NAG1 transmission shift issue

[Garland66]

Hello everyone!
I'm almost at my wits' end with this one, so hopefully someone can shed some light! I have a 2012 Unlimited Sahara with the 3.6L and Nag 1 auto transmission. I have had a myriad of wiring issues with this one. Recently, I am having an issue where the trans will go to limp mode and stays in second gear, but does not throw any codes. I do not feel like its a mechanical issue, but an electronic or electrical issue. I have been inside the transmission through the valve body and the pan was really clean super minimal deposits on pan magnet. I replaced the conductor plate while I was in there. I have a bidirectional scanner that allows live data monitoring and module resetting. Here are some things I have noticed during my diagnosis:

• While monitoring the TCM module, the output shaft speed is reading at the top of the range WITH THE ENGINE OFF! This reading is a calculation of both rear wheel speed sensors (all 4 if 4WD is engaged) and the n3 out put speed sensor from the conductor plate. The n3 sensor and all wheel speed sensors are reading correctly.
• The gear shift indication does not allow manual shift selection.
• It will throw a P0731 - Gear Ratio Error in 1st code. I feel this is a symptom code not a cause after the trans stays in second gear and it comes in after a stop and start (it's taking off in 2nd gear....of course the ratio is off)
• I have gotten lost communication codes across modules connected to the Canbus C circuit, but those went away.

Here is what I've done to remedy / diagnose:

• I did some continuity tests on the canbus circuit (plan do do more)
• I took it to a professional shop for diagnosis; they said it was an issue inside the transmission. I replaced the conductor plate with a better one. Did not help
• I also may have an ABS module issue, but again, the wheel speed sensors are working properly. I actually purchased a rebuilt one (took a chance) but couldn't write the vin to it. Took it to dealer they said it was DOA. They also said they updated the programming on a couple of modules. I'm beginning to think they may have corrupted something, but they will never admit that.

This is very frustrating! This is a hobby jeep but it is in very good shape. I just want to get this resolved...thanks in advance!!

 

[pc1p]

I can't speak specifically to the Nag1, but there are many examples of similar issues due to poor grounds and/or corroded battery terminals/connections. Before going any further, I would absolutely start there...

 

[Chris]

Hello everyone!
I'm almost at my wits' end with this one, so hopefully someone can shed some light! I have a 2012 Unlimited Sahara with the 3.6L and Nag 1 auto transmission. I have had a myriad of wiring issues with this one. Recently, I am having an issue where the trans will go to limp mode and stays in second gear, but does not throw any codes. I do not feel like its a mechanical issue, but an electronic or electrical issue. I have been inside the transmission through the valve body and the pan was really clean super minimal deposits on pan magnet. I replaced the conductor plate while I was in there. I have a bidirectional scanner that allows live data monitoring and module resetting. Here are some things I have noticed during my diagnosis:

• While monitoring the TCM module, the output shaft speed is reading at the top of the range WITH THE ENGINE OFF! This reading is a calculation of both rear wheel speed sensors (all 4 if 4WD is engaged) and the n3 out put speed sensor from the conductor plate. The n3 sensor and all wheel speed sensors are reading correctly.
• The gear shift indication does not allow manual shift selection.
• It will throw a P0731 - Gear Ratio Error in 1st code. I feel this is a symptom code not a cause after the trans stays in second gear and it comes in after a stop and start (it's taking off in 2nd gear....of course the ratio is off)
• I have gotten lost communication codes across modules connected to the Canbus C circuit, but those went away.

Here is what I've done to remedy / diagnose:

• I did some continuity tests on the canbus circuit (plan do do more)
• I took it to a professional shop for diagnosis; they said it was an issue inside the transmission. I replaced the conductor plate with a better one. Did not help
• I also may have an ABS module issue, but again, the wheel speed sensors are working properly. I actually purchased a rebuilt one (took a chance) but couldn't write the vin to it. Took it to dealer they said it was DOA. They also said they updated the programming on a couple of modules. I'm beginning to think they may have corrupted something, but they will never admit that.

This is very frustrating! This is a hobby jeep but it is in very good shape. I just want to get this resolved...thanks in advance!!

The NAG1 transmission can be tricky, especially when electrical issues are involved.

From your description, it seems like you've covered a lot of ground. Since the transmission is going into limp mode, and you’re not seeing codes that directly point to a transmission fault, focusing on the CANBUS communication might be the way forward. The irregular output shaft speed reading while the engine is off stands out and could be indicative of a feedback issue with the TCM or even a wiring fault somewhere in the transmission loom.

Here are a few thoughts and suggestions that might help narrow down the issue further:

• Check Ground Connections: A bad ground can cause all sorts of electronic misbehavior. Ensure that all grounds related to the transmission control module and the related sensors are clean and secure.
• Inspect Wiring and Connectors: Since you've had other wiring issues, I would recommend closely inspecting the wiring harness leading to the transmission and ABS module. Look for chafed wires, poor connections, or corrosion that could create intermittent issues.
• TCM Testing: Since you have a bidirectional scanner, if you can, try to explicitly command the TCM to shift through its ranges. This might give you more insights into whether the TCM is functioning as expected.
• Verify ABS Module Installation: If you suspect the ABS module is also influencing the transmission performance, consider double-checking its installation and whether any other modules might need reprogramming after its replacement.
• Look for Updates or Technical Service Bulletins (TSBs): Sometimes there are factory recommendations or updates for specific issues with the NAG1 transmission and associated systems that may not be widely known.
• Revisit Professional Help: If you’re still stuck, considering a second opinion from another transmission specialist who specifically works with the NAG1 may give you new insights and solutions.

Best of luck! Keep us updated on your progress!