The Celica transmission is very, very different from that in your Volvo. The Overdrive unit is bolted on to the back of your transmission, almost like an afterthought. It uses an electrically operated valve to pass transmission fluid into the core, engaging the overdrive. When it's cold and the transmission has not warmed up, the fluid may not be able to flow well enough to build up the necessary pressure to engage the unit (or to keep it engaged). However, it's more often an electrical problem. When you press the button, does the light come on? Every time? That's the first place to look. You may change the fluid and screen, too, as a good PM measure, but the OD unit is typically more sensitive to fluid level than fluid quality. See the FAQ for a very detailed description of the unit, common problems, and their fixes.