KS: X? - MOTOR · 2015. 7. 7. · the CKP, though some systems may al-low the CMP to substitute in...

18 July 2015

CRACKERJACK

PCM DIAGNOSIS:

A SURPRISE IN EVERY BOX?

BY SAM BELL

The PCM is one of avehicle’s most expensivecomponents, underlining

the importance of anaccurate diagnosis.

These proven techniqueswill prevent unwelcome

surprises.

Pg_EDIT_PCM:Pg_EDIT_ 6/18/15 6:35 AM

Iam one of the vanishing breed of old-timers in our industry whocan remember the days before the advent of computerized en-gine controls. One of the first such systems used in a mass-pro-duced car available in the U.S. market was the Bosch D-Jetronicsystem found in the 1969 VW Type 3, introduced the previousyear in Germany. This was a MAP-based setup which used a set

of “subpoints” in the distributor to control injection timing.The control unit was analog, not digital, and was remarkably robust

for its day. Nevertheless, most dealerships kept one in stock, not becausethey went bad, but so that their techs could plug in the proverbial“known-good unit” early on in the diagnostic process. Once a fresh unithad failed to correct the problem, techs began more systematic trou-bleshooting. If the fresh unit did seem to be the cure, smart techs knewthere was a final step—plugging the original back in. It turned out moreoften than not that the real problem had been resolved by the mechani-cal cleaning action of connection and disconnection!

Every time a new technology debuts on the automotive stage, there’sa tendency to blame it for whatever problems are encountered. Those ofus who have grown old in this business will remember when PCV, EGR,electronic ignition and fuel injection were blamed for problems that hadnothing to do with them. Not that there weren’t some legitimateteething pains along the way as those technologies matured!

Because we now have nearly 20 years of experience with OBD II-equipped vehicles, we no longer reflexively blame the PCM when weencounter problems, but it’s safe to say that even today an unnervinglyhigh proportion of PCM replacements are unnecessary. Manufacturers,remanufacturers and OEM warranty analyses all confirm this. For exam-ple, one OEM Technical Training Manager reported that his Tech Linestaff believes “ECMs, BCMs, IPDMs, TCMs and anything else endingin M are often replaced without proper diagnosis!” In fact, with the ex-ception of a few well-documented pattern failures (2005-09 Ford coildrivers issues, for example) probably only about 40% or fewer of today’sPCM replacements are actually necessary. That’s somewhat better thanthe odds of hitting 0 or 00 on a roulette wheel, though not as good as theodds on red, black, even or odd.

My friend Brian Mann runs a mobile diagnostic and reprogrammingservice in my area. He reports seeing a great deal of variation in the levelof electrical troubleshooting expertise among shops in his customer base.“There are shops that many times I know—and tell them—that a [re-placement] PCM will probably not fix the issues, and they respond withsomething like ‘we’ll start with the PCM’ or ‘the PCM controls every-thing.’ Approximately 60% of the time the replacement actually fixes theissues that the technician is attempting to solve.”

Conceptually, one of the biggest problems is that the diagnosis of afaulty PCM is usually arrived at by a process of elimination. That meanschecking out all the other possible causes of the symptoms you’re tryingto fix. Arriving at a PCM diagnosis by process of elimination is tricky in-deed, perhaps even more so for a conscientious tech who may second-guess himself ’til the cows come home!

The tough question I want to examine is this: How do you know whento replace the PCM in a modern vehicle? And how do you know whennot to? While there are some relatively easily identifiable instances thatare pretty straightforward—flood, for example—most PCM failures aresomewhat harder to diagnose. How do we arrive at the diagnosis in anefficient manner? I am by no means the first to pose this question, northe first to try to answer it; it’s probably been asked several times amonth for nearly 20 years on iATN alone.

Let’s begin with some basic tests that should always be done:

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Codes. We’ll start with DTCs. Incases where there are many codes pre-sent, your first working hypothesisshould be that they probably all have acommon cause.

