I have a 1998 expy. 4.6l there is a button on the gear shifter that turns the overdirve off and on. the light at this button is flashing, what does that mean?

 

It means you have a problem.... How bad depends on if there is a code in the computer because of it. You will need a code reader that is capable of reading trans codes too.

How does the truck drive/shift? That could be a clue to the problem too.

Good luck!

Mitch