Where is plug to the converter on a 3800 "89 Buick Lesabre?

The *440T4 or the 4T60 automatic overdrive transmission* will be damaged if the overdrive gear is disabled. The older THM 180 torque converter lock-up solenoid could be disconnected without damaging the transmission.. That’s not the case for 1989. You may not have a *transmission problem* - recommend having experienced mechanic test drive vehicle. Torque converter *shudder* results in the same sensation as running across rumble strips on the highway .. TEST > ‘IF’ the torque converter is causing the problem(?) While at crushing speed and the problem you’re having is present .. Lightly depressing the brake pedal will UN-LOCK the torque converter - *torque converter shudder* (IF present) WILL disappear! Good luck with it!