Long a staple of cheesysci-fi movies and horrorfilms, the very notion ofbrain transplants con-jures up images of badlyunbalanced “scientists”

obsessively working late into the nightin their secret underground laborato-ries. After all the necessary connec-tions have been made, a switch isthrown. While lightning crackles out-side, voltages surge and the “patient” isrevived! That’s almost how it looked atmy shop one recent evening.

A little background is in order. The1967 Volkswagen Type 3 was the firstmass-market car in the U.S. to utilizean electronic control unit (ECU) toregulate fuel delivery. The last vehicles

with nonelectronically controlled car-buretors were produced in the mid-1980s, and by 1990, all new vehiclessold in this country featured electroni-cally controlled fuel and ignition sys-tems. The so-called brain box was hereto stay. Despite some early problems,the vast majority of automotive ECUsin use in the 21st century are quite ro-bust and reliable.

This article deals with fairly compli-cated diagnostic issues. It is not, infact, for the faint of heart, but for toptechnicians who don’t flinch awayfrom complicated work. Where I re-fer to the schematic diagrams repre-sented in Figs. 1-5, please take thetime to carefully examine the circuitsin question. It will help you do auto-

30 May 2008

BRAIN TRANSPLANTS BY THE

NUMB3RSBY SAM BELL

Due to the expense involved, the decision to

replace a vehicle’s brain should not be taken

lightly. Following the necessary pre-op

investigative procedures will verify the necessity,

then assure the success of the operation.

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31May 2008

motive brain surgery on vehicles thatmay roll into your bay next month...orperhaps tomorrow.

Neuroanatomy:Sensors &Motor NervesBrains and brain-boxes alike needsome way to perceive the worldaround them. We humans rely on vi-

sion, hearing, touch, smell and taste.Computers have to make do with allkinds of sensors—temperature andpressure sensors, position and speedsensors, specialized oxygen or air/fuelsensors, knock sensors and a host ofassorted miscellaneous sensors reflect-ing simple binary states (think brakeon/off or a/c request) or sophisticated

multimodal states (think yaw, pitch,roll and acceleration).

Fig. 3 on page 37 shows a portion ofthe powertrain control module (PCM)wiring schematic for a 1999 ToyotaCorolla VE. There are five sections inall. Let’s look at a few of the input cir-cuits in detail. At terminal VC/wire 1B(arrow A) we see a 5-volt reference sig-nal sent out to be shared by the MAP,TPS and VPS (tank vapor pressure sen-sor). Each of these sensors also shares asensor ground, E2/9B (Fig. 4), via thesplice at IB, and each provides its ownsignal return (at PIM/2B, VTA/11B andPTNK/7B, respectively.) It’s clear thatnone of these signals can ever exceedthe reference voltage even if un-plugged, and that their minimum possi-ble values must always equal or exceedground voltage. The total voltage ineach circuit (that is, the sum of the volt-age between the reference voltage andthe signal, added to the voltage be-tween the signal and the ground) isequal to the original reference voltage.Thus these three sensors are examplesof voltage-splitting circuits.

Take a look at the ECT and IAT sen-sor circuits at terminals THW/4B andTHA/3B (arrows B and C). Each hasits own 5-volt reference feed. Thistime, each circuit features a negative

32 May 2008

BRAIN TRANSPLANTS

Fig. 1 Fig. 2

Inoperative dash lights? Maybe it’s just the rheostat. Think again. It’s not thatsimple anymore. This “driver’s multiplex control unit” is supposed to send areference voltage to the rheostat, then use the return signal to drive an inter-nal amplifier to control the dash illumination. The new unit (blue) is on the left,the brain of the old one is on the right, in front of the fuse panel it mounts to.