This is a good time to look carefullyat the wiring diagram. For example, ablown fuse might cause numerous opencircuit DTCs. (Of course, you’ll stillhave to find the source of the overloadthat caused the fuse to blow in the firstplace.) Tip: A lot of fault codes, like aToyota P0606 “ECM Processor Fail-ure,” may appear to point to an internalmodule failure, but actually are morelikely the result of some other failure,such as a faulty rear O2 sensor in this in-

stance. In some cases, a fault for ashorted actuator component may beyour first clue to a PCM failure. Useyour bidirectional scanner to verify thePCM’s ability to actuate the new ventsolenoid or other widget as part of yourinitial diagnosis. If the PCM won’t actu-ate a noid light, it certainly won’t runanything else. Probably more than 95%of the time, a PCM that will light a noidwill function normally once the new ac-tuator is installed. (But keep reading tolearn of a Jeep where that didn’t holdtrue!)

Communications issues. You mayencounter a raft of U codes, or you mayhave trouble connecting with one or

more modules. It can be very temptingto jump to the conclusion that the PCMmust be bad if you can’t talk to it. Whilethat is a possibility, there are number ofother tests that should be conductedfirst. Among the most common causesof communication problems is an openPCM power or ground circuit. CAN-bus faults can arise when one or moremodules are internally shorted or whennetwork wiring is compromised. Tip:Look for aftermarket electrical/elec-tronic devices which may have been im-properly installed. It’s not uncommon tofind that someone has used one branchof a CAN bus as a handy “ground” forthat new radio.

CRACKERJACK PCM DIAGNOSIS: A SURPRISE IN EVERY BOX?

20 July 2015

That’s what I say when I dropsomething. Or when I drop theball on something I should have

caught earlier on. In this case, I’m re-ferring to voltage drops, primarily onthe power or ground supply circuits.

Ideally, you want to backprobeeach power connection and measureits voltage in relation to battery posi-tive while the circuit is under its max-imum load, usually during cranking.How much drop is too much? Mostexperts say anything over 200 milli-volts (mV) is a potential problem, butI’ve seen many so-called known-good vehicles where even twice thatmuch is normal. It’s very easy to getbogged down because of a faulty as-sumption or a lack of data fromknown-good vehicles. If you don’thave the data, you may be able tofind it via your repair informationsource or online. If you still can’t findthe specs you need, try to find aknown-good vehicle of the samemodel, realizing that there may besome year-to-year variations.

Don’t forget the ground side ofthe circuit. Here, you’ll measure thedrop between the battery negativeterminal and the PCM ground, againby backprobing the connection un-der maximum load. The same limitsand cautions apply.

Unfortunately, it’s often impossibleto backprobe those connections, andtrying to take those measurementsduring cranking complicates matters

further. Fortunately, there are goodsolutions. One of my favorites in-volves the use of LOADpro DynamicTest Leads, which apply a safe loadlevel to any voltage circuit. Don’t relyon any open circuit test, like a merevoltage reading on a disconnectedharness. Voltage drops require currentflow, and current flow requires a load.Nor can you rely on a high-impedancetest light, since it requires only a tinycurrent to make it shine. Of course, azero meter reading or a dark testlight does mean exactly what you’dthink, so check those fuses!

If you don’t have something likethe LOADpro leads, you’ll need to ap-ply a dummy load to the disconnect-ed harness. One generally safe optionis to check the schematic to verify thefuse rating of the supply circuityou’re testing. You want to apply aload that draws approximately halfas much current as the fuse rating. A55W headlamp bulb may be just thething for most 10A circuits, as it willdraw roughly 4A. Make sure the hotbulb doesn’t burn you.

Using a correct terminal into theharness, connect the load via ajumper to one terminal, with a sec-ond jumper to the ground connectorin the harness. If the lamp lightsbrightly, measure the voltage be-tween its terminals and compare thatto the voltage across the battery itself.If they agree, you’re all set. Move onto the next circuit and repeat. If the

voltage you read at the load differsfrom that at the battery, you’ll haveto do the traditional voltage dropmeasurements on both power andground sides of the load you’ve wiredin. As always, the load must be on;when there’s no current flow, therecannot be any voltage drop. Measuresupply voltage drop from B� to thepositive connection at the load; mea-sure ground drop from B� to thenegative connection at the load.

