Mitsubishi Repair: tranny problems?, automatic transmission problem, coolant temperature

Question
I have a 1993 Plymouth laser rs,automatic transmission.Problem is the car takes about six miles before it locks up in overdrive,up to tweleve miles when its colder,like under 50 degrees.When its 70 or so it will shit into overdrive faster but still not right away.I think the car does need a thermostat would that affect the tranny going into overdrive?Could it be an overdrive controller?  

Answer
Chris,
Replacing the thermostat is a good idea if your coolant temperature does not increase after driving for a few minutes (i.e. your heater does not start to blow hot air after a few minutes).  But that shouldn't effect your overdrive problem.

You should have your transmission computer inspected and if you have not had your trasmission fluid and filter replaced within the past 60,000 miles, I would highly recommend you have it done.  Dirty and old tranny fluid can cause shifting problems.  Check your owners manual to be sure you use the correct type of transmission fluid.  Any mechanic should be able to perform the fluid and filter change, but you may want to take your car to a transmission shop since they specialize in transmissions better than the average mechanic.
Good luck!