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temperature coefficient (NTC) ther-mistor and a sensor ground, again atE2, via the IB splice. The thermistorsfunction as voltage-dropping resistorsto report operational temperaturesneeded to adjust the base spark timingand fueling programs. At very coldtemperatures, or when the circuits areopen, the reference voltage will bedropped only a little or notat all. Very hot conditions ora shorted sensor circuit willresult in substantial or totalvoltage drop. Such circuitsare called voltage-droppingcircuits.

Arrow D at NE/4A (Fig. 5)points out a third type of cir-cuit used for the crankshaftposition sensor (CKP). Thisinput takes the form of a sup-plied AC voltage signal whosefrequency varies with enginespeed. A “missing tooth” re-luctor provides additional in-formation on the basis ofwhich the crankshaft’s exactposition can be calculated.Toyota labels this sensor NE,which stands for “Number,Engine”—or, as you and Iwould think of it, rpm. TheG2 or CMP signal is similar,

but features only one hump. These areexamples of voltage trigger circuits.

Next, look at the circuit indicated byarrow E at STA/11C (Fig. 3). This is anexample of a pull-up circuit. An appliedvoltage detected here indicates that thestarter is engaged. Finally, look at OX1,the upstream oxygen sensor terminal,shown here with arrow F at 8B (Fig. 4).

This is a pull-up/pull-down circuit. Avalue above the PCM’s internally gen-erated reference voltage (.45 volt forthis circuit) is interpreted as a pull-uprich condition, while a voltage lowerthan .45 is a pull-down indicator show-ing a lean condition.

Our human brains rely on motornerves to carry out our intentions as we

interact with our surround-ings. Similarly, automotivebrain boxes need some wayto implement the decisionsthey reach based on theirprogramming and the inputsthey have received.

Fig. 4 also shows a few ofthe actuators under thePCM’s control. Arrow 1 at#10, #20, #30 and #40 (12A,25A, 11A and 24A, respec-tively) indicates the injectors.A separate driver within thePCM completes each injec-tor’s ground circuit, causingthe injector to spray a pre-cisely calculated amount offuel just outside the intakevalves. Similar drivers areused to actuate a variety ofelectromagnetic relays andsolenoids.

The twin DIS coil packs

37May 2008

BRAIN TRANSPLANTS

Fig. 3 Fig. 4

Fig. 5

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are under the control of integrated ignit-er modules which, in turn, are con-trolled by their own PCM driver circuitsin Fig. 5 at IGT1/20A and IGT2/19A(arrows 2 and 3).

The idle air control valve in Fig. 2features a rotary vane. The vane posi-tion is controlled by a duty-cycled mag-netic coil, responding to the commandsof a driver at RSO/10A (arrow 4).

The Diagnostic ConundrumFollow me as I relate the Corolla’s sto-ry. The car was towed in with the com-plaint “died while driving.” Naturally,the car started and ran as soon as thetow truck dropped it off. There wereno codes or pending codes in memory,and the data stream values all seemednormal both KOEO and KOER.

Great, I thought. It’s cold and snow-ing, and I have a car with an intermit-tent dying problem. Good thing my cellphone’s charged up! I drove around for20 minutes and returned to the shop. I

let the car idle in the back bay (with anexhaust hose on it, of course!) for 21⁄2hours, when it thoughtfully stalled tenminutes before closing time.

There were no codes, of course, andthe stall was not preceded by anyroughness or unusual noises. Rather, itseemed almost as if someone had sim-ply switched the ignition off. This time,though, the Corolla didn’t restart. Wequickly determined that there was nospark and not even a signal from thePCM to the coil and igniter assem-blies. It was a diagnostic conundrum.

Were we missing some critical in-put, like CKP or CMP? Was it an igni-tion switch problem? A faulty ground?By now it was past closing, so I decid-ed to pack it in and try to pin it downthe next day.

