transmissions:pg edit 5/22/12 8:02 am page 1 … in isuzu npr trucks. this transmission is very...

Diagnostic trouble codes(DTCs), when present,are quite helpful in diag-nosing any type of vehiclemalfunction, includingautomatic transmission

troubles. There are times, however,when these codes can be misleading,ambiguous or just plain confusing. Thiscan be the case with either manufactur-er-specific codes or—more common-ly—generic codes.

Take lessons we learned many yearsago with generic codes P0740, P0741,P0742 and P0743 related to the torqueconverter clutch (TCC). These codes

are related to electrical or mechanicalproblems that may occur with the con-verter clutch system. The misleading as-pect is that these codes can signal eitheran electrical fault or a mechanical faultor both.

A P0740 code, for example, is notbound to one specific meaning, and thediagnostic approaches toward mechani-cal and electrical faults are completelydifferent. A 1995 vehicle with a 4T60-Etransmission would produce a P0740code for a TCC mechanical fault, whilewith a TAAT in a 1996 Saturn would in-dicate an electrical problem. Additional-ly, there are some cases where the man-

TRANSMISSIONDTCs: ONLY PART OF THE STORY

BY WAYNE COLONNA

Taken at face value, transmission diagnostictrouble code descriptions can provide

direction, or possibly lead you astray. Whenavailable, information from other sources

also should be used to fill in the gaps.

20 June 2012

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TRANSMISSIONS:Pg_EDIT_ 5/22/12 8:02 AM

21June 2012


ufacturers define this code to encom-pass mechanical and/or electrical prob-lems, such as with the GF4EAT trans-mission in 1997-98 Mazda vehicles.

Once one became wise to this seem-ingly arbitrary definition assignment forthese codes, a great deal more effortwent into finding the correct definitionof the code before launching into a di-agnostic routine.

I have since called this aspect of ourlives in the shop “diagnosing diagnostictrouble codes,” or “diagnosing troublecodes.” From this perspective, one candevelop a broader view by reading thestory the list of codes produced is telling.A code priority list can then be devel-oped, meaning you can determine whichcode may be responsible for another andbegin a diagnostic approach, startingwith the most offending code present.

Fig. 1 at right shows the codes re-trieved by a scan tool from a GM vehi-cle. They’re already listed how I wouldarrange them in my priority list. With-out question, a system voltage malfunc-tion would have to be resolved first be-fore any other diagnostics. If after thishas been resolved the output circuitcode (Manufacturer Controlled Com-puter Output Circuit) remained, thiswould be my next diagnostic approachbefore tackling the transmission shiftmalfunction codes.

Fig. 2, however, shows a list of codesin a scan tool that may be more difficultto prioritize for a focused diagnostic ap-proach. At first glance you may considerthe P0726 (Engine speed input circuitperformance) code as your first priority.But I think the scan tool has already pri-oritized the code list. If the solenoidused to control the TCC is stuck on, itwill certainly have an effect on enginespeed performance and could easilyproduce both the P0744 and P0726.

Another example would be any Fordvehicle equipped with a 5R55N/S/Wtransmission. It’s not uncommon for thistransmission to blow out the OverdriveServo, as seen in Figs. 3 and 4 on page23. This servo operates the overdriveband used to provide 2nd and 5th gears,which, if they became inoperative,would produce gear ratio codes P0732(2nd gear ratio error) and P0735 (5thgear ratio error), respectively. This will

also produce line pressure solenoids Band C performance codes P0775 andP0975 as well. This is a very good exam-ple of how one code will set another.But we still have yet to determine theroot cause of the problem.

The damage to the servo seen in Fig.4 is usually indicative of high line pres-sure. Sometimes we see amongst thesetransmission codes a P0171 and P0174for a lean bank 1 and 2 issue. Manytechs may be quite familiar with thesecodes and what would cause them. Amass airflow sensor might be one of thefirst possibilities you’d consider. This, of

course, is an engine load device whosesignal is critical to line pressure controlin the transmission. In this case, al-though I have a visible problem withthe servo which I know I can fix withoutpulling the transmission, P0171 andP0174 would be at the top of my priori-ty diagnostic list. This is the most likelyroot cause of the P0732, P0735, P0775and P0975 codes.

