the flyback voltage.

As the magnetic field falls back into

36 January 2005

FUEL INJECTOR WAVEFORMS, SLICED EXTRA THIN

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the injector winding, the ener-gy loops through the clippingcircuit. This allows the currentto diminish at a set rate. Thelower the voltage level set bythe zener diode, the more en-ergy that will be allowed toloop through the circuit. If anordinary diode were used in-stead of a zener diode, itwould allow the largestamount of stored energy toloop through the circuit.

A diode will allow thestored energy to loop until itreaches source voltage—inthis case, 12 volts. This wouldallow the injector the longestperiod of time to close. Thehigher the zener diode volt-age, the shorter the period oftime allowed for the injectorto close. This is due to theenergy looping through thecircuit being cut off early bythe voltage rating of the zen-er diode.

If a 65-volt zener diodewere used, the energy loopingthrough the circuit would be stopped at65 volts. This is 53 volts sooner than aconventional diode, which allows the

energy to continue to loop through thecircuit until it reaches 12 volts. So theenergy from a zener diode with a higher

voltage rating will shut off theenergy looping through thecircuit sooner, allowing afaster pintle closing rate. Like-wise, the energy from a lowerrated zener diode will allowthe energy to loop longer,causing a slower pintle closingrate. The closing rate is set bythe zener diode voltage, whichis matched to the injector de-sign.

When the pintle starts tofall through the magneticfield, the field is distorted anda voltage is induced (Fig. 7).The delay in closing voltagecan be seen in waveform seg-ment H in Fig. 2. The delayis the source of the injectorclosing bump in the wave-form. Notice that when cur-rent is not flowing throughthe injector circuit, the pintlemovement induces a positivevoltage (waveform segment Iin Fig. 2). Note: Some injectorwaveforms will not exhibitthis bump.

The PCM-commanded ON time isthe time between segments B and Fin Fig. 2. However, this is not thetrue injector opening time. The trueinjector opening time is from humpD to the closing bump I (indicated bymeasurement J). In a good circuit,the time to get from D to I will equalthe PCM’s command time from seg-ments B to F (indicated by measure-ment E).

Now that we understand the datacontained in a saturation injectorwaveform, we can use it to see whatmay have failed within the circuit.Let’s begin by analyzing two injectorwaveforms (Fig. 8 on page 40). Atfirst glance, both of these waveformslook good. However, one has a prob-lem. Can you see it?

Look at segment A in both wave-forms. The PCM has commanded theinjector driver ON. The voltage ex-hibits an abrupt change and falls nearground on both patterns (segment B inboth waveforms). Current begins toflow through the injector winding andcounter voltage begins to rise off

38 January 2005


Fig. 3

Fig. 4

ground (segment C in both waveforms).Notice there’s more rise in the countervoltage on segment C in the upperwaveform in Fig. 8 than in segment Cin the lower waveform.

Next, the PCM calculates the cor-rect ON time and commands the in-jector driver OFF. This causes anabrupt rise in the voltage (segment Din both waveforms). The voltage risespast the open circuit level and peaksout at the point where it’s clipped at65 volts (segment E in both wave-forms).

Notice that in segment E of the up-per waveform in Fig. 8 there’s morespace between the rising edge and thefalling edge of the flyback voltagethan in segment E of the lower wave-form. This is an indication that themagnetic field of the injector circuitrepresented by the upper waveformin Fig. 8 has more stored energy.Since the magnetic field is built bythe current flowing through the wind-ing, this shows that the other injectorcircuit (lower waveform) has a resis-tance problem.

As the voltage falls back to theopen circuit voltage, you can see theinjector pintle closing bump. Noticethat segment F in Fig. 8’s upperwaveform shows a longer closing timeby 2.5 microseconds (�S) than seg-ment E in the lower waveform. Thisis because the magnetic field has

more energy and can correctly con-trol the pintle closing rate.

