Taken at face value, transmission diagnostic trouble code ...

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BY WAYNE COLONNA Taken at face value, transmission diagnostic trouble code descriptions can provide direction, or possibly lead you astray. When available, information from other sources also should be used to fill in the gaps.

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iagnostic trouble codes (DTCs), when present, are quite helpful in diagnosing any type of vehicle malfunction, including automatic transmission troubles. There are times, however, when these codes can be misleading, ambiguous or just plain confusing. This can be the case with either manufacturer-specific codes or—more commonly—generic codes. Take lessons we learned many years ago with generic codes P0740, P0741, P0742 and P0743 related to the torque converter clutch (TCC). These codes

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are related to electrical or mechanical problems that may occur with the converter clutch system. The misleading aspect is that these codes can signal either an electrical fault or a mechanical fault or both. A P0740 code, for example, is not bound to one specific meaning, and the diagnostic approaches toward mechanical and electrical faults are completely different. A 1995 vehicle with a 4T60-E transmission would produce a P0740 code for a TCC mechanical fault, while with a TAAT in a 1996 Saturn would indicate an electrical problem. Additionally, there are some cases where the man-

Photoillustraion: Harold A. Perry; images: Wieck Media & Thinkstock

TRANSMISSION DTCs: ONLY PART OF THE STORY


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ufacturers define this code to encompass mechanical and/or electrical problems, such as with the GF4EAT transmission in 1997-98 Mazda vehicles. Once one became wise to this seemingly arbitrary definition assignment for these codes, a great deal more effort went into finding the correct definition of the code before launching into a diagnostic routine. I have since called this aspect of our lives in the shop “diagnosing diagnostic trouble codes,” or “diagnosing trouble codes.” From this perspective, one can develop a broader view by reading the story the list of codes produced is telling. A code priority list can then be developed, meaning you can determine which code may be responsible for another and begin a diagnostic approach, starting with the most offending code present. Fig. 1 at right shows the codes retrieved by a scan tool from a GM vehicle. They’re already listed how I would arrange them in my priority list. Without question, a system voltage malfunction would have to be resolved first before any other diagnostics. If after this has been resolved the output circuit code (Manufacturer Controlled Computer Output Circuit) remained, this would be my next diagnostic approach before tackling the transmission shift malfunction codes. Fig. 2, however, shows a list of codes in a scan tool that may be more difficult to prioritize for a focused diagnostic approach. At first glance you may consider the P0726 (Engine speed input circuit performance) code as your first priority. But I think the scan tool has already prioritized the code list. If the solenoid used to control the TCC is stuck on, it will certainly have an effect on engine speed performance and could easily produce both the P0744 and P0726. Another example would be any Ford vehicle equipped with a 5R55N/S/W transmission. It’s not uncommon for this transmission to blow out the Overdrive Servo, as seen in Figs. 3 and 4 on page 23. This servo operates the overdrive band used to provide 2nd and 5th gears, which, if they became inoperative, would produce gear ratio codes P0732 (2nd gear ratio error) and P0735 (5th gear ratio error), respectively. This will

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Photos & diagrams: Wayne Colonna

TRANSMISSION DTCs: ONLY PART OF THE STORY

Fig. 1 also produce line pressure solenoids B and C performance codes P0775 and P0975 as well. This is a very good example of how one code will set another. But we still have yet to determine the root cause of the problem. The damage to the servo seen in Fig. 4 is usually indicative of high line pressure. Sometimes we see amongst these transmission codes a P0171 and P0174 for a lean bank 1 and 2 issue. Many techs may be quite familiar with these codes and what would cause them. A mass airflow sensor might be one of the first possibilities you’d consider. This, of

Fig. 2

course, is an engine load device whose signal is critical to line pressure control in the transmission. In this case, although I have a visible problem with the servo which I know I can fix without pulling the transmission, P0171 and P0174 would be at the top of my priority diagnostic list. This is the most likely root cause of the P0732, P0735, P0775 and P0975 codes. Without getting the load issue resolved, I will more than likely see this job come back with either a broken servo again or a complaint of harsh shifting at best. But this begs the question: What if


