As I recently wrote, we encountered a “P0757-2-3 SOLENOID” code in our 2012 Jeep Wrangler Rubicon Unlimited (JK) on a trip. This came with a refusal to shift out of 2nd or 3rd gear, and eventually a check engine light. The service data instructions say to check all the wires between the TCM and the transmission for open or short, as well as the resistance of the solenoid coil in question. If all of those check out good (which they did), then replace the TCM. I did that and the problem returned.
As I mentioned before, the problem seemed to originally manifest when I hit a bump in the road. While testing the “new” TCM, the same thing happened: the error state occurred right after a bump in the road. Pulling over and resetting codes fixed the problem, and then it manifested again on the next bump. While this could be an intermittent fault in the solenoid coil itself, I was starting to suspect the wiring again.
Even though I tried testing for open and short-to-another-circuit conditions on the wiring harness while jiggling the wires, I was unable to find any sort of problem. Service data does not provide a very clear routing of the wiring harness, and zooming in the grainy picture seemed to indicate that the harness left the kick panel under the steering wheel (where the TCM is) and punched through the transmission tunnel on top of the transmission itself. However, that’s not the case – it goes into the main engine harness, through the firewall into the engine bay, and then down alongside the bell housing. Checking for issues in this area, I found the probable culprit.
The C100/C106 connector normally resides right next to the battery in the stock location and has the transmission control/monitor wires in it. Since I had installed the MORE dual battery tray years ago, that harness no longer had a good place to sit and was sort of laying next to the battery. Over time, the factory plastic loom has disintegrated from the heat, exposing the wires directly and also allowing the harness to slip down between the battery tray and the engine head. It was here I caught a clear view of the problem: the wires had been pinched and their insulation destroyed.
One of the conditions for setting the P0757 code is “shorted to another circuit” which really means “shorted to something other than ground”. The big yellow wire in the front of the above frayed bundle is a 12AWG ignition power circuit. In addition, several others had bare copper exposed, including the LightGreen/Yellow(stripe) wire, which is the 2-3 solenoid circuit:
It seems clear to me that being pinched where it was, the wires were not quite in contact with each other, but close. As soon as I hit a good enough bump, the engine would rock against the bundle and squeeze the wires together, causing the fault condition.
I separated the wiring harness a little further so I could re-route it, repaired all the wires that had been exposed, wrapped them in fresh loom, and bundled everything back up. It can now be secured to the battery tray well above the valve cover, out of harm’s way so hopefully that won’t happen again. Obviously my fault for not securing this well enough when the original mod was made, but at the time the loom was healthy and it seemed to stay well out of the way on its own.