I have a 740 turbo wagon. I love the car. But as long as I've owned it I have not been able to get the overdrive to work properly. While driving I hit the button , the light on the dash comes on and it engages the 5th gear(overdrive). It will stay engaged only anywhere from 2 min. to 30 min. Then the RPMS will shoot up to the 4th gear level. As soon as I let off the gas it will drop down considerably below the 5th gear level. As soon as I hit the gas again the RPMS will shoot back up to the 4th gear level. And the light stays on the whole time. I brought the car to my local Volvo mechanic. He replaced the relay ,soleniod, and soleniod connecors. And cleaned the filters. To no avail. Then he told me it must be the internal clutch or pistons. I pulled the entire overdrive off the vehicle and pulled it apart and brought it to him. He found nothing wrong with it. He did not bother to install the new driven plate or piston seals--he insisted they were fine. ($350-non refundable for these parts) I even pulled the high pressure filter and pan filter a 3rd time and cleaned them, and refilled with ATF type F fluid. The only thing was, the mechanic could not get off some circular metal plug that he said contained some spring ball-bearing, he refered to it as a pressure relief valve and insisted it is never the problem. I am at my wits end and I pray somebody out there has some insight for me. Thank you and God Bless.