Now let’s magnify the bottomcounter voltage in both waveforms inFig. 8 and further analyze them (Fig.9). These waveforms look quite differ-ent at first glance. They’re not like thewaveforms in Fig. 8, where the appear-

39January 2005

Fig. 6

Fig. 5 Fig. 7

ance seems very similar. In segment A(both waveforms in Fig. 9), the PCMcommands the injector driver closed.This creates an abrupt drop in voltage.The initial voltage drop comes veryclose to ground (segment B). The cur-rent starts to flow through the injector

winding, making a counter voltage andcausing the voltage drop at the bottomof the waveform. Note: The scope wasset at 2 volts per division for the upperwaveform in Fig. 9 and .5 volt per divi-sion for the lower waveform.

We can see the injector pintle open

in segment C of both waveforms in Fig.9. However, the opening time is de-layed by 1.9mS in the lower waveform.The counter voltage difference fromthe upper to the lower waveform is .65volt. Since the counter voltage mirrorsthe change of current flow, the lowerwaveform has considerably less currentflowing through the injector circuit.

Due to the diminished rate ofchange of the current flowing throughthe lower waveform, the magneticfield takes longer to build. This delayin the magnetic field causes the pintlespring to keep the pintle seatedlonger. The magnetic field must havemore energy than the mechanicalforce of the spring to overcome it andopen the injector pintle.

The resistance within the lowerwaveform in Fig. 9 is not in the con-trol side of the circuit. Since thescope lead is at the injector, you canclearly see the voltage drop on theground side of the circuit is good. Itwould be necessary to check the pow-er side of the injector circuit, the in-jector connector and the injector coilwinding to find the problem.

In this example, the problem wasdue to high resistance in the injectorcoil winding, which caused the pintleopening and closing delay. The prob-lem is very hard to see until you mag-nify the counter voltage in the injectorwaveform. At first glance, one mightnot see a difference between the up-per and lower waveforms in Fig. 8, orbetween cylinder 1 and cylinder 2. Butupon closer inspection, we can see thatthere’s a delay of 2.15mS. In the upperwaveform, the actual injector openingtime is 3200�S, or 3.2mS. In the lowerwaveform, the actual injector openingtime is 1100�S, or 1.1mS. This is one-third the fuel delivery and would causea lean misfire and rough idle complaintthat can be easily overlooked.

As you can see, when the letters ofa new electrical language can belinked together and understood, theycan tell a story about what has gonewrong within a circuit.

40 January 2005



Fig. 8

Fig. 9

The process of choosingthe right diagnostic scantool for the shop is chal-lenging, to say the least.Prior to 1996, the choiceswere simple: the Snap-on

MT2500, the OTC Monitor 4000, theVetronix Mastertech or any of the OEscan tools, if you could get them. The in-troduction of the OBD II generic stan-dard in 1996 and subsequent updates tothe OBD II standard, like the singleCAN communications protocol, have re-sulted in an increased number of com-panies introducing scan tools. The up-dated EPA Clean Air legislation andNASTF’s efforts have made access tothe factory scan tools a little easier. InMay, I scanned MOTOR Magazine andseveral other publications and countedmore than 60 advertisements for scantools. It was the most heavily advertisedtype of equipment in those magazines.

The most commonly asked questionat the PWR Training events I conductis: “What scan tool should I buy?” Thehonest answer to the question is: “Asmany as you can afford.” Why? BecauseI don’t believe a do-it-all scan tool existsand we’re not likely to see one anytimesoon. Some aftermarket scan tool man-ufacturers may claim to have the totalsolution, but what you may find is anadequate engine diagnostic scan toolthat falls short in body and chassis sys-tem diagnosis. The diagnostic capabili-ties of the GM Tech 2, Ford NGS andChrysler DRB III are difficult to dupli-cate in an all-inclusive aftermarket com-bination scan tool.

With all this confusion, how do youknow which scan tool will provide thegreatest value? There are many factorsto consider before you upgrade or pur-chase a new scan tool. This article willfocus on developing a strategy that willmake the decision a little easier.

