My 1987 Grand Marquis with overdrive tranny will only shift from first...

[christmas_tree_farmer]

...into second gear.What can cause this? I've had no prior indications of any tranny trouble,it just suddenly started on return trip from a local store.

[MARLON SEPPALA]

Sounds like you have a elctronic solenoid that controls your valve body shift modulator. Checked or changed.
Typically an internal part of the transmission a partial disassemble required just to see it. the part itself is not expensive. around thirty bucks. It's getting it installed that cost's my brother did his own took him all weekend.