If there’s an unacceptable drop ateither connection, you can verify thatfixing it will eventually solve yourproblems by temporarily bypassingthe affected power or ground circuitby backprobing it using a fusedjumper. You’ll still have to find and fixthe actual source of the voltage dropeventually, but the bypass will showyou whether that’s all you need. Notethat there may be times when there’sno voltage at all on one of the supplyterminals. Consult your wiringschematic; in many cases the supplyvoltage passes through a relay whichmay not be energized due to thePCM harness being already discon-nected to allow your testing. In suchcases, you can restore the currentpath by applying the missing signal(usually a ground) to the relay’sswitching leg via the correct harnessterminal. Tip: Internal relay contactsare a frequently overlooked site forvoltage drops on the power side, es-pecially in high-mileage vehicles.

‘Oops!’


Power & ground. Electronic de-vices require an adequate supply ofelectrical power, and that includes theground supply circuit as well. While youcan begin with a rudimentary open-cir-cuit voltage check, you’ll eventuallyhave to check for voltage drops in bothpower and ground circuits. Resistancetests are never an adequate substitute(see “Oops!” on page 20). And don’toverlook the importance of a good bat-tery in a modern automobile; a faultybattery can cause an avalanche of codesand other problems.

Reference voltage (Vref). In mostmodern vehicles, the PCM supplies acarefully regulated reference voltage tovarious sensor circuits, such as TPS,MAP or MAF, ECT and others. This isusually very close to 5.0V and can bemeasured on one terminal of a discon-nected ECT harness, for example. Ifyou have access to datastream, you maybe able to read what the computer isseeing via your scan tool. (If your inter-face doesn’t provide voltage readings,check the PID for the coolant tempsensor. Disconnected, it should readsomething very close to �40°F.)

Even if communications are down,you should still be able to measure Vrefwith your DMM. Consult the appropri-ate wiring diagram for the vehicleyou’re servicing and note that, in someinstances, there may be multiple Vrefcircuits. Make sure that each displays

something very close to the nominal val-ue. If the value you find is substantiallyhigher, your problem may be a badPCM ground, so double-check beforecondemning the control unit. If the val-ue is very low, it’s likely something hasshorted to ground. Start by unpluggingsensors while monitoring Vref. If thatfails to reveal the culprit, check the har-ness for damage, such as a chafe-through short circuit to ground. If thevoltage remains stubbornly low, it’s timeto double-check powers and grounds,and only then to order up a new PCM.Tip: Most PCMs are equipped withmultiple power and ground circuits; youmust check all of them.

RPM. I’m using this term in its

broadest sense to include any input thatindicates actual engine motion. Typical-ly, for OBD II vehicles, this comes viathe CKP, though some systems may al-low the CMP to substitute in the eventof a CKP fault. As a rule, the PCMshould respond in some way (like trig-gering spark and injection) when it de-tects an engine rpm signal. If there’s noresponse, the fault may still lie in an im-mobilizer or antitheft system ratherthan in the PCM itself, so don’t jump toa premature conclusion.

Pin-outs. For most vehicles, thereare published known-good voltage val-ues for each pin of the ECU connec-tion. In some cases, these values maychange substantially as operating condi-tions vary, so be sure to read the notes,paying special attention to conditionssuch as KOEO (key on, engine off), “atidle” or “above 2000 rpm on decel.” Tip:A scope or graphing multimeter can beyour best friend for looking at and ana-lyzing fast-changing voltage signals.

Harness tests. Internal power-limit-ing circuits may kick in to prevent ther-mal overload if the PCM is trying to op-erate a shorted injector, solenoid or oth-er component. Resistance or currenttesting of each actuator circuit may berequired. If chasing an intermittentproblem, an inductive ammeter currentprobe (amp clamp) may be required, asmany such faults arise only during pro-longed operation. Hook up your amp


Left: This Mercedes TCM circuit board spent too long underwater. Efforts at resuscitation ultimately failed, and an organ transplant wasrequired. Right: The corroded pins seen here, and the rest of the internal circuitry from the vehicle’s engine controller, responded to care-ful cleaning, but replacement might just as easily have been required. In this instance, the customer got lucky.