I have broken scan tools into threegeneral categories: OBD II generic scantools, aftermarket combination scantools and vehicle manufacturer scantools. Let’s take a look at the categories:

OBD II Generic Scan ToolsEvery shop should own at least two in-expensive OBD II generic scan tools.Why? Due to the imperfect imple-mentation of the OBD II specification

20 July 2005

by some vehicle and scan tool manu-facturers, one scan tool may not com-municate with a particular vehicle, butanother one will. There are three basictypes of OBD II generic scan tools:

•Dedicated scan tools such as theAutoXray EZ-Scan 6000, Injecto-Clean CJ-15 and SPX/OTC ScanPro.

•PDA-based tools, including thoseoffered by InjectoClean, AutoEngi-nuity and EASE, among others.These tools may operate on the PalmOS or Pocket PC platforms.

•PC-based OBD II generic toolsfrom InjectoClean, AutoEnginuity,EASE and others offer PC-based so-lutions.

Each type has benefits and limita-tions. The dedicated and PDA unitsare designed to be inexpensive, sim-ple and quick diagnostic scan tools. Ilike using them to retrieve faultcodes, check readiness status andtake a quick look at high-priority pa-rameters. (If you’re not sure whichdata parameters are important, youmight want to review my “Interpret-ing Generic Scan Data” article in theMarch 2005 issue of MOTOR.)

The limitations of OBD II genericscan tools usually relate to small dis-play screens and slow sample rates oflive scan data. If you’re willing to in-vest a little more money, the EASEOBD II generic PC-based scan toolhas several very useful features.Those I find invaluable are unlimiteddata recording and customized datagraphing. This is the tool I use to di-agnose difficult intermittent drive-ability problems.

Aftermarket CombinationScan ToolsIf you’re operating a general repairshop and service many vehicle makesand models, it makes sense to own atleast one combination scan tool. Ex-amples of this type of tool include theSPX/OTC Genisys, Snap-on Solus,EASE PC-based combination scantool, etc. These scan tools generallywork best when diagnosing enginecontrol systems on domestic vehicles.Some scan tool manufacturers alsooffer upgrades for Asian and Euro-pean vehicles.

Some aftermarket scan tools are

21July 2005

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SCAN TOOLASSESSMENT

BY BOB PATTENGALE

Are you getting everything possible

out when you plug it in? Here’s how

to evaluate your current scan tool

capabilities, determine shop needs

and plan purchase decisions.

manufacturer-specific. For example,the Vetronix Mastertech works well onToyota and Honda vehicles, and Baummakes tools that work well on Euro-pean vehicles. There are other exam-ples, but the point is that when lookingfor the right scan tool, knowing whatvehicle makes you work on or don’twork on can simplify your options.

Some aftermarket manufacturersmay claim their scan tools can diag-nose a particular system, but once youbegin the diagnostic process, youmight be missing some parametersand bidirectional controls that wouldbe available on a factory scan tool. Inan example I came across recently,one scan tool manufacturer’s market-ing materials claimed to offer air bagdiagnostics for Nissan vehicles. Thescan tool was connected to a 1997 Nis-san Altima. In this case, the scan tooldid not actually communicate with theair bag module, but did provide infor-

mation on how to retrieve the faultcodes manually. Although retrievingthe fault codes was a good first step,the fault code diagnosis required addi-tional tests that needed to be per-formed with the Nissan factory scantool. Most of us have run into similarissues, and it can be frustrating. Theimportant point here is to understandthe limitations of the all-in-one tool. Alist of aftermarket scan tool manufac-turers, with website information, isgiven on page 28.

Vehicle ManufacturerScan ToolsIf yours is a specialized repair facilityor you work on a particular make ofvehicle more than 40% of the time,you should own the factory scan toolfor that vehicle maker. This will pro-vide the best overall coverage for allvehicle systems, including powertrain,air bag, climate control, etc. The limi-

tations to this solution are cost andthe learning curve for each tool.

