Hello. I am experiencing a problem of not having overdrive on my automatic transmission. All the other gears work fine without any problems. This is not the original transmission and was rebuilt by another person. When the vehicle is put into overdrive, it will behave exactly as if it is in third gear with the exception when the throttle is let off - the rpm's then settle very low as if it is in overdrive. It was suggested that maybe the previous owner maybe plugged the valve to prevent overdrive from activating. Somehow this is a performance booster? If anyone has any suggestions on how to troubleshoot or repair the problem, it will be much appreciated. Thank you.