As a test, while you're driving at a steady speed on the highway, manually shift out of overdrive. The switch is the middle one in the group of switches to the right of the instrument cluster. Press the switch again so it goes back into overdrive, then, while holding the gas pedal steady, use your left foot to lightly tap the brake pedal. Engine speed will pick up for a couple of seconds, then go back down. You will feel a big difference in the way these two things feel.
In the first procedure, the transmission really is going into third gear. The change is real obvious. In the second procedure, the transmission is staying in fourth gear, (overdrive), but the torque converter is unlocking. Chrysler was the first manufacturer to start using lockup torque converters in the mid 1970s to increase fuel mileage. They were strictly hydraulically-controlled. Your lockup system is computer-controlled electrically. It unlocks in preparation for coming to a stop so it won't stall the engine, and it will unlock if the engine is running hot. As for coming to a stop, the computer reacts to two things. Removing your foot from the gas pedal, or applying the brake pedal will trigger the converter to unlock.
Common causes of the torque converter unlocking are a brake light switch out-of-adjustment, and a bad spot on the throttle position sensor. When the problem starts to occur, pull the brake pedal up with your toe. If the problem stops, readjust the brake light switch.
If the transmission is actually going into third gear, your mechanic will connect a hand-held computer that will talk back and forth with the transmission computer. It will display any related diagnostic fault codes and will show the gear you have selected. If the selection doesn't agree with the shift lever position, the computer is either at fault, or more likely, the range selector sensor switch on the transmission is defective.
Sunday, February 21st, 2010 AT 11:02 AM