One potential solution to the costissue is to work with other shops inthe area. For example, three shopscould pool their resources to pur-chase a group of factory scan toolslike the Ford NGS, GM Tech 2 andChrysler DRB III, then share themas needed. Another option, whichworked well for one shop, is to pur-chase several factory scan tools, thenrent them out to other shops. See thebox “Vehicle Manufacturer ScanTools” on page 26 for contact infor-mation for the various vehicle manu-facturers. This list was compiled fromthe NASTF service information ma-trix (www.nastf.org).

Each shop may need a combinationof scan tools from all three categoriesto make the diagnostic process as effi-cient and successful as possible. Thereare a few questions you need to con-

22 July 2005

SCAN TOOL ASSESSMENT

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sider before making the next scan tooldecision. For example, should youpurchase an all-in-one diagnostic plat-form or separate tools? The trendfrom many equipment manufacturersis to combine all the diagnostic toolsinto one platform. This is designed tosave money and simplify the learningcurve. Although this may seem like agood idea, there are some issues thatneed to be considered.

Also, what happens if the all-in-onetool malfunctions? If you purchased acombined scanner/scope/gas analyzer,you now have no diagnostic equip-ment available. Are the combinedplatform items as good as dedicatedtools? In order to combine all the fea-tures and keep the cost down, somecorners may need to be cut. The bestexample is the micro gas analyzersthat are available for many of the all-in-one tools. The accuracy, reliabilityand expected life are reduced, when

24 July 2005


No. of % ofCarmaker Jobs TotalAcura . . . . . . . . . . . . . . . . . . .14 . . . . . .1%Audi . . . . . . . . . . . . . . . . . . . . . . . . . . . . .0%BMW . . . . . . . . . . . . . . . . . . .11 . . . . . .1%Chrysler/Dodge/Jeep . . . . . . . .4 . . . . . .0%Ford/Lincoln/Mercury . . . . . . .6 . . . . . .1%Geo . . . . . . . . . . . . . . . . . . . . . .8 . . . . . .1%GM: Buick, Cadillac,

Chevrolet, GMC, Oldsmobile, Pontiac, HUMMER . . . . . . . . . . . . .2 . . . . . .0%

Honda . . . . . . . . . . . . . . . . .165 . . . . .14%Hyundai . . . . . . . . . . . . . . . . . .9 . . . . . .1%Infiniti . . . . . . . . . . . . . . . . . .21 . . . . . .2%Isuzu . . . . . . . . . . . . . . . . . . . .54 . . . . . .5%Jaguar . . . . . . . . . . . . . . . . . . . . . . . . . . .0%Kia . . . . . . . . . . . . . . . . . . . . . .4 . . . . . .0%Land Rover . . . . . . . . . . . . . . . . . . . . . . .0%Lexus . . . . . . . . . . . . . . . . . . .38 . . . . . .3%Mazda . . . . . . . . . . . . . . . . . .79 . . . . . .7%Mercedes-Benz . . . . . . . . . . . . . . . . . . . .0%Mini . . . . . . . . . . . . . . . . . . . . . . . . . . . . .0%

No. of % ofCarmaker Jobs TotalMitsubishi . . . . . . . . . . . . . . .26 . . . . . .2%Nissan . . . . . . . . . . . . . . . . . .186 . . . . .16%Porsche . . . . . . . . . . . . . . . . . . . . . . . . . .0%Saab . . . . . . . . . . . . . . . . . . . . . . . . . . . . .0%Scion . . . . . . . . . . . . . . . . . . . . . . . . . . . .0%Saturn . . . . . . . . . . . . . . . . . . . . . . . . . . .0%Subaru . . . . . . . . . . . . . . . . . .41 . . . . . .4%Suzuki . . . . . . . . . . . . . . . . . . .5 . . . . . .0%Toyota . . . . . . . . . . . . . . . . .414 . . . . .36%Volkswagen . . . . . . . . . . . . . .60 . . . . . .5%Volvo . . . . . . . . . . . . . . . . . . . .2 . . . . . .0%

Total . . . . . . . . . . . . . . . . .1149

Vehicle Model Year 1991-1995 . . . . . . . . . . . . . .3691996-2000 . . . . . . . . . . . . . .4192001-2005 . . . . . . . . . . . . . .228

Total . . . . . . . . . . . . . . . . .1016

Vehicle Repair Matrix

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compared to dedicated full-size gasanalyzers. The all-in-one tool is, how-ever, a good choice for a shop thatneeds an extra backup tool or to pro-vide options for multiple technicians.

