Q: What would cause my transmission to not shift

One cause to rule out is low fluid level or a plugged transmission fluid filter. Delayed shifts can also be caused by internal hydraulic leaks in the valve body or failing internal transmission seals found on clutch pack pistons and piston o-rings. External pressure gauge testing can be useful in the diagnosis. Other possibilities external to the transmission include faults in the MAP sensor, throttle position sensor, and vehicle speed sensor. If the faults are external, such as in the aforementioned sensors, of course it will be readily restorable. However, due to the age of the transmission if there are internal leaks, faulty clutches and bands, valve body problems, and the like it will be more cost effective to remove the transmission and rebuild it than address, piecemeal, a variety of deficiencies. If this is the original transmission, it’s near the end of its service life. Rebuilding is no big deal, indeed technically it is one of the easier systems on the car to deal with. If you desire that the shift problem be diagnosed, the recommended service is a shift diagnostic.