Chrysler Repair: P1684 and Transmission limp mode, grand caravan, dodge dealer

Hi Kevin, I am at wits end with my Dodge dealer. A recent transmission flush seemed to create a problem with my 2003 Grand Caravan 3.3L.  The transmission sticks in second gear when I start the van, but if I put it in park, turn off the ignition and re-start the van, the transmission behaves normally.  On occasion at highway speed 50 mph in 4th gear, it will rev out of gear, and drop hard into 2nd gear and stick again.  My dealer has had the van for 6 days on 3 occasions and cannot solve the problem.  They need help and I am now searching the internet looking for advice.  I saw a recent similar post on this website that looked promising.  The only fault code on the transmission is a P1684.

Hi Bryan,
The 1684 can occur by simply disconnectint the battery and that is detected by the PCM, but
if that code is attributed to be from the TCM, then it could be a problem with the battery positive supply to the TCM or the grounding of the module. The 12V+ supply to the TCM is from fuse 15 in the power box in the engine compartement. It would be good to check that fuse to be sure it doesn't have a subtle crack in it wire that is causing an intermittent flow of current to the TCM. It is 20 amp size, and located just to the rear/left of the transmission control relay. The current flows to pin 56 of the TCM plug on a yellow/red wire. That pin of the plug should show 12v at all times. Try shaking the wire harness to see if it provokes a loss of voltage momentaritly.
The ground circuit is duplicated from both pins 53 and 57 to a nearby splice and from there on a wire directly back to the - post of the battery. That is not a likely cause of current loss but you could verify the continuity from those pins of the TCM plug to the - post clamp, with both the plug and the clamp removed, and similarly shake the harness.
Of course simply disconnecting the battery will also cause the code which will only self-erase after 50-100 key on-off cycles. Other than that keep looking for codes. You might be able to get codes by using the ignition key:"on-off-on-off-on and leave on" doing that in 5 seconds or less elapsed time. Then watch the odometer window of the cluster to see if the mileage reading changes to show 4 digit numbers which would be fault codes. If that in not productive you can often get a free readout using a plug-in code reader at an Autozone parts store. Let me know any new transmission codes (in the 0700's or 1700's) and we'll go from there.
PS: Sorry for the delay in answering but I just found your qustion in the 'pool' to which it had been referred by Kevin. Please 'rate' my answer, and where you see the question about 'volunteer of the month' please consider giving me a 'yes' vote. Thank you.