The TH350C transmission did feature a lock-up torque converter. The lock-up clutch will apply in third gear at cruising speeds typically around 40+ MPH. The lock-up clutch internally locks the turbine to the front cover of the converter, which provides a true 1:1 drive ratio to the input shaft and eliminates all converter slippage. The lock-up feature was designed to help improve fuel efficiency and reduce transmission operation temperature in steady-state cruising conditions. When the lock-up applies, it can feel like an additional forward driving range is being applied, giving it that sense or feel of "overdrive", although it's actually just a true 1:1 ratio through high gear in the TH350C.