The next day, the car idled flawlesslyfor over four hours before suddenly dy-ing. Again, there was no spark. As we sawpreviously in Fig. 5, the PCM controlsthe coil packs through their respective ig-

niters via two separate wires. The oddsof both signals disappearing by coinci-dence were incredibly small. Scoping theCKP (the waveform in Fig. 6 on page72) and CMP sensors showed apparentlynormal waveforms even during crankingwith no spark. I hadn’t yet accessed thePCM to verify that the signals were mak-ing it all the way there, so I couldn’t ruleout a wiring problem. Or could I? I pow-ered up my scanner again, thinking Icould check the rpm PID during crank-ing, but I was unable to establish com-munication with the PCM. I tried againwith two other scan tools, but there wasstill no joy. I nearly overlooked the ab-sence of the MIL, but finally noted thatit was off, even with the key on, and hadbeen so ever since the stall.

I examined the schematics carefully.Could something be shorting out thedata line to the DLC? Could the PCMbe losing power? As I swapped theEFI and horn relays in the underhoodjunction box, I noticed that one of the

BRAIN TRANSPLANTS

box’s bolts was missing. Was this aclue? Would it turn out to be related tothe large dent in the left front fender?I knew a main power supply harnessran through this area. Maybe it hadbeen damaged in whatever misadven-ture had befallen the fender. Alas,careful inspection showed no evidenceof wiring damage, and extensive wig-gling and manipulation of the harnessfailed to restore the car to life.

Throughout, I could not reestablishcommunication with the PCM. I didscope the data line, however, and dis-covered a constant 12-volt reading (ex-cept when my various scan tools wouldtoggle it to ground during their at-tempts to establish communication).That meant the data bus was not short-ed to ground, though a current-limitedshort to power remained a theoreticalpossibility.

It seemed like a good time to checkfor TSBs; I came across none thatseemed relevant. I logged on to iATN

to check the archived fixes for possiblesimilar problems and found one for abad EFI relay and one for a bad igni-tion switch. Swapping the relays hadyielded no relief, and a voltage checkat the traditional Toyota underhood di-agnostic connector (DLC-1) indicatedthat both the ignition switch and therelay were working at least to thatpoint in the wiring.

Now I had at least two reasons to ac-cess the PCM: I needed to check thatthe crank and cam sensor signals werein fact arriving, and I needed to verifythat power and ground were present.

I scoped the various power andground inputs during cranking andfound no significant voltage drops. Thecam and crank sensor signals lookedfine when checked directly at thePCM. I reviewed the current statusand had my son Josh, the service man-ager, call the customer to authorize an-other block of diagnostic time.

In essence, diagnosing a faulty con-

trol module often relies on a process ofelimination. If all the requisite inputsare verified and the necessary outputsare still missing or insufficient, thePCM must be at fault. But proving anegative is a tricky proposition at best,and constitutes the first conundrum,especially when “substitute known-good part” means “buy known-goodpart first!”

It was time to do “pin-outs,” me-thodically checking each PCM termi-nal for the correct power, ground orsignal as the key is switched to ON orSTART. Many manufacturers providepin-out charts for their PCMs, or youcan work your way through theschematics, provided you have suffi-cient expertise or experience to deter-mine which values or signals are validand which are not.

Many technicians dread doing pin-outs, probably because it looks like alot of tedious and time-consumingwork. However, I find that the actual

Circle #20

procedure usually takes far less time toperform than to read about. A helpfultip is to write down your results on theprinted procedure, or right on theschematic, as you go. In the event ofcomplications, this will make futureanalysis much easier than if you simplycheck off each item as “in spec.”

As I progressed from pin to pin, thePCM suddenly revived. I wiggled allthe harnesses and connectors I mighthave disturbed in my testing, but thistime always maintained scan commu-nication. I tried gently tapping on thePCM housing, drumming my fingerson the box, even gently flexing it,(though I had no idea how it could beflexed in the installed position, as themounting seemed quite stress-freeand secure). This is the kind of scena-rio I detest. I knew I hadn’t yet iden-tified the problem, and it seemed un-likely that I would until it reappear-ed, as the car now ran quite well. The

conundrum continued to baffle me.I thought if I took another look at

the data stream, maybe I could find aclue I had previously overlooked. Allthe running values seemed normal. Ichecked the KOEO PIDs next (Fig. 8on page 73). Again, everything seemedfine. The MIL was now on, but wentoff once the engine was started. Then,after only a few minutes this time, thecar stalled again.

