Solve more transmission concerns in-house with these diagnostic tips

Oct. 1, 2018
With the proliferation of computer-controlled transmissions over the last three decades, there is no reason a competent drivability diagnostic technician can’t apply his or her skills to transmission diagnosis.

Years ago, while I was working for a particular shop, we used to view Volkswagens like vampires. We would hold our hands up and use our index fingers to make a cross to repel the evil beings and send them down the road to another shop. After a while I attended some training, purchased a very reasonably priced scan tool that works well on VW/Audi products and dove in. Persistence lead me to realize “these are just cars after all!” My point is attitude. Once we decide to get over the fear of the unknown and start working towards a goal we usually attain said goal. Now the shop still works on VW’s and Audi’s to this day. No need to push those cars out the door.

Transmission diagnostics are no different. I realize that transmission rebuilding is a specialty by itself and many shops do not perform these tasks in-house. However, with the proliferation of computer-controlled transmissions over the last three decades, there is no reason a competent drivability diagnostic technician can’t apply his or her skills to transmission diagnosis. Don’t ship the car to the transmission shop right off the bat. Pull up your boot straps, adjust your attitude and give it a shot. You might be surprised how many transmission issues can be resolved without major transmission overhaul.

Draw a line

The basic theory is to “draw a line” between an electrical issue and a hydraulic/mechanical issue. If the failure ends up being electrical (wiring, a solenoid or a module for example) then the repair can usually be performed in-house. If all of the electrical components test good then a hydraulic/mechanical issue would be the cause of the transmission issue. At this point, your individual shop can decide if the problem will be dealt with in-house, a remanufactured unit will be installed or the vehicle will be sent to a transmission shop for a rebuild.

As with any diagnosis, a basic knowledge of the system and logical procedure should be followed to avoid going down the proverbial rabbit hole. Figure 1 is a rough flowchart that should lead you down the correct path on most applications. The first step is to do a little research on the basics of how the transmission works, paying attention specifically to areas related to the particular transmission complaint. Technical Service Bulletins (TSBs), reflashes, solenoid apply charts and code setting criteria are some of the areas to focus on here. Second, the fluid level should be checked. Feel free to mix up steps 1 and 2. Step 3, if it hasn’t been done already, is to connect a scan tool and gather codes and any other pertinent data. From there, we need to make a decision on where to go next.

Engine issues must be resolved first. If the engine is not operating correctly it needs to be fixed before we can move forward. Remember, engine operation can impact transmission operation; rarely does transmission operation impact the operation of the engine. Once we have covered the initial steps our path will depend on what data we have obtained to this point. Sometimes, our task may be a simple as scoping the operation of a shift solenoid, while other times we may need to be more creative with our testing techniques.

Figure 1 — A basic flowchart to lead you down a logical transmission diagnostic path

Putting the plan into action

Let’s explore a few broken cars to illustrate the process. The first car will be a 2008 Ford Focus. It has already had its transmission rebuilt and an aftermarket remanufactured valve body has been installed to attempt to resolve a MIL illumination and shifting issue. The PCM has stored DTCs; P0751 – SSA Performance or Stuck Off and P0972 – SSA Control Circuit Range/Performance. During the research phase, it is determined that neither of these codes are set by a circuit fault for SSA (Shift Solenoid A). They have been set due to a performance issue with the shift associated with SSA. It can also be noted that there are no engine related DTCs and the engine is operating correctly.

Research found a TSB that discussed the possibility of a worn servo rod bore that can be repaired by installing a bushing to correct the bore wear and subsequent fluid pressure loss. Could this bushing have been missed during the transmission rebuild? Our task now will be to determine if SSA is performing correctly using the same testing we apply to fuel injectors. After all, shift solenoids and fuel injectors are both just solenoids, correct?

