Delivering the Goods - August - 2018

Why Doesn’t My 6T40 Shift Right?

Exploring the purpose for the Service Fast Learn Adapts procedure.

By now, most builders in major metropolitan areas have developed a process for building the 6T40 successfully. They’ve tailored their parts order to the failure observed inside the unit. When they need to replace the TECHM, they know to arrange for programming to maintain a reasonable delivery date.

With everything happening like clockwork, there’s still room for something to go wrong with these units. Sometimes it’s a detail missed during diagnosis. Other times, it’s something wrong that shows up on the final test drive.

Even using an OEM TECHM and having it programmed at the dealership, problems can occur. When there are issues, it’s important that you keep the entire vehicle in mind before deciding to replace parts or remove the transmission. When issues are related to shift quality, the first item to check would be the transmission adaptive learning.

Adaptive computer systems play an important role in a transmission’s operation. When it comes to diagnosing transmission related complaints, don’t ignore the details imposed, whether you’re looking for a customer complaint for the first time or performing a final road test.

Frozen adaptive values, or adaptive value resets due to power loss, power fluctuations, or bad ground or power connections are a few major situations you need to take into consideration before condemning a transmission for exhibiting shift problems or codes.

With this in mind, our standard procedures for evaluating the integrity of the electrical system and the computer systems for codes has become our benchmark starting point before laying a wrench on any late model vehicle. From there, you can establish routines and procedures that’ll minimize your chances of missing fundamental problems that may lead to a misdiagnosis.

Unfortunately, even best diagnostic processes are compromised when the OEM decides to “change the game” without notifying the aftermarket (or at least making the information available). Let’s visit such a case with GM’s 6T40 family of transmissions.

The 6T40 transmission family includes the 6T30, 6T40, 6T45, and 6T50 units. These units vary in torque capacity and, in the case of the 6T30, case size (figure 1). Hydraulic, mechanical, and electrical fundamentals are the same.

These units are the “little brother” to the 6T70 family. In fact, the 6T70 unit came on the market a year earlier. So GM incorporated the technological learning curve into the 6T40 family of units when they came on line in 2008.

One major consideration is the Service Fast Learn Adapts procedure. In the early stages of the 6T70, this function was available and was part of the repair process prior to a final test drive.

Due to non-specified internal issues, it’s no longer available on the GDS2 (OEM) scan tool. Many aftermarket scan tools have this function available, but never try to perform the Service Fast Learn Adapts procedure on the 6T70 family transmissions.

For the 6T40 family, the OEM procedure is to perform the Service Fast Learn Adapts procedure after any of these repairs:

  • Transmission overhaul, replacement, or internal service
  • Valve body repair or replacement
  • Speed sensor replacement
  • Control solenoid valve assembly (TECHM) replacement
  • TCM software or calibration update
  • Any service in response to a shift quality problem

This procedure was set from the original production date of this unit and it hasn’t changed. It’s important to perform a fast learn because it allows this unit to calibrate the shift control components more precisely, before driving the vehicle, to prevent premature unit failure.

The Service Fast Learn Adapts procedure allows the transmission control module (TCM) to learn initial, individual clutch characteristics. This information is translated into adaptive data cells, which the TCM uses for clutch control during shifts. These cells change continuously throughout the life of the transmission to provide consistent shift quality as the transmission components wear and age.

The Service Fast Learn Adapts procedure is a scan tool-driven process that’s available on the OEM GDS2 scan tool and most other popular aftermarket scan tools. Access to fast learn requires the VIN entry or VIN read mode on your scan tool.

Using a Snap-on brand tool (one of the more popular brands), access transmission data, and then select FUNCTIONAL TESTS (figure 2). You’ll be able to select FAST LEARN from the menu. Other aftermarket scan tools will provide a similar procedure.

Prior to initializing the procedure, make sure the vehicle meets these conditions:

  • Drive wheels are blocked
  • Parking brake is applied
  • Service brake is applied
  • 0.0% throttle and no external engine RPM control
  • Transmission fluid temperature (TFT) is between 158º-239ºF (70º-115ºC). Refer to the chart in figure 3, for the required transmission temperature range, based on the generation unit you’re servicing.
  • Transmission gear selector has been cycled from park to reverse three times to purge air from the reverse clutches

Once you’ve met these vehicle conditions, initialize the Service Fast Learn Adapts procedure on your scan tool. Your scan tool will provide instructions throughout the process. Follow the prompts as required. Once the process is complete:

  • Turn the key off.
  • Exit to the main screen on your scan tool, turn it off and unplug it from the DLC.
  • Open and then close the driver’s side door. This cycles the Retained Accessory Power (RAP) module on the vehicle, and indicates a completed ignition cycle.
  • Restart the engine to finish the procedure.

Failure to follow this procedure may leave the vehicle in a neutral state (no movement in any range). Leaving the scan tool plugged in can keep the TCM from cycling off by providing an external power source. Also, if at any time during the procedure the vehicle doesn’t meet the required conditions, Service Fast Learn Adapts may abort and you may need to restart the process from the beginning.

If this occurs, the transmission will remain in a neutral state until the controller shuts down:

  • Turn the key off.
  • Remove the scan tool.
  • Open and close the driver’s side door.
  • Wait for more than 30 seconds prior to retrying the procedure.

If the procedure fails repeatedly, you may have exceed the limit that engineering set in the system, and you may have an internal transmission issue.

If you followed all the procedures correctly and the vehicle remains in neutral, disconnect the battery negative lead for 5-10 minutes and then reconnect it. Drive and reverse engagement should return.

The Service Fast Learn Adapts procedure may not run or abort because of other conditions. It’s important to use your scan tool and other items to check for:

  • TFT out of range, faulty (refer to chart in figure 3)
  • Faulty brakes or brake switch
  • Active DTCs
  • Closed throttle and engine RPM increases above 1500 RPM while initializing the procedure
  • Internal mode switch (IMS) reading incorrectly or not functioning
  • Line pressure control isn’t within 145 PSI (1000 kPa) of commanded pressure • VSS signal present while performing procedure or excessive vehicle vibration
  • Faulty clutch clearance, clutch piston, or other internal hydraulic leak

Once you’ve completed the Service Fast Learn Adapts procedure, clear DTCs (if any set) and test drive the vehicle to make sure shift quality is good.

I know there are technicians who’ve never performed this procedure and have delivered vehicles successfully and are still having success. This information isn’t to change your procedures. The purpose behind this is to make you aware of a process that could save you from needlessly replacing parts and pulling transmissions.

It’s also to advise you that it does indeed fix some shift quality-related issues, especially when there are no codes present. Recent cases that came through the ATRA Hotline and other technical circles have shown modest shift quality issues as well as flared and slipping shifts resolved by simply performing the Service Fast Learn Adapts procedure.

One more point that comes into play when you skip the procedure is whether the transmission ever successfully adapts, especially when you release the vehicle to a customer who may not experience driving conditions that will allow adaptation to occur naturally.

Remember, the adapt cells are constantly changing as you drive the vehicle. But the adaptation only occurs once the transmission reaches operating temperature. So, if the customer only drives very short distances, that may not allow enough time to trigger the adaptive values to adjust, which could cause premature transmission failure.

Another case would be a commuter that lives close to the highway. The transmission may not reach operating temperature until it’s on the highway in 6th gear. If the person’s job is close enough to the highway or they use Park-n-Ride, there’s very little opportunity for the transmission to adapt shift transitions at all. In this case, the customer would most likely notice shift quality issues if he stops driving his normal, routine route.

So the next time your 6T40 transmission isn’t shifting just right, check your adapts. Your solution might be a scroll and a few clicks away!