Mercury Grand Marquis transmission converter shutter issue in overdrive requiring fluid exchange on Mercury Grand Marquis

Some Mercury Grand Marquis owners report experiencing a transmission shudder issue, particularly in overdrive, where the vehicle shakes or trembles during shifting or when driving uphill. This shudder, often noted during downshifting from 4th to 3rd gear, may be related to torque converter clutch chatter and can sometimes be alleviated by turning off overdrive. Many owners find that changing the transmission fluid is a crucial first step, with a complete flush using a professional transmission flush machine recommended due to the system holding approximately 14 quarts of oil. While some have had temporary relief by adding products like Instant Shudder Fixx, the issue may return. Additionally, the introduction of the Mercon V transmission fluid spec in 1998 aimed to reduce lockup clutch chatter, and newer fluids like Mercon LV might also help. If the shudder persists after fluid changes, further investigation into potential mechanical issues, such as a broken overdrive servo retaining snap ring or problems with the torque converter, may be necessary. Environmental factors, such as wet or cold conditions, and issues with the transmission shift sensor connection could also contribute to the shuddering problem.