When I went to restart it, the MILremained on. At last, a break in thecase! The freeze frame for DTC P0100(IAT circuit malfunction) showed boththe ECT and the IAT at �40°F. Like-wise, DTC P0115 (ECT circuit mal-function) had set as well. All of this to-gether indicated a high voltage in eachof the temperature sensor circuits, soeither the sensor ground at E2 musthave failed or the PCM had failed tosee it. Yet I had checked it and found itgood while the car was still in its no-

start mode. That meant I must be deal-ing with an internal fault.

I thought that installing a freshPCM was the next step, but was notsure that was the end of the wholething, because I still couldn’t see whythe PCM wouldn’t run the igniters justbecause it was missing that particularsensor ground. Furthermore, E2, thesensor ground in question, serves asthe ground for the MAP, TPS and FTP(fuel tank pressure) sensors, yet MAPappeared at a plausible level for astalling engine (20 in. of mercury) infreeze frame.

I knew that going forward withoutreplacing the PCM would be fruitless,yet I also knew that I still hadn’t arrivedat the root cause of the fault. I didn’twant to take a chance on damaging anew PCM. What if there really was awiring problem or an intermittentlyshorted component? I personally knewmore than one tech who had replaced a

40 May 2008

BRAIN TRANSPLANTS

Circle #21

bad PCM, only to fry the new one because he hadn’t cor-rected the root cause of the failure. My dilemma grewsharper when we learned that a new PCM carried a pricetag of about $1200. Toyota does not offer a remanufacturedunit for this vehicle, but we were able to source one from areputable supplier with whom we had success in the past.

Circuit Verification & TestingJosh sold the customer on a replacement PCM as a neces-sary step, but first I wanted to finish the pin-out sequenceI’d started earlier. Pin by pin I checked through all thewiring connected to the PCM. I tugged and wiggled thewhole harness and each of its individual branches. I flexedconnectors. I monitored current draw to each solenoid, re-lay, igniter and injector under the PCM’s command, usingtwo lab scopes, two low-amp probes and four DMMs, allwith MIN/MAX capture features activated. I used a varietyof jumper wires, power probes and bidirectional scannersto actuate components. I was still operating with the origi-nal PCM, hoping to catch whatever gremlins and glitchesmight be lurking.

Yet every single actuator was on its best behavior. Ilashed the PCM down with a couple of tie wraps and drovethe Corolla on every errand I needed to run. I let the caridle for hours at a time. It ran flawlessly.

By now a couple of days had passed and the replace-ment PCM had arrived. I disconnected the battery, a criti-cal step required before any brain transplant, to avoid volt-age spikes. I gave the original unit a last sniff test (see“Something’s Rotten . . . ” on page 74) and still didn’t de-tect the characteristic burnt electronics odor that wouldhave stopped me from plugging in the new one.

With the new PCM connected though not yet bolted in,I cycled the key to the ON position. The rest of the idiotlights came on, but the MIL remained dark. I switched theignition off. Throwing all caution to the wind, I engaged thestarter. I was back to a crank/no-start condition. Wonderful.

I had become pretty familiar with each wire by thistime, so testing promised to be somewhat faster, at least.The data line once again showed a steady 12-volt trace onone of the scopes. The MIL remained stubbornly unlit.Power and ground connections, including E2, were allgood. The crank and cam signals were fine. I swappedback in the original PCM and started over on my pin-outs.And suddenly, the car started as I was verifying that thestarter signal was being received.