Without getting the load issue re-solved, I will more than likely see this jobcome back with either a broken servoagain or a complaint of harsh shifting atbest. But this begs the question: What if

TRANSMISSION DTCs: ONLY PART OF THE STORY

Fig. 1

Fig. 2

22 June 2012

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there were no codes indicating a load sig-nal issue? Although the servo breakingmay be due to other reasons, the mostcommon reason is high line pressure.The questions that really need to be askedare: Why would there be a high linepressure problem? Is it an engine load is-sue that has yet to reveal itself in a codethat’s causing the high line pressure is-sue? Answering these types of questionswill provide a better chance of achievinga logical, prioritized diagnostic routine.

Looking at Mode 6 would be my nextmove. If Mode 6 shows a clean bill ofhealth regarding engine load, the trans-

mission itself would now be consid-ered. Stuck valves or malfunctioningpressure control solenoids would nowbe considered as the root cause of thistransmission failure.

Determining which code may havecaused another is not always as easy as itsounds. Take, for example, an incident Iexperienced with an Acura 3.2 TL thathad both a P0740 code for a perfor-mance failure of the converter clutchand a traction control system (TCS)code 31-2 engine retard command(PFINH) signal. One tech may thinkthey’re two separate issues; another may

think one is affecting the other, particu-larly since often we have seen various is-sues with the antilock braking systemaffect transmission operation: late up-shifts, no upshifts, loss of high gear,downshift clunks . . . the list goes on.

So in addition to handling all engine-related codes before diagnosing transmis-sion issues, we now need to include allABS-related complaints beforehand aswell. But as with most things in life, noteverything is this black & white. Some-times, engine-related issues have noth-ing to do with transmission issues, andthe same holds true for the ABS system.

In this particular scenario with theAcura, the P0740 is what actuallycaused the 31-2 to occur. There’s nomanufacturer literature to inform youof this strategy. In fact, we’ve seen TCScode 36-2 (TPS output signal malfunc-tion [THLOUT]) produce code 31-2.So it appears that if there are any issuescompromising engine load, the 31-2 willset, indicating that the TCS is unable toinitiate an engine retard while sometype of engine load problem exists. Thisunderstanding would then point all di-agnostic efforts toward resolving theP0740 code rather than working out the31-2 code, which would be futile.

Prioritizing groups of codes with theunderstanding of how one may set an-other and being clear about arbitrarydefinitions given to generic codes be-gins to remove some of the associatedambiguity that may exist with variouscodes. Misleading codes can lead totime wasted diagnosing an area of thevehicle that need not be investigated.That time is rarely compensated for.

Another technique I have developedin diagnosing diagnostic codes is tocompare codes among different manu-facturers using the same transmission.For example, the Aisin Seiki AS68RCsix-speed rear-wheel-drive transmissionfor medium-duty trucks used in Dodgecommercial chassis packages with theL6 6.7L diesel engine is also called theAS6 in the Mitsubishi Fuso, theM036A6 in the Nissan UD and theA465 in Isuzu NPR trucks.

This transmission is very similar tothe Allison 1000/2000 series transmis-sions in that they both have pressureswitches that close after a solenoid oper-

Fig. 3

Fig. 4

23June 2012


ates a valve in the valve body (Fig. 5, atright). A pressure switch code settingdoes not necessarily mean the pressureswitch is faulty. A pressure switch codein the Dodge and Mitsubishi transmis-sions usually indicates a stuck valve or apoorly performing solenoid. Not know-ing this can be very misleading in termsof determining your diagnostic ap-proach. Mitsubishi generic codes forthese pressure switches do just that—mislead you into thinking that you haveonly a pressure switch issue. Dodge, onthe other hand, uses the same genericcodes but refers to them as solenoidperformance codes.

By comparing the Mitsubishi pres-sure switch code list with the Dodge so-lenoid performance code list (Fig. 6, be-low), a fuller picture begins to emerge.Mitsubishi assigns generic code P0746to a Pressure Switch 1 malfunction,while Dodge describes this as being aPressure Control Solenoid A Perfor-mance issue. Both are right, yet both re-main a bit vague, since neither one in-cludes the valve being stroked by the so-lenoid that will provide pressure to closethe switch. Admittedly, if one has accessto the Dodge material that explains thetheory of operation, this vital piece of in-formation can be discovered. This, inpart, is how Dodge explains P0746:

The Linear Solenoid 1 is used to con-trol input pressure to the ControlValve 1. The Control Valve 1 uses theinput to modulate the line pressure tothe appropriate supply pressure forthe clutch. The TCM verifies the oper-ation of the Control Valve 1 by moni-toring the Pressure Switch 1. If a faultis detected, the transmission will gointo limp-in mode, the Torque Con-verter Clutch (TCC) will be disabled,the limp-in gear will vary based onthe current gear and other conditionsand the MIL will be illuminated.

