The transmission computer uses input, output, and pressure sensors to monitor transmission operation. Since it has gear ratio information programmed in, it know exactly how fast the axle should be turning for any given engine speed. If there is a tiny discrepancy, due either to slippage of the clutch packs or the sensors, the computer will default to "limp-in" mode and stay there until the ignition switch is cycled off and back on to reset the system. In limp-in mode, you can select park, reverse, neutral, and drive. In drive, it will stay in 2nd gear so you can drive the car to the shop.
At 200,000 miles, it's almost certainly worn clutch plates inside the transmission. One of the drawbacks of this design is that the computer constantly updates shift points to maintain the solid shift feel of a new car. On older cars, you could notice the shifts get sloppy over thousands of miles and many years. You don't have the luxury of an awareness that your car is aging. Instead, due to the constant updating by the computer, the shifts feel solid until the day there's too much clutch plate wear for the computer to overcome. You don't get any warning that the wear is taking place until the computer detects it and puts it in second gear.
Saturday, April 18th, 2009 AT 1:34 AM