The shift solenoid resistance is obtained from service information and, in this case, SSA and SSB should both be between 10.9 and 26.2 ohms. We will scope both SSA and SSB so we can compare a known good solenoid (SSB) to our suspect solenoid (SSA.) The wiring diagrams are consulted and it is determined that both solenoids are power side switched. The easiest place to access the wiring is right at the PCM which is located next to the battery so our connections will be made there. Figure 2 shows the scope capture obtained while cycling the solenoids on and off with the Ford IDS scan tool. SSB was cycled on and off first to establish a baseline followed by the cycling of SSA. The scope connections are; Channel A (Blue) is connected to SSA voltage on the switched side, Channel B (Red) is a current probe measuring the amperage through SSA, Channel C (Green) is connected to SSB voltage on the switched side and Channel D (Purple) is a current probe measuring the amperage through SSB.

Figure 2 — A scope capture is taken while exercising two shift solenoids to compare a known good to a questionable one. Exactly the same as fuel injectors

Almost everything in this capture mirrors one taken of a fuel injector. The main difference is the voltage captures appear to be upside down. This is because the shift solenoids in this application are not ground side switched like most fuel injectors, they are controlled by switching power on and off. Regardless of this difference, both solenoid drivers in the PCM can be seen switching on and off cleanly. We can also see that both shift solenoids are building magnetic fields because they both have inductive kicks when the PCM driver shuts off. Both solenoid current captures show about a 750 milliamp current draw, which by using basic Ohms law, confirms that both solenoids are in an acceptable resistance range. An extra bonus without doing any serious analysis - both the good SSB and the suspect SSA look exactly the same.

At this point it is fair to say that electrically (wiring, solenoid and PCM), everything seems to be functioning correctly. Additionally, very light “clicks” can be heard coming from the transmission while performing this test which confirms the solenoids are mechanically moving. Actually, the Ford diagnostic chart suggests jumping power and ground to the transmission connector and listening for a click. We used the scan tool but the same result was obtained. Can we now “Draw the Line?” There has to be an internal hydraulic/mechanical issue causing the MIL illumination and shifting issue. Time to send it back to the transmission shop to see what they may have overlooked.

Testing a GMC and a Nissan

Our second vehicle, a 2012 GMC Canyon, is a perfect example of how the injector scoping techniques we are familiar with apply to transmission shift solenoids. The shop involved followed a diagnostic chart that had them use a test light to check and see if the solenoid control in the PCM was functioning. When the solenoid was commanded on the test lamp illuminated. The solenoid was then replaced and the late shift issue still existed. Scoping the solenoid operation revealed the real cause of the issue. Figure 3 shows the 1-2 shift solenoid voltage on channel A (Blue) and the 1-2 shift solenoid current on Channel B (Red.)

Figure 3 — The solenoid driver in the PCM is not turning on and off quickly enough

The scope capture shows that the shift solenoid driver in the PCM is taking a long time to pull the control wire to ground causing a slow/late solenoid opening. The PCM driver is also taking a long time to turn off causing another slow solenoid switching event. In addition, the slow current release is not allowing the magnetic field in the solenoid to collapse quickly and is the reason there is no inductive kick in the voltage capture. It should be noted that this “slow” solenoid driver would have illuminated a test lamp and led to an inaccurate diagnosis. Time to draw the line. A new PCM was installed, programmed and the shifting issue was resolved.

The third vehicle we will explore is a 2004 Nissan Maxima that has had the transmission rebuilt and now has a late 1-2 shift much like our previous example. The shop contacted me to perform a TCM initialization which should be done after a major repair. The initialization completed and the late shift remained. Next, the shop asked me to perform a TCM update. The calibration in the TCM was outdated so I programmed the TCM with the latest calibration from Nissan. Again, the late shift complaint remained. The next step is to continue our flowchart (Fig. 1).

Since our first few steps have been completed and no DTC’s are present, it is time to be creative. Research on the particular transmission, using the solenoid apply chart (Fig. 4) confirms that shift solenoid A switches from ON to OFF in order to obtain our questionable 1-2 shift. When shift solenoid A turns OFF it vents pilot pressure and allows a shift valve in the valve body to move which actually accomplishes the shift.