With this information in hand, a soliddiagnostic routine can now be deter-mined. If you didn’t have this informa-tion, you could still compare a Mit-subishi code list with a Dodge list. Bydoing so, it becomes quite apparenthow the two together augment eachother. Even with a piece of the puzzle

missing, the code list comparison willoffer a more successful diagnostic ap-proach than if one had not taken thetime to make this comparison.

Comparing code lists of differentmanufacturers using the same transmis-sion can also reveal misleading informa-tion provided by the manufacturer.Look again at the chart in Fig. 6. Noticethat Mitsubishi assigned one generic

code (P0761) to two pressure switches(5 and 6). Yet Dodge assigns a P0761 toa performance issue with shift solenoid3 (SS3) and a P0766 with shift solenoid4 (SS4). If you were not comparingcode lists, this would not jump out atyou, and you’d be unaware of this ap-parent discrepancy. So why the discrep-ancy between the two manufacturers?What’s going on here?


Fig. 5

Fig. 6

24 June 2012


Let’s begin by taking a look atthe theory of operation Dodge pro-vides for SS3 and SS4. First, SS3:

The TCM verifies the opera-tion of the Shift (On/Off) Valve3 by monitoring the PressureSwitch 5. If a fault is detected,the transmission will go intolimp-in mode, the Torque Con-verter Clutch (TCC) will bedisabled, the limp-in gear willvary based on the current gearand other conditions and theMIL will be illuminated.

Next, the theory of operationfor SS4:

The TCM verifies the opera-tion of the Shift (On/Off) Valve4 by monitoring the PressureSwitch 6. If a fault is detected,the transmission will go intolimp-in mode, the Torque Con-verter Clutch (TCC) will bedisabled, the limp-in gear willvary based on the current gearand other conditions and the

subishi’s information is wrong. So howdoes one clear up this discrepancy?Learn by trial & error in parts replace-ment to see what works? Unfortunately,this can be a spot a tech can find him-self in if there are no other pieces of in-formation to work with.

In this case, a hydraulic sche -matic would have to be acquired tosee what’s actually occurring insidethe valve body. Once this schemat-ic is located and obtained (Fig. 7, atleft), it reveals that SS4 has nothingto do with closing Pressure Switch6. In fact, it’s SS3 that closes Pres-sure Switch 6. So Mitsubishi has itright. What the code definitiondoesn’t reveal is how SS3 closesboth Pressure Switches 5 and 6.The hydraulics revealed that SS3strokes Shift Valve 3 to close Pres-sure Switch 5 and it strokes ShiftValve 4 to close Pressure Switch 6.

The point being made here isnot to explain how the AS68RCtransmission operates as much asthe benefit gained by comparingcode charts from different manu-facturers using the same trans-mission—particularly with gener-ic codes. It might not fill in all theholes, but it offers up greaterscope of what’s going on and cancaution you should an apparentdiscrepancy come to light.

One last point of interest re-garding this Aisin AS68RC transmission.Mitsubishi designated fault code P0706for a problem with Pressure Switch 8.Interestingly enough, Dodge assigned itas a Transmission Range Sensor Ration -ality fault. What’s happening here is thatthe computer is watching the transmis-sion range sensor (TRS) signal when theselector lever is placed in Drive (Fig. 8).Once in Drive, line pressure from themanual valve is used to close PressureSwitch 8 as a verification signal that thisoperation was successful. This meansthat when a Drive signal is sent from theTRS and Pressure Switch 8 does notclose, code will P0706 set.

In this case, Dodge’s definition ofP0706 stands more of a chance of deter-mining a correct diagnostic approachthan Mitsubishi’s, as a malfunctioningTRS can certainly produce this code.Looking only at Mitsubishi’s code defin-ition, you may find yourself changing allkinds of parts except the TRS.


26 June 2012

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

MIL will be illuminated.

So Dodge says that SS3 is responsiblefor closing Pressure Switch 5 while SS4closes Pressure Switch 6, each throughtheir respective shift valves. Logically,then, you’d begin to think that Mit-

Fig. 7

Fig. 8

PSW 6 PSW 5

ShiftValve 4

S3 On

ShiftValve 3

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X

X

X

X

Manual Valve

PD N R

PSW 8

45-50 psi


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