Sounds like your internal mode switch (IMS) is beginning to fail. Starting in 2009, GM replaced the manual lever position sensor (MLPS) and pressure switch manifold assembly with the IMS. It's located on the underside of the pan and is integral to the rooster comb assembly... You drop the pan, remove the selector shaft-to-IMS roll pin then disengage the IMS assembly from the selector shaft. I use diag cutters to pull those roll pins.

Here's a couple pics for reference

I also did a tear down and inspection video for YT on this same transmission - you can check that out for a walk-through on removing the linkage...Installation is simply in reverse...

I also did a tear down and inspection video

 

Thanks for the quick response.
First of all, is it safe to say this will likely progress to a point where the transmission won't change gears or could it stay like this for a long time and simply be a nusiance?

If I'm understanding you correctly, the IMS assembly can be replaced with transmission in place, without removing transmission or the valve body components?

 

Thanks for the quick response.
First of all, is it safe to say this will likely progress to a point where the transmission won't change gears or could it stay like this for a long time and simply be a nusiance?

If I'm understanding you correctly, the IMS assembly can be replaced with transmission in place, without removing transmission or the valve body components?

If the IMS complety fails, you wont be able to start your truck. Scan the pcm for IMS related (or any) DTCs.

You can replace it without dropping the valve body but it's not as easy as it is once it's out of the way. Roll pin punch and hammer or diag cutters to remove the roll pin...the new IMS will come with a roll pin.

 

...and you probably want high-temp antiseize. You may wish to get new gaskets/seals for the exhaust also (as I'm assuming you are disconnecting the exhaust pipes from the manifolds), as you don't want exhaust leaks before the o2 sensors.

 

Since I don't have to remove the valve body assembly, I must be able to remove the old IMS assembly (once the shaft is pulled out) and install the new assembly with the Guide Plate left in place. I guess I will have to blindly feel that the end seats properly under the guide plate........is this correct?

I find the pan bolt torque should be 9 ft lbs. Is there a torque rating for the single bolt that secures the Detent Roller? Also, do I need to install locktite onto that bolt?

 

Since I don't have to remove the valve body assembly, I must be able to remove the old IMS assembly (once the shaft is pulled out) and install the new assembly with the Guide Plate left in place. I guess I will have to blindly feel that the end seats properly under the guide plate........is this correct?

I find the pan bolt torque should be 9 ft lbs. Is there a torque rating for the single bolt that secures the Detent Roller? Also, do I need to install locktite onto that bolt?

98-100 inch lbs on the pan bolts. 15 ft lbs on the detent roller bolt. You can remove the parking pawl guide plate if you want/makes it easier. 12-15 ft lbs on those bolts on put-back, if you do end up removing it.

 

Looks like my problem may be in the wire harness. I haven't changed the IMS switch yet. Moving the wire harnesses around appear to have made the failures to disappear. I couldn't find any definitive chaffed wires, but the wire loom is shot. Maybe wire(s) were grounding out? A lot of the wiring is too hard to inspect due to their location. I have carefully re-wrapped the wiring around the transmission and so far gear indicator is working properly. My reverse camera and my cruise control are both working again as well. I hadn't noticed the cruise was not working until after I posted the above issues. I suppose time will tell if it will return.

 

A quick follow-up on this issue, I haven't resolved this issue yet. It started failing again around 2-3 weeks after the above posting. I've went through the wire harness and looked for broken or cut shielding and inspected the plugs at the transmission and at the TCM. Aside from needing to install wire loom and electrical tape to dress of up the harness, no obvious issues have been found. The failure is very erratic. It fails in drive and reverse and sometimes it holds steady, sometimes it drops in and out and sometimes it drops out for extended periods. Throughout the failures, it doesn't affect drivability, but it does affect the backup camera in reverse and when it fails in drive, it causes the cruise control to drop out.

I have done more research and watched more YouTube videos and assuming there isn't something in the wiring I have missed, I'm down to the IMS or TCM. I watched a YouTube video where another person was having the loss of indication at the dash (like I'm having), but his truck was also in limp mode. His indication was in a hard failure though, unlike my failure.

The cost of the IMS switch and the TCM are about the same........$150. The TCM is easier and less intrusive, but I'm still leaning toward the IMS switch as the more likely candidate. I can't find the schematic online, so I'm still not clear how this circuit flows. In my mind, I place the truck into drive via the column mounted gear shifter, it uses a cable and linkages to actuate the shift lever on the side of the transmition, which rotates the selector shaft and causes the IMS to change position. The IMS switch sends a signal to the TCM informing it of the gear position and the TCM would then send a signal to the dash to provide a visual confirmation of the gear selected. I suppose the IMS could send a signal to the TCM and the dash (in parallel), but that seems less likely. It confuses me that the truck drives fine during a failure, but still affects reverse camera and cruise control function. Both the backup camera and the cruise control are aftermarket add-ons, so I'm not sure where they get their inputs from exactly.

Does the failure as I describe it eliminate one or the other component as a root cause?

 

I thought I would bump this to the top just in case someone with the answer missed the posting.

 

A quick follow-up for others to learn from my experience. I changed the IMS switch and then the fault changed from intermittently dropping out to every other gear not showing up at all. Ohmed out the wires from the IMS back to the TCM plug and eventually traced fault to a signal wire between tranny and tcm. The wire loom of main engine harness had crumbled and the wire harness rubbed into the steel support bracket at the back of the engine. This was shorty the 5v signal to ground. The sad thing is that I had dressed up a lot of the wiring harness last year, but had stopped at this point due to difficulty of access. If I had followed through, I could have avoided a lot of work.