The dash on this Subaru Forester displays “Er55” or maybe “Er SS.” Either way, it signals aproblem. The PCM was not able to communi-cate with any of a number of scan tools.

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Circle #21

clamp to your DMM, engage theMin/Max function (preferably with anaudible alert) and warm things up. Trywiggle-testing the harness to see if youcan induce the failure in your shop orparking lot, where it’s much easier todeal with than by the side of a busyroad.

Actuator tests. This is really a fol-low-up on the harness tests you justconducted. Unless you know exactlyhow and why the original computer wasdamaged, you should never plug in anew one without verifying that all actua-tors draw no more than their maximumspecified current. Failure to run thesetests may result in ruining a new or re-placement unit in “short” order.

What’s Next?Even if everything you’ve checked sofar has been in spec, somewhere alongthe way you should probably price out areplacement PCM. If it’s an applicationwhere you can get a quality remanufac-tured unit for, say, $150, and there’s not

too much additional labor involved, itmight be worth it to plug one in for di-agnostic purposes. (If it solves the prob-lem, so be it. If it doesn’t, you’ve proba-bly effectively eliminated the computeras a cause of the symptoms.) This iseven more true if you’re working on a


24 July 2015

vehicle where PCM failures are knownto be common.The same consideration applies even

more forcefully if you service a lot ofother vehicles just like it. On the otherhand, if it’s a unit that cannot be trans-ferred to another vehicle after initializa-tion, or one that costs $500 or more,you’re gambling pretty heavily withyour customer’s money as well as withyour own reputation and bottom line.Where replacement costs are high, youmay also want to check whether any ofthe well-regarded rebuilders will bench-test your unit and let you know if theyfind a problem. This usually means ty-ing up the vehicle for a few more days,but may be acceptable to both you andyour customer. Unfortunately, there aresome units that are obsolete, too highlyspecialized or for which there are no ac-ceptable remanufacturers, so this maynot always be a viable option.Once in a while you may encounter a

faulty replacement unit, or a situation inwhich an underlying but still undiag-

Here’s a close-up of the black mark on oneof the CAN bus pins from the Subaru’s en-gine controller. The cause: A lightningstrike on a nearby utility pole.


nosed problem has damaged the originalunit and your replacement module aswell. My good friend Mark Lauretig,who runs a nearby shop, reported an in-stance where only the third replacement(used, not remanufactured) PCM solveda problem in a 2001 Blazer. Cover yourbases and stick to your guns, he advises.

Is there a better way to diagnose asuspected PCM failure? Might thedealer have something that could help?Not generally, unless you consider theirparking lots to be a tool. The most com-mon dealership diagnostic procedurewhen a PCM failure is suspected is sim-ple substitution. If you have a known-good unit from the vehicle you just ser-viced an hour ago but that won’t bepicked up until the end of the day, or,better yet, from a used car on the lot,substitution can be quicker than anyother course of action, or at least so itwould seem.

There are, as usual, quirks and excep-tions. Some units will need to be over-written, reinitialized, reprogrammed orotherwise made usable as part of theswap. This is particularly needed whenan antitheft or immobilizer system is in-volved. And, for some makes and mod-

els, such a swap is simply impossible, aswhen a VIN mismatch on a single-writemodule triggers a no-start.

Case NotesJeep goes from bad to worse. After wehad correctly diagnosed a faulty PCMthat resulted in a chronic misfire due to aweak injector driver, we replaced it with a

remanufactured unit from a well-respect-ed supplier, only to discover that we hadsomehow created a crank/no-start/no-spark condition. The replacement mod-ule would easily cause a test light at-tached from B+ to coil negative to flashwhile cranking, but there was no sparkfrom the coil. A scope check revealedthat the remanufactured unit didn’t pull


28 July 2015

When testing circuits, choosethe correct pins to matchthe terminals of any discon-

nected harness plugs. Oversize pinsused for testing can and will spreadconnector terminals, possibly result-ing in lasting problems. Backprobing(as shown here) is usually the safestprocedure, but may not always bepractical. For those times whenfront-probing is the best option, Ikeep an assortment of very small-di-ameter dressmaker’s pins to supple-ment the many styles and sizes ofterminals we already keep in stockfor harness repairs. We also routinelysave odd terminals from old sectionsof harness. These can be invaluablefor testing other vehicles later.

