Why would the