Mitsubishi Repair: Transaxle Problem, automatic transaxle, mitsubishi eclipse

Question
My son drives a 1993 Mitsubishi Eclipse with a 2.0 liter engine and an automatic transaxle (3 speed with overdrive). It's got about 150K miles on it and until recently was running great. As he was driving a few days ago, the transaxle suddenly stopped delivering power to the wheels. The engine kept running fine and the gearshift lever was still in D, but the car was simply coasting. He parked the car and we've since tried to figure out what's wrong. Here are the symptoms:
1) The engine runs fine.
2) The shift lever operates perfectly normally. The transaxle linkage also seems to be completely normal.
3) With the engine running I can move the shift lever through all positions (P, R, N, D, 2, 1) but the transaxle never shifts into any gear.
4) With the engine off and the ignition key in the on position, I can hear an electrical or electronic buzzing noise as we move the shift lever into each of the various gear positions. The buzzing stops when we shift into Park or Neutral.

Is there some sort of transaxle-related electrical or electronic component that may be causing our problems?


Answer
David,
There are several actuators and solenoids that all control the correct gear the car is supposed to be in relative to the throttle pedal position and speed.  The transmission also has it's own computer that controls the shifting.  It's called the TCU (Transaxle Control Unit).  If the TCU is malfunctioning, it will not know what gear to shift the car into when the lever is shifted.  You should get the TCU inspected for damage.  The problem may also be as simple as a wiring harness  that could have come loose or may be shorting out.  Have those items checked before spending money on having the transmission rebuilt.
Good luck!