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'95 Buick Park Ave won't stay in overdrive

Once my Park Ave has shifted into overdrive or fourth gear it will run down the interstate at 70 MPH at under 2000 RPM. (It will also get a great 30 MPG, but that’s not the issue.) Problem is if I step down on it to pass from say 55 to 65 and it shifts out of OD it will not shift back into OD regardless of whether or not I lift my foot completely off the gas. I can’t think the tranny is shot. There is no check engine light on. What’s causing this?

Try clicking the OD switch off then back on. If it goes back into OD then it’s basically a minor problem but you will need a good transmission mechanic to fix it. Has the transmission filter and fluid been changed on a regular basis? transman?

Dropping the gear selector down into straight drive and back to OD doesn’t make it go back into OD. Neither does coming to a complete stop and shifting into park and back. The fluid was changed 18 months and
20K miles ago.

This is where a trans shop is going to need to come in because for some reason, either electronic or hydraulic, the 4th clutch doesn’t want to re-engage. Since it seems to happen after you kick it down that could be an indicator of something like a TPS. The only way to know if its internal or an external source is to have a trans shop check it out. Unless they can show you something definitive, make sure you get a second opinion.