JAC91SC

In the midst of our 9 cars, My wife has a 1990 Lx 2.3 automatic. A few weeks ago it just out of nowhere stopped shifting into OD. someone suggested that I replace the Modulator valve. I went to an aftermarket parts store, because the dealership says it is obsolete (the computer...I work in the parts department...) pull it in the garage with everything but OD... change the Modulator Valve..back it out and go for a test drive.... I now have nothing past 2nd gear and it is not shifting its self... if I put the gearshift in OD or D it goes to 1st gear, but when winding it out I have to shift it to 2nd for it to change gears.... I am at wits end with it and am hoping it doesn't need a rebuild since the t bird just blew a head gasket.... any insight?