We recently had something weird happen with our 2012 Jeep Wrangler Rubicon (JK), and I figured I better document the symptom and fix for posterity. We have the WA580/NAG1 5-speed auto transmission and the 3.6L Pentastar engine.
The first symptom may be unrelated, but it seems like too much of a coincidence to omit. We started the vehicle to drive home and noticed that the front power windows were entirely non-responsive. The rears worked fine, but the fronts were totally dead, like a fuse was out. As we drove, we also noticed that the transmission wasn’t shifting out of 2nd or 3rd gear (it was hard to tell without a reader) and it would not respond to an attempt to manually shift. We pulled over, power cycled the car, and everything seemed back to normal.
As we were driving home we suddenly got a check engine light and the instrument cluster went dim (as it does). This seemed to correlate to a moderate bump in the road, but it’s hard to say for sure. We again noted that the transmission seemed stuck in 2nd or 3rd gear (i.e. limp mode).
Once we got it into the garage, I checked for trouble codes and found a P0757 which identifies in a generic reader as “Shift Solenoid ‘B’ Stuck On”. However, the Jeep-specific code reads as “P0757-2-3 SOLENOID”. Basically, something wrong with the transmission solenoid responsible for triggering the shift from 2nd gear to 3rd. I followed the service data procedure for diagnosis, which involved checking the wiring between the Transmission Control Module (TCM) and the solenoid pack in the transmission, as well as checking the resistance of the 2-3 solenoid coil. That all checked out (5 ohms exactly for the coil). Service data called for replacing the TCM under those circumstances because the only thing left is the output driver in the TCM that controls that solenoid.
So, I ordered a new (to me) TCM (part number 05150729AE) from eBay and put it in. Service data says that replacing the TCM requires resetting the adaptive learning settings and re-training. I reset those (using JScan) and went for a drive. Almost immediately, it was clear that the transmission still wasn’t shifting. No check engine light (yet) but it was still in limp mode. I drove easy for a while until the transmission temperature got above the required level for the learning to start, in case it remained in limp mode until that point and then started working, but it did not.
In a parking lot I pulled codes from the TCM and found a new one: P0731 which is “P0731-GEAR RATIO ERROR IN 1ST”. From a quick google, it seemed that most people get this when they re-gear and don’t update the gear ratio in the ECU. I thought maybe this new TCM doesn’t know our gear ratio, having just been transplanted into a different vehicle (with a non-standard ratio), and that it was thus unhappy and the error made sense.
Poking around in JScan, I saw that the gear ratio in the ECU was still set properly for our setup (4.56) and didn’t see any way to tell the TCM about the ratio. I figured maybe I’d try re-programming the ratio to the same thing, which I did. Once finished, JScan said that I needed to go run the “Initialize EGS” procedure to “transfer powertrain information to the TCM”. That seemed promising, in that it fit with my guess about needing this transplanted TCM to learn the attributes of the new vehicle.
I ran that procedure, which required me to cycle the ignition a couple times. After that, I started up and immediately had 1st gear and proper shifting afterwards. Drove around a bit and then back home without throwing any more codes or exhibiting any weird behavior.
Fixed?