Many salvage yards provide theentire mating connector with used-module purchases; these can turnout to be as valuable as the modulesthemselves in some cases. Also, re-

member that spread connector ter-minals are a very real possibility ifyou’re not the first person to havelooked into a problem vehicle. If youencounter an intermittent runningproblem, especially one that be-comes more pronounced with heator vibration, this is an area you’llwant to rule out fairly early in the di-agnostic process.

Most manufacturers issue a termi-nal test pin kit to their dealers andspecify how much “drag” thereshould be between male and femalepins. When in doubt, experiment us-ing a known-good terminal with thecorrect mating pin. Tip: You can of-ten correct for the effects of a previ-ously spread female terminal by care-fully adding a slight amount of bendor twist to the corresponding malepin. This is often more efficient thantrying to tweak the female terminalback to its original form.

Cautionary Note

Use appropriately thin pins when back-probing circuits. Here we’re checking theharness for a voltage drop in a Chrysleroxygen sensor heater circuit.

The black bars added to this screen capture point up the difference in response betweenthe two upstream O2 sensors. The PCM bore ultimate responsibility; it simply couldn’t sup-ply enough power for the bank 2 upstream heater to function properly.


the coil all the way to ground. The offset of approximately 1.3Vwas just high enough to prevent proper spark formation. A sec-ond replacement unit solved the problem immediately.

Corolla stalling. Despite intermittent stalling and loss ofscan tool communications, PCM replacement was not re-quired in this instance. Vref was intermittently shorting toground internally within the tank pressure sensor. The PCMuses Vref to communicate. A voltage check at a more accessi-ble sensor revealed the Vref fault when the scanner lost com-munication. After that it was “just” a question of unpluggingsensors one by one until the culprit was found.

2001 Mitsubishi Galant crank/no start. This car came toa friend’s shop with a cam sensor code. Finding no cam sensorpattern with his scope on cranking, my friend ordered a newcam sensor from a local parts jobber. He installed the new partbut, although the car would start and run for a few seconds, itwouldn’t stay running. And the same code returned.He scoped the new part and found a clear, recognizable

pattern. A boneyard computer also reacted the same way.Reasoning that this was likely an antitheft issue, he had thecar towed to the local dealership for them to reprogram theimmobilizer. It wouldn’t be much of a story if that had fixedeverything. It didn’t.Swamped at his own shop and figuring that the dealer

techs had had much more experience on this particular mod-el, my friend asked them to diagnose the problem. Knowing

that a faulty crankshaft sensor can result in the same symp-toms, and finding that the CKP was a non-OEM part, thedealer techs replaced it. No joy. Next, they sold him a“known-good” used computer and the requisite program-ming. Still no joy. Finally, working side-by-side with a goodused car from their sales lot, they eventually swapped in acam sensor. This solved the problem immediately. My friendspent a bit over $900 to fix a problem he could have fixed thefirst time if he had simply ordered up an OE CMP. Soundslike the automotive version of Russian roulette!

Mercedes S500 no-start. Towed in from a body shopwhere “the battery went dead after they replaced the head-lights,” this case turned out to be anything but simple. It didn’trespond to a jump, stubbornly refusing even to crank. Theshifter was stuck in Park. The battery needed replacement.The replacement headlights were not wired into the CAN sys-tem as the originals had been, although this eventually turnedout to be irrelevant. There were recurrent communicationscodes in multiple modules, and the transmission control mod-ule (TCM)was unable to communicate with the scan tool at all.When I removed the TCM for inspection, I found it had al-

most two inches of water inside! I cleaned and dried it, whichallowed me to move the shifter out of Park for the first time.The engine control module also showed corrosion damage onseveral pins, but I was able to clean it up by disassembling it,spraying it down with a baking soda and water solution, thengently cleaning it with a soft nylon brush. Replacing the TCMwith a used unit allowed the car to start and run.

