1990 Reatta want shift into Drive or Overdrive

[LarryK]

82 year old that bought 2 Reatta's recently needs your help.  I have changed transmission fluid and filter.  It will start in either drive or overdrive and is fine until it shifts and then nothing.  I then have to put it in 2nd to keep moving.  I feel it is also probably starting in Second. It is solid in 2nd with no slipping.  The fluid was dirty but no metal fragments etc.  Please advise what I should try next.  The car is too nice and fun to drive not to be able to use it in this condition.   Any help from anyone would be appreciated and I have a second vehicle that I can swap parts on and willing to try most anything to get it road worthy.  I look forward to hearing from anyone willing to help.   Thanks.  Larry K

[Ronnie]

11 minutes ago, LarryK said:

I have changed transmission fluid and filter. 

Did your problem start when you changed the fluid or was it already giving trouble shifting before you changed it?

[Padgett]

I'd try a can of Seafoam Transtune. It can free up sticking shuttles.

[LarryK]

The problem was there when I bought it.  I did use the Seafoam Transtune before I changed the transmission fluid and filter.  Larry K

[DAVES89]

The only thing I can think of is the vacuum modulator, but that only helps with the harshness of the shifts.

[Padgett]

Modulator also controls when it shifts. Can use diagnostics to see what gear it is supposed to be in. Can pull the vacuum line and see if tranny fluid drips out.