1995 g20

i posted one transmission problem before but now im a lil more specific. could a vaccum modulater, or a vehicle speed sensor keep a vehicle from being shifting into overdrive. i can shift on the column from first to second then when i try to put it in overdrive it like shifts into neutral. i was told to check the modulater and the vss but if its a waste of time id appreciate the help.

i posted one transmission problem before but now im a lil more specific. could a vaccum modulater, or a vehicle speed sensor keep a vehicle from being shifting into overdrive. i can shift on the column from first to second then when i try to put it in overdrive it like shifts into neutral. i was told to check the modulater and the vss but if its a waste of time id appreciate the help.

No Modulater valve
The shifting is computer controled.
Get a transmission capable scanner and check for codes and transmission shifting and post back code no.
How is the fluid and fluid level?
Have you dropped the pan and checked the filter?