2002 Chrysler 300M Special. A close friend and retiredtechnician bought this car as a project. Among its many prob-lems was an intermittent condition that caused the transmis-sion to go into a fail-safe or limp-home mode, in which it op-erated only in 2nd gear. Both the TCM and the ECM share ahousing, but apparently they sleep in separate bedrooms;there was an occasional “controller conflict” and a U0402code indicating implausible data from the TCM. The transperformed perfectly most of the time, with no hint of prob-lems. When it did act up, shutting the car off and restarting italways made it work normally.A remanufactured PCM resolved this problem, but there

were other issues as well. Fresh intake manifold O-ringscured a chronic rough idle, and replacing all four oxygen sen-sors brought warm engine performance back into an accept-able range. But there was a persistent MIL on with a recur-rent P0030 indicating a fault in the bank 1 upstream O2 sen-sor heater circuit.Scan data (see the screen capture on page 28) showed

closed-loop performance in less than 30 seconds after a coldstart, with about 1.6A of current flow at full system voltage.By contrast, the bank 2 sensor heater, not throwing anycodes, showed an unloaded voltage of only 4.17V and wasvery late to achieve closed-loop. Running a temporaryjumper from the ASD relay output to the B2S1 heaterbrought it into closed-loop as quickly as the other bank, butset a new code. Careful testing and inspection eliminated thepossibility of a harness or connector fault. The solution: yetanother remanufactured PCM.


Circle #11

30 July 2015


Circle #19

2009 Subaru Forester with 34,000 miles. This SUV hadbeen parked outside during a colossal thunderstorm. When itwouldn’t start the next morning, Dad tried jump-starting it.When that failed, it was towed into a local body shop. After acouple of days there, it was towed to my shop. Like the Mer-cedes S500 discussed on page 30, it arrived with the batterycompletely discharged. The shift-lock override had been de-ployed and the shifter was now stuck in Neutral. Even depress-ing the override again would not allow Park to be selected.

The vehicle would not crank even with a good jump box inplace, although jumping power directly to the starter solenoidresulted in strong cranking. Once a fresh battery was in-stalled, the instrument cluster displayed a cryptic “Er SS” or,possibly, “Er 55”; the LCD display was ambiguous. The PCMwould not communicate with any of the five scan tools I tried.

This vehicle runs on a CAN-bus system, and the total resis-tance of the bus was substantially lower than spec. Unplug-ging the comm connectors from the TCM, the IPC, the PCMand other modules revealed an unexpected change when thePCM was connected or disconnected. Scan communicationwith some of the other modules became possible with thePCM disconnected. I replaced the PCM with a used unit(which had to be reprogrammed to the immobilizer) and theForester then started and ran, albeit with the transmission inlimp-home mode.

Inspection revealed a tiny blackening of one of the twoCAN bus terminals in the old PCM. Although there was noovert evidence of a direct jolt to the SUV, I believe that whena reported bolt of lightning hit the ground cable on the utilitypole next to where the vehicle had sat during the storm, it in-duced a high-voltage spike into the vehicle’s wiring.

ConclusionsModern PCMs are generally very reliable, although somemay lack appropriate internal (current-limiting) circuitry toprevent power transistor failure in the event of excessive actu-ator draw. Because most PCM failure diagnoses for faults oth-er than nonactuation of a coil or injector driver circuit are theresult of ruling out all other possible causes, there are manypossibilities for misdiagnosis.

Solid troubleshooting skills and careful attention to wiringschematics can help tilt the diagnostic odds in your favor, butin some instances, only installation of a known-good replace-ment unit can confirm (or disprove) the diagnosis. Maintain-ing clear communications with your customer throughout thediagnostic process will help avoid misunderstandings.

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


KS: X? - MOTOR · 2015. 7. 7. · the CKP, though some systems may al-low the CMP to substitute in...

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