Figure 4 — A 2004 Nissan Maxima solenoid apply chart and a visual of the solenoid valve

The task now is to determine if the solenoid is operating correctly or if there is a hydraulic issue in the valve body. Scoping the solenoid while commanding it ON and OFF with be the first step and is easily obtained because the TCM is located in the passenger’s side kick panel. The wiring diagram confirms that the solenoid is power side switched so our voltage capture will appear upside down just like the capture from the Focus (Fig. 2.)

In figure 5, channel A (Blue) is connected to shift solenoid A current and channel B (Red) is connected to shift solenoid A voltage. The solenoid is then cycled ON and OFF. Both the voltage and current captures confirm proper electrical operation of the TCM, wiring and solenoid. In addition, a pintle bump can be seen if we were to zoom in on the beginning of the current capture which confirms the solenoid is mechanically opening and closing.

Figure 5 — Scoping shift solenoid A confirms both electrical and mechanical operation of the solenoid.

At this point we have one more thing to confirm: is the shifting issue due to an incorrect command from the TCM or an issue with the valve in the valve body? To accomplish this, we will have to be a little bit creative. When using a Picoscope the space bar will freeze the capture. What if we were to drive the vehicle with our scope still connected and observe when the shift command happens? Then, could we compare this information to when the shift actually occurs? That is exactly what we will try.

In the next scope capture, figure 6, our scope is connected in the same manner as our last capture. The vehicle has been driven until the 1-2 shift is felt and the scope is stopped immediately. The scope’s time cursors are then used to measure the time between the shift command and the shift feel. If the time difference were to be extremely short then a late shift command might be suspected. In this case, it took almost 2 seconds for the physical shift to occur after the solenoid had changed state. This confirms that there is a hydraulic issue inside the transmission. Can we draw the line? Our failing Maxima had a shift valve that was binding in its bore. A new Nissan valve body, although more expensive than a remanufactured unit, was installed and the shifting issue was resolved.

Figure 6 — The time measured between the command for the solenoid to turn off and the time of the actual shift was excessive

Time To Draw A Few Lines Of Your Own

In all three of our case study vehicles we loosely followed the flow chart (Fig. 1) and, with knowledge we already possess and a little creativity, we were able to determine if the faults were electrical or hydraulic/mechanical in nature. It has been my experience, not a recorded statistic, that the electrical versus hydraulic/mechanical fault ratio is about 50/50. Even if your shop does not perform mechanical repairs on transmissions, this would mean you would be able to keep about half of the transmission repairs, and associated profits, in your building.

Because of the importance of the research required for some of these diagnoses, I would be remiss if I were not to mention some additional resources to find the information we may need. We are probably all familiar with service information sources such as AllData, Mitchell, Shop Key, etc. and professional websites such as International Automotive Technicians Network (iATN) and Identifix so I will not go into any of those. However, there are additional resources that are specifically geared (pun intended) to transmissions.

Transmission associations like the Automatic Transmission Rebuilders Association (ATRA) and the Automatic Transmission Service Group (ATSG) have their own websites and publications. Some of these are free resources while others may require memberships. Many of these groups and associations offer training around the country that can be beneficial even if you are not a rebuilder. In addition, they often make service and training manuals available for purchase or they can sometimes be found in downloadable versions on-line. There are also companies that specialize in transmission parts, Sonnax just to name one, that can also be a valuable resource because they offer updated products to solve common issues. Websites for companies like these usually have valuable information that could aid in your diagnosis of a particular transmission. The last resource I would like to mention is a good local transmission rebuilder. Having someone that is sharp on the internals of a transmission can often be the right person to call and bounce a few ideas off of before going too deep in a diagnosis.

To summarize, basic knowledge of transmissions, techniques we already use for other diagnoses, some research and a logical diagnostic approach will get us a long way adjusting our attitudes towards automatic transmissions and give us a jump start on our transmission diagnostics. The “fear” of a transmission will slowly fade, the work will remain in your shop and you will benefit from the additional profit and customer confidence.

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