I was slated to leave town for a week on my annual skitrip the next day. I was sure the car wasn’t fixed. Somehow,I was not surprised to find the Corolla at the shop when Ireturned. It had, I learned, left for a day or two, but thenhad died while being driven and was towed back to ourshop. Joe, my lead tech, found time to expose the PCMagain and do some testing, and I let him bring me up tospeed. The replacement PCM was unable or unwilling tocommunicate under any circumstances, and would not runthe car. When it was plugged in, the MIL remained off at

42 May 2008

BRAIN TRANSPLANTS

Circle #23continued on page 72

all times, and DLC pin 7 showed asteady 12-volt flat-line.

Joe had checked with some knowl-edgeable techs at our local Toyota deal-ership, but no one had ever seen or evenheard of a similar problem. I recalledhearing of a couple of instances where ashorted VSS had disrupted communica-tions, but unplugging its connector onthis car quickly ruled that out.

By this time, we were again runningthe car on the original PCM, but asecond replacement unit had alreadybeen ordered. We remained confidentthat our original analysis had been cor-rect, but were convinced there wasstill more to the story.

Joe had even set the car on stands sowe could run it in gear without tying upone of the lifts. It wasn’t until the nextday, as we were running it on the stands,that it again stalled. Communicationsagain failed, and this time we were luckyenough to find that VC (more common-ly known as VRef, or reference voltage)had dropped to 1.5 volts. We unpluggedthe MAP with no change, then the TPS.Still no change. When Joe unpluggedthe tank pressure sensor under the car,voltage suddenly returned to normal,the MIL reappeared and the car startedright up. He reconnected the sensor, butthe car continued to run.

We now had a strong inkling thatthe tank pressure sensor was probablythe site of an intermittent short, or par-tial short, to ground. But we still lackedproof. Wiggling the harness to the evapcanister was unrevealing. Looking atthe schematic, we decided to accessthe BC1 connector by the rear wheelarch (see photo on page 74).

After only a few more minutes ofrunning, the car died again, and aDMM on the VC terminal showed 1.5volts. I quickly unplugged the BC1 con-nector, the reading on the DMM shotback to its previous 5-volt level and thecar restarted. I plugged the connectorback in immediately and watched VCdrop again as the car died. Because theplug was now readily accessible, wecould make or break the circuit quicklyand repeatedly with consistent resultswithout the pressure sensor coolingenough to mask the problem.

We examined the harness between

the BC1 connector and the canister andfound it intact. It was clearly time to re-new the pressure sensor. Inquiry re-vealed that the entire canister assemblywas available for only about $20 morethan the cost of the sensor alone. Weknew from past experience that Corol-las of this vintage often experience a V4evap bypass VSV (Vacuum SwitchingValve, or solenoid) failure, so we couldeliminate or postpone that probable is-sue with a whole new assembly.

Josh put together an estimate, calledthe customer and sold the job after I as-sured him that this would take care of

everything. We installed the new canis-ter assembly and the second replace-ment brain. Joe and I were confidentthat we had fixed the car, and he took ithome that night, a 65-mile one-way trip.

Joe was on time the next morning.“Hey, that car shut off only twice onme!” he said. “Once when I got homeand turned it off, and once this morn-ing when I parked it out back.”

Postmortem &Pathology ReportIf not for the requirement of returningthe PCM (and the first replacement)still sealed as cores, I would give a fullpathology report. In this case, however,we’ll have to settle for a “best guess

72 May 2008

BRAIN TRANSPLANTS

Circle #41

Fig. 6 These low-voltage pulses fromthe PCM are used to trigger the ignitersbuilt into each coil pack. (The 4.1-voltamplitude seen here is a known-goodvalue. A few vehicles work at even low-er levels, though some may slightly ex-ceed the nominal 5-volt specification.)

