Defaulting to second gear is "limp" mode that allows you to drive slowly to the repair shop without needing a tow truck. It should not come out of limp mode until you turn the ignition switch off and restart the engine.
As a general rule, if it goes right to second gear as soon as you shift into gear, the problem is sensor-related. If it goes to limp-in during or right after an up-shift, it is more likely to be slippage in one of the clutch packs. That requires a rebuild.
Once it goes to limp mode, there will be a diagnostic fault code stored in the Transmission Computer. You need to find a mechanic with a scanner that can access that computer to read the code(s). That will get you to the circuit or system that needs to be diagnosed. Also, the Chrysler DRB3 scanner and some aftermarket scanners will read out a set of four numbers called the "clutch volume index", (CVI). They represent the volume of fluid it takes to apply each clutch pack. An experienced transmission specialist can tell by those numbers how much wear has taken place and whether it's time for a rebuild. You can't go by the quality or feel of the shifts like we did for decades because the computer continuously updates how it applies and releases the clutch packs to maintain the like-new shift feel, ... Until the day comes when it can't update enough to compensate for the wear. The result is the transmission doesn't wear out any faster than they did in the past, but you don't have the months or years of warning that the wear is taking place. The sudden failure comes as a surprise.
Monday, November 14th, 2011 AT 7:47 PM