Finally, should you purchase OEMor aftermarket tools? This is a diffi-cult decision to make, but in manycases the OEM tool will provide themost comprehensive capabilities for agiven vehicle make. Aftermarket toolsare designed to combine features, re-duce cost and simplify the user learn-ing curve. This is where a specializedrepair shop has an advantage over ageneral repair shop. The specializedrepair facility normally services vehi-cles that fall into a range of years,makes and models. In this case, pur-chasing the factory diagnostic scantool is the best choice.

The next couple of questions to an-swer are: Do you know if you alreadyhave the correct scan tools? And if not,how do you decide what to purchasenext? The first step to answering thesequestions is to evaluate the specificneeds of your shop. This may take a lit-tle time, but the exercise will help withother shop decisions, like repair infor-mation and training requirements. Be-gin by reviewing the vehicles repairedduring the last year. Your shop man-agement program may be able to pro-vide this information electronically. Ifnot, manually review the invoices. Theinformation collected should includevehicle year, make and mileage.

If you want to spend the extratime, consider documenting the typeof diagnostic work that was per-formed on these vehicles—for exam-ple, powertrain, air bag, antilockbrake system, etc. This informationwill be used to determine what typesof scan tools will provide the greatestbenefit for the shop. If you would likethe sample spreadsheet in electronicformat, visit the PWR Training websiteat www.pwrtraining.com/resources.

The box “Vehicle Repair Matrix”on page 24 shows information thatwas compiled from an import serviceshop in the Tucson area. As you cansee, this shop focuses on Asian andEuropean vehicles. Also, Toyota,Honda and their subsidiaries repre-

26 July 2005


Carmaker Scan Tool Contact Phone Website

Acura (see Honda) Audi . . . . . . . . . . . . . . . . . . . . .Equipment Solutions . . . . . . . .1-800-892-9650 . . . . . . . . .N/ABMW . . . . . . . . . . . . . . . . . . . . .Central Letter Shop . . . . . . . . .1-800-695-0079 . . . . . . . . .N/AChrysler/Dodge/Jeep . . . . . .SPX . . . . . . . . . . . . . . . . . . . . . .1-800-801-542 . . . . . . . . . .N/AFord/Lincoln/Mercury . . . . . .N/A . . . . . . . . . . . . . . . . . . . . . .1-800-ROTUNDA . . . . . . . .www.motorcraft.comGM: Buick, Cadillac,

Chevrolet, Geo,GMC, HUMMER,Oldsmobile, Pontiac . . .SPX Kent-Moore . . . . . . . . . . . .1-800-825-5886 . . . . . . . . .www.gmtechinfo.com