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scenario” to explain what went awry in-side the original brain. At first blush itappears most likely that the IAT andECT sensor circuit malfunctions weresymptomatic of either an open in theE2 ground or an unusual voltage“drop”—actually a voltage rise aboveground—affecting their branches ofthe E2 sensor ground circuit. Such aground rise would also help to explain

the somewhat higher than expectedMAP reading in freeze frame.

But would an E2 voltage drop explainthe loss of MIL control, data communi-cations and the loss of the IGT signals aswell? This seems unlikely in that E2 isnot only a ground to the PCM but is alsoa ground from the PCM to the sensorcircuits it serves. The other grounds tothe PCM are indicated by arrow 7 inFig. 4, but they all share the IB groundsplice with E2 as well. All of thesegrounds originate at EC, the multipleground terminal located at the left sideof the cylinder head (see photo on page75). I had scoped each of those groundsduring the crank/no-start phase as theyreached the PCM at their respective ter-minals (E1, E01, E02 and E03) and hadfound them all equal at 96 millivolts(mV). I had rechecked them duringcranking when the engine did start, andhad found the same values, within 3mV.

The various branch grounds ema-nating from EC are designed to pro-vide sufficient ground current to pow-er all of the actuators under the PCM’scommand. It’s possible the E2 groundis internally tied to the ground used bythe IGT drivers and by the SIL (datasignal) driver, yet the short through thetank sensor that is putatively responsi-ble for pulling E2 high is only a shortto reference voltage. Reference voltageis a high-impedance circuit with onlyabout 600 milliamps (mA) of availablecurrent, so even a direct short herewould reduce the available current on-ly to IGT1, IGT2 and SIL, resulting in,at most, a 3.5-volt drop in signal ampli-tude, and not the observed completeabsence of the ground signals to the ig-niters and at DLC pin 7.

Additionally, even when the tankpressure sensor was acting up, the ref-erence voltage at VC did not drop fullyto ground, instead remaining at a 1.5-volt level. If we then calculate the volt-age drop at E2, it would be no morethan 3.5 volts. This will not account forthe �40° readings of the IAT and ECT.The most likely explanation for theP0100 and P0115 codes, then, is an in-ternal interruption of the ground leg ofthe internal circuitry used to pull the12-volt PCM power down to the 5-voltlevel used as reference voltage for both

73May 2008

Circle #42

Fig. 8 This is a normal scan datawaveform during communication. Thiscapture was made by backprobing pin7 of the DLC with a scan tool connect-ed. The PCM pulls the signal to ground,varying both pulse width and frequen-cy to transmit the requested data.There’s a constant flat-line signal atthe system voltage level with noground pulses during normal opera-tion when no scan tool is connected.

Fig. 7 Toyota calls this signal “NE,” forNumber, Engine. You and I recognize itas CKP (crankshaft position sensor).The “missing tooth” signature providesa recognizable reference point fromwhich the PCM can precisely calculatesubsequent speed and position.

temperature-sensing circuits. Eachwould then have read full system volt-age, which would still be calculated as�40° in data stream.

Alternatively, the analog-to-digitalconverters serving both IAT and ECTmay have lost a shared internal circuitpath, causing both to be read as full high.Yet this explanation appears logically lesslikely in that it requires additional as-sumptions. (Another possible explana-tion, an intermittently bad connection atIB, has to be ruled out on the basis thatthe car was ultimately fixed without anyfurther attention to the IB splice.)

How would this account for the lossof signal from the SIL and IGT drivers?Occam’s razor, a principle of logicalanalysis, states that the explanation of agiven problem is most likely the onethat requires the fewest assumptions.Our previous analysis indicates that anintermittently failing internal groundleg likely caused the temperature sen-sor circuit malfunctions. Applying Oc-cam’s razor, it seems most probablethat the same internal open in a groundleg deprived the IGT and SIL driversof their needed current.

