700r4 won't shift into overdrive?

Hey folks, I have a '93 Buick Roadmaster 5.7 litre that runs and works fine. Except, the 700r4 transmission will not ever shift into overdrive. I know the linkage is okay. I have tried to shift manually but it still never attempts to go into overdrive. It does have a TV cable that I know is adjusted properly. I have worked with 4l60e's before but I have no experience with the older 700's. Can anybody give me some pointers before I go lurking in the valve body? Thanks for all help!

I am sorry I dont have any pointers but u sound like u know a few things about trans.my 96 lost 3rd. 1st and second work great but 3rd just slips untill it shifts into 4th I think I got the car up to about70 after it went into 4th and the rpms did not sound like they were too high