Honda . . . . . . . . . . . . . . . . . . .Teradyne . . . . . . . . . . . . . . . . . .1-800-210-8699 . . . . . . . . .www.serviceexpress.honda.comHonda . . . . . . . . . . . . . . . . . . .Vetronix . . . . . . . . . . . . . . . . . .1-800-321-4889 . . . . . . . . .www.vetronix.comHyundai . . . . . . . . . . . . . . . . . .SPX Kent-Moore . . . . . . . . . . . .1-800-336-6687 . . . . . . . . .www.dealerequipment.comInfiniti (see Nissan)Isuzu . . . . . . . . . . . . . . . . . . . . .SPX Kent-Moore . . . . . . . . . . . .1-800-345-2233 . . . . . . . . .www.isuzutechinfo.comJaguar . . . . . . . . . . . . . . . . . . .N/A . . . . . . . . . . . . . . . . . . . . . .N/A . . . . . . . . . . . . . . . . . .www.jaguartechinfo.comKia . . . . . . . . . . . . . . . . . . . . . .N/A . . . . . . . . . . . . . . . . . . . . . .1-866-542-8665 . . . . . . . . .www.kiatechinfo.comLand Rover . . . . . . . . . . . . . . .N/A . . . . . . . . . . . . . . . . . . . . . .N/A . . . . . . . . . . . . . . . . . .www.landrovertechinfo.comLexus (see Toyota)Mazda . . . . . . . . . . . . . . . . . . .Hickok Customer Care . . . . . . .1-800-342-5080 . . . . . . . . .www.mazdatechinfo.comMercedes-Benz . . . . . . . . . . . .N/A . . . . . . . . . . . . . . . . . . . . . .1-800-FOR-MERCEDES . . .www.startekinfo.comMini . . . . . . . . . . . . . . . . . . . . .N/A . . . . . . . . . . . . . . . . . . . . . .N/A . . . . . . . . . . . . . . . . . .www.minitechinfo.comMitsubishi . . . . . . . . . . . . . . . .SPX . . . . . . . . . . . . . . . . . . . . . .1-888-727-6672 . . . . . . . . .www.mitsubishitechinfo.comNissan . . . . . . . . . . . . . . . . . . .SPX . . . . . . . . . . . . . . . . . . . . . .1-800-662-2001 . . . . . . . . .www.nissantechinfo.comPorsche . . . . . . . . . . . . . . . . . .N/A . . . . . . . . . . . . . . . . . . . . . .N/A . . . . . . . . . . . . . . . . . .www.techinfo.porsche.comSaab . . . . . . . . . . . . . . . . . . . . .SPX . . . . . . . . . . . . . . . . . . . . . .1-800-345-2233 . . . . . . . . .www.saabtechinfo.comScion (see Toyota)Saturn . . . . . . . . . . . . . . . . . . .SPX . . . . . . . . . . . . . . . . . . . . . .1-800-533-6127 . . . . . . . . .www.gmtechinfo.comSubaru . . . . . . . . . . . . . . . . . . .SPX Kent-Moore . . . . . . . . . . . .1-866-213-4690 . . . . . . . . .www.techinfo.subaru.comSuzuki . . . . . . . . . . . . . . . . . . .SPX Kent-Moore . . . . . . . . . . . .1-800-345-2233 . . . . . . . . .www.suzukitechinfo.comToyota . . . . . . . . . . . . . . . . . . .SPX . . . . . . . . . . . . . . . . . . . . . .1-800-933-8335 . . . . . . . . .N/AToyota . . . . . . . . . . . . . . . . . . .Vetronix . . . . . . . . . . . . . . . . . .1-800-321-4889 . . . . . . . . .www.techinfo.toyota.comVolkswagen . . . . . . . . . . . . . .Equipment Solutions . . . . . . . .1-800-892-9650 . . . . . . . . .erwin.volkswagen.deVolvo . . . . . . . . . . . . . . . . . . . .SPX . . . . . . . . . . . . . . . . . . . . . .1-800-345-3399 . . . . . . . . .www.volvotechinfo.com

Vehicle Manufacturer Scan Tools

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sent more than 50% of the shop’s re-pair volume. So what does this infor-mation tell us? First, this shop needsto look for scan tools that provide thegreatest coverage for Asian and Euro-pean vehicles. Second, based on theinformation, they may want to con-sider purchasing a scan tool that willprovide the best coverage for Toyotaand Honda vehicles. This data can al-so be used to help make other shopmanagement decisions.

Here’s something else to considerwhen selecting scan tools and diag-nostic equipment for the shop: Howdo you know if a piece of shop equip-ment will stand the test of time anddeliver the best value? Most diagnos-tic equipment will fall into one of twocategories—equipment that defies ob-solescence and equipment that be-comes obsolete.