Razor Burn(out)If the argument would hold up, Oc-cam’s razor would then shave us evencloser by connecting the short-circuit-ed pressure sensor to the stalling, and

then the stalling to the ground fault.Perhaps the ground legs for regulatingthe voltages at VC, THA and THW areall branches of an internal ground sup-ply circuit that also supplies theswitched ground for the SIL and IGTdrivers, and all of these ground legs to-gether constitute the output from a

74 May 2008

Connector BC1, located just abovethe inflated wedge, allows quick dis-connection and reconnection of thefuel tank pressure sensor.

BRAIN TRANSPLANTS

Circle #43

Sometimes, like Hamlet, you have torely on your olfactory prowess to

sniff out a problem. Don’t be afraid tosniff any suspect electronic unit. If youdetect the characteristic aftermath ofburning electronics, look carefully forany circuit or component that mighthave caused the overload. These willoften be stepper-type idle speed mo-tors, or shorted relays or solenoids.Less frequently, the problem will bedue to wiring damage caused by im-proper installation, abrasion, rodenthandiwork or even faulty insulation.

If you replace the control unit with-out correcting the cause of the origi-nal failure, you’re setting yourself upfor a comeback. Adopt this rule in-

Where there’s smoke, there’s . . . smell!Just as you wouldn’t want the PCM toignore any of its critical sensors, youshouldn’t ignore the evidence yournose collects while examining a ques-tionable controller.

‘Something’s Rotten . . . ’

single, internal power-limited transis-tor driver. If that’s the case, then per-haps the short in the tank sensor wasjust enough to cause the power-limit-ing feature to kick in. In which case,perhaps we didn’t need the new PCMat all. Still, with barely more than halfan amp of current available from VC,

the hypothetical current limiting forour hypothetical driver must kick in atan awfully low level.

Subsequent investigation and exper-imentation has confirmed that shortingthe VC terminal fully to ground is suf-ficient to cause an immediate stall, in-cluding the loss of SIL and the twoIGT signals. It did not, however, resultin anomalous IAT or ECT readings,setting only a code for the MAP sensorI disconnected in the process.

I still haven’t found a logical explana-tion that ties the �40° readings on thetemp sensors to the rest of the faults,however, so I have to believe there reallywas an internal PCM fault that was prob-ably independent of the external sensorcircuit short through the tank pressuresensor. If you have a spare PCM, I’ll behappy to tackle the internal circuitry.

Diagnostic RecapWhile the actual mechanics of automo-tive brain surgery are not particularlydifficult, it’s important to rememberthat all scan tools should be discon-nected from the PCM in the key-offposition, and that the battery should bedisconnected prior to the operation,and all capacitors fully discharged.Electrostatic discharge can damagesensitive components. Use of agrounding strap is advised.

Scrupulous checking of all actuatorsand their circuitry is a must. There aremany modern vehicles whose proces-sors still do not incorporate current-lim-iting protection for all internal drivers.Shorted components or wiring candamage or destroy a control module.

As complex as the diagnostics in theCorolla proved to be, the basic patternis still relatively simple. We need to re-search the circuitry, then verify that allthe correct inputs are being received,and that adequate power and groundare available. Next we have to verifythat all actuators and their circuits areserviceable. Finally, we have to deter-mine whether the failure is internal orexternal to the unit in question. Then,it’s out with the bone saw!

75May 2008

This photo shows the “EC” gangground at the end of the cylinderhead. Its branches supply ground tothe PCM inside the vehicle.

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

Circle #44

stead: Whenever your analysis leadsyou even to consider the possibilitythat one faulty component mayhave damaged another, prepareyour customer for just such an even-tuality. If it turns out that, say, ashorted idle speed motor has over-loaded the PCM, your customer willalready have a conceptual frame-work allowing him to process, un-derstand and accept such a diagno-sis. Ultimately, having come to termswith the situation, your customer willthank you for your thoroughness.Meanwhile, if it turns out that therewas no external root fault, your cus-tomer will still be happy at havingdodged that bullet.