Staying in or jumping back to second gear is "limp mode" which allows you to drive slowly to a repair shop without needing a tow truck. It's important at this point to not disconnect the battery or let it run dead because there will be valuable diagnostic fault codes in the Transmission Computer.
You must turn the ignition switch off, then restart the engine, like you observed, to get it out of limp mode.
There are two speed sensors that could be responsible, but also other electrical problems as well as slippage in a clutch pack. The computer knows that for a given engine speed, the axles must be rotating at a specific speed when in a certain gear. If those two speed sensors don't agree, due to clutch pack slippage, it defaults to limp mode.
Besides the fault codes, if your mechanic has the Chrysler DRB3 scanner, he can read the "clutch volume index", (CVI). That is a set of four numbers corresponding to the volume of fluid it takes to apply each clutch pack. Based on those numbers, an experienced mechanic can determine the percent of life left in each one, and the likelihood of one of them starting to slip soon.
A lot of independent shops have the Chrysler scanner because with an extra plug-in card, they will let them do emissions-related diagnostics on any brand of car sold in the U.S. Starting with '96 models.
Sunday, March 15th, 2015 AT 7:34 PM