Examples of equipment that defiesobsolescence are vacuum gauges,

smoke machines, DMMs, fuel pres-sure gauges, lab scopes and gas ana-lyzers, among others. Once theseitems are purchased, they normallyneed to be replaced only whenthey’re worn out or broken. The mostcommon example of equipment thatmay become obsolete is the diagnos-tic scan tool. Each time a manufac-turer releases a new vehicle with en-hanced electronic features, a scantool will need to be updated or, insome cases, replaced.

Equipment that defies obsoles-cence forms the backbone of diagnos-tics, but the scan tool is the heart,and the most valuable piece of diag-nostic equipment in the shop today.Scan tools should be updated as oftenas possible and replaced when that isno longer a practical option.

The next step is locating scan toolsthat might meet your shop’s needs.How do you know if a particular scan

tool is right for your shop? Often asalesperson will show a new scan tool orproduct that is interesting, but may notbe right for your shop. The goal here isto ensure that a decision to purchase isbased on true need and not emotion.The following questions should be an-swered long before you arrange productdemonstrations. This list is not all-inclu-sive, but contains some of the most im-portant decision-making questions:

•What scan tool do I want and whydo I need it?

•What vehicles can this scan toolservice?

28 July 2005


Circle #18

Actron Manufacturing/KalEquip www.kalequip.comAutoEnginuitywww.autoenginuity.comAutologicwww.autologicco.comAutomotive Electronics Services www.aeswave.comAutoTapwww.autotap.comAutoXraywww.autoxray.comBaum Tools Unlimitedwww.baumtools.comBlue Streak Electronicswww.bsecorp.comDavis Instrumentswww.davisnet.comEASE Diagnosticswww.obd2.comEquus Products/Innova www.iequus.comGxTwww.gxtauto.comInjectoCleanwww.injectoclean.comInterro Systemswww.interro.comLaunch Techwww.cnlaunch.comSnap-on Toolswww.snapon.comSPX/OTCwww.otctools.comTeradyne Diagnostic Solutionswww.teradyne-ds.comVetronixwww.vetronix.comWaekonwww.waekon.com

Aftermarket Scan ToolManufacturers

•Are these the same vehicles iden-tified as the most popular on your ve-hicle repair spreadsheet?

•Will this scan tool pay for itself orsave money or time in the diagnosticprocess? Take the time to perform arealistic ROI calculation to determinethe true return on investment.

•Will my technicians use this scantool? If the answer is no, you need toask why.

•Can my technicians use this scantool?

•What training will be required atthe time of purchase, and down theroad?

•What are the likely update costsfor this scan tool?

•What are the limitations and ca-pabilities of this scan tool?

After answering these questionshonestly and completely, the nextstep is to research and locate the po-tential products that fit your shop’sneeds. This can be accomplished in a

number of ways, including throughmagazine advertisements or by mak-ing contact with various equipmentcompanies. The most valuable sourceis other shop owners or equipmentusers who have similar interests.

An often overlooked research re-source is the International AutomotiveTechnicians Network (iATN) atwww.iATN.net, a network of over50,000 shop owners and technicianswith over a million years of shared in-formation and experience. A sponsoringiATN member has the ability to searchthe entire database for comments abouttools and equipment. If you don’t findan answer in the database, you can posta question to the members. In most cas-es you’ll receive answers within a fewhours. One of the iATN forums specifi-cally focuses on Snap-on products. Nev-er rely solely on what a salesperson tellsyou about his product. If you can’t findothers who use and believe in the prod-uct, don’t buy it.

Advancing vehicle technology isguaranteed to accelerate changes indiagnostic scan tools. How do youcope? First, make sure the scan toolsyou select best suit the needs of yourshop. Next, start adjusting yourequipment purchase budget to placean increased emphasis on scan tools.Third, use creative methods for pur-chasing equipment, including work-ing with other shop owners. Finally,keep your technicians trained, tomaximize their abilities with the scantools on hand.

Perhaps you were hoping for a directanswer to the complex question “Whatscan tool should I buy?” The bottomline is only you know the exact situationin your shop. I know that if you use thesteps discussed here, your chances ofsuccess will be greatly improved.

30 July 2005



Circle #20

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