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Fig. 3 there were no codes indicating a load signal issue? Although the servo breaking may be due to other reasons, the most common reason is high line pressure. The questions that really need to be asked are: Why would there be a high line pressure problem? Is it an engine load issue that has yet to reveal itself in a code that’s causing the high line pressure issue? Answering these types of questions will provide a better chance of achieving a logical, prioritized diagnostic routine. Looking at Mode 6 would be my next move. If Mode 6 shows a clean bill of health regarding engine load, the trans-

Fig. 4

mission itself would now be considered. Stuck valves or malfunctioning pressure control solenoids would now be considered as the root cause of this transmission failure. Determining which code may have caused another is not always as easy as it sounds. Take, for example, an incident I experienced with an Acura 3.2 TL that had both a P0740 code for a performance failure of the converter clutch and a traction control system (TCS) code 31-2 engine retard command (PFINH) signal. One tech may think they’re two separate issues; another may

think one is affecting the other, particularly since often we have seen various issues with the antilock braking system affect transmission operation: late upshifts, no upshifts, loss of high gear, downshift clunks . . . the list goes on. So in addition to handling all enginerelated codes before diagnosing transmission issues, we now need to include all ABS-related complaints beforehand as well. But as with most things in life, not everything is this black & white. Sometimes, engine-related issues have nothing to do with transmission issues, and the same holds true for the ABS system. In this particular scenario with the Acura, the P0740 is what actually caused the 31-2 to occur. There’s no manufacturer literature to inform you of this strategy. In fact, we’ve seen TCS code 36-2 (TPS output signal malfunction [THLOUT]) produce code 31-2. So it appears that if there are any issues compromising engine load, the 31-2 will set, indicating that the TCS is unable to initiate an engine retard while some type of engine load problem exists. This understanding would then point all diagnostic efforts toward resolving the P0740 code rather than working out the 31-2 code, which would be futile. Prioritizing groups of codes with the understanding of how one may set another and being clear about arbitrary definitions given to generic codes begins to remove some of the associated ambiguity that may exist with various codes. Misleading codes can lead to time wasted diagnosing an area of the vehicle that need not be investigated. That time is rarely compensated for. Another technique I have developed in diagnosing diagnostic codes is to compare codes among different manufacturers using the same transmission. For example, the Aisin Seiki AS68RC six-speed rear-wheel-drive transmission for medium-duty trucks used in Dodge commercial chassis packages with the L6 6.7L diesel engine is also called the AS6 in the Mitsubishi Fuso, the M036A6 in the Nissan UD and the A465 in Isuzu NPR trucks. This transmission is very similar to the Allison 1000/2000 series transmissions in that they both have pressure switches that close after a solenoid oper-

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TRANSMISSION DTCs: ONLY PART OF THE STORY ates a valve in the valve body (Fig. 5, at right). A pressure switch code setting does not necessarily mean the pressure switch is faulty. A pressure switch code in the Dodge and Mitsubishi transmissions usually indicates a stuck valve or a poorly performing solenoid. Not knowing this can be very misleading in terms of determining your diagnostic approach. Mitsubishi generic codes for these pressure switches do just that— mislead you into thinking that you have only a pressure switch issue. Dodge, on the other hand, uses the same generic codes but refers to them as solenoid performance codes. By comparing the Mitsubishi pressure switch code list with the Dodge solenoid performance code list (Fig. 6, below), a fuller picture begins to emerge. Mitsubishi assigns generic code P0746 to a Pressure Switch 1 malfunction, while Dodge describes this as being a Pressure Control Solenoid A Performance issue. Both are right, yet both remain a bit vague, since neither one includes the valve being stroked by the solenoid that will provide pressure to close the switch. Admittedly, if one has access to the Dodge material that explains the theory of operation, this vital piece of information can be discovered. This, in part, is how Dodge explains P0746: The Linear Solenoid 1 is used to control input pressure to the Control Valve 1. The Control Valve 1 uses the input to modulate the line pressure to the appropriate supply pressure for the clutch. The TCM verifies the operation of the Control Valve 1 by monitoring the Pressure Switch 1. If a fault is detected, the transmission will go into limp-in mode, the Torque Converter Clutch (TCC) will be disabled, the limp-in gear will vary based on the current gear and other conditions and the MIL will be illuminated.

