you need to find a good OBD 1 scanner that has jap software for mitsu.
Clear any codes (probably by unhooking battery, then attemp to drive it. Then scan the trans control module for codes , repair/diagnose from there
Guessing at it wont fix it.You need to get down and diagnose the issue.
Also you may want to perform a pinout test on the shift solenoids, you will need an EWD for that. Verify the signal from the TCM unit to the shift solenoids is also a must, so a solenoid state chart will be required to determine if the TCM is outputting correctly or at all.
Piece of cake.
BTW if the speedometr isnt working, that means the VSS may not be outputting, that will also cause a no upshift, most other faults will leave the trans in second gear by default.
Fact is stranger than Fiction