On old cars, they had a vacuum modulator on the trans for shifting. It would look sort of like a vacuum advance on a distributor. but it would be in the side of the trans usually close to the back. Newer cars use the TCM to control that now.
It doesn't shift into overdrive. The computer says shift modular and the auto parts store is saying vacuum modulator. My husband would have to put it back on the computer to get the code. Is there anyway we can the blue print of transmission to see the location to figure out if it electrical or vacuum? Picture will also help with replacing the part.
having the code that the computer sees as a problem would be the first thing to examine. everything else would be guessing. when ever you scan a vehicle you should write down all the codes to get. sometimes you may have a code that conflicts with other codes. and sometimes you may have things happen that are from the computer shutting things down in what is called limp mode. (in some cars if your check engine light comes on certain things are shut down to help insure that the engine can keep running. cruise control, AC, overdrive are all things that may be shut down if the check engine light is on)