With this information in hand, a solid diagnostic routine can now be determined. If you didn’t have this information, you could still compare a Mitsubishi code list with a Dodge list. By doing so, it becomes quite apparent how the two together augment each other. Even with a piece of the puzzle

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Fig. 5 missing, the code list comparison will offer a more successful diagnostic approach than if one had not taken the time to make this comparison. Comparing code lists of different manufacturers using the same transmission can also reveal misleading information provided by the manufacturer. Look again at the chart in Fig. 6. Notice that Mitsubishi assigned one generic

Fig. 6

code (P0761) to two pressure switches (5 and 6). Yet Dodge assigns a P0761 to a performance issue with shift solenoid 3 (SS3) and a P0766 with shift solenoid 4 (SS4). If you were not comparing code lists, this would not jump out at you, and you’d be unaware of this apparent discrepancy. So why the discrepancy between the two manufacturers? What’s going on here?


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TRANSMISSION DTCs: ONLY PART OF THE STORY Let’s begin by taking a look at the theory of operation Dodge provides for SS3 and SS4. First, SS3:

In this case, a hydraulic schematic would have to be acquired to see what’s actually occurring inside S3 On the valve body. Once this schematThe TCM verifies the operaic is located and obtained (Fig. 7, at left), it reveals that SS4 has nothing tion of the Shift (On/Off) Valve to do with closing Pressure Switch 3 by monitoring the Pressure 6. In fact, it’s SS3 that closes PresSwitch 5. If a fault is detected, sure Switch 6. So Mitsubishi has it the transmission will go into right. What the code definition limp-in mode, the Torque ConShift Shift doesn’t reveal is how SS3 closes verter Clutch (TCC) will be Valve 4 Valve 3 both Pressure Switches 5 and 6. disabled, the limp-in gear will X The hydraulics revealed that SS3 vary based on the current gear strokes Shift Valve 3 to close Presand other conditions and the sure Switch 5 and it strokes Shift MIL will be illuminated. Valve 4 to close Pressure Switch 6. The point being made here is Next, the theory of operation not to explain how the AS68RC for SS4: transmission operates as much as X The TCM verifies the operathe benefit gained by comparing code charts from different manution of the Shift (On/Off) Valve X facturers using the same trans4 by monitoring the Pressure X mission—particularly with generSwitch 6. If a fault is detected, X ic codes. It might not fill in all the the transmission will go into holes, but it offers up greater limp-in mode, the Torque Conscope of what’s going on and can verter Clutch (TCC) will be caution you should an apparent disabled, the limp-in gear will Fig. 7 discrepancy come to light. vary based on the current gear One last point of interest reand other conditions and the MIL will be illuminated. subishi’s information is wrong. So how garding this Aisin AS68RC transmission. does one clear up this discrepancy? Mitsubishi designated fault code P0706 So Dodge says that SS3 is responsible Learn by trial & error in parts replace- for a problem with Pressure Switch 8. for closing Pressure Switch 5 while SS4 ment to see what works? Unfortunately, Interestingly enough, Dodge assigned it closes Pressure Switch 6, each through this can be a spot a tech can find him- as a Transmission Range Sensor Rationtheir respective shift valves. Logically, self in if there are no other pieces of in- ality fault. What’s happening here is that then, you’d begin to think that Mit- formation to work with. the computer is watching the transmission range sensor (TRS) signal when the P selector lever is placed in Drive (Fig. 8). D N R Once in Drive, line pressure from the Manual Valve manual valve is used to close Pressure Switch 8 as a verification signal that this operation was successful. This means that when a Drive signal is sent from the TRS and Pressure Switch 8 does not close, code will P0706 set. PSW 8 In this case, Dodge’s definition of P0706 stands more of a chance of determining a correct diagnostic approach than Mitsubishi’s, as a malfunctioning TRS can certainly produce this code. Looking only at Mitsubishi’s code defin45-50 psi ition, you may find yourself changing all kinds of parts except the TRS. PSW 6

PSW 5

This article can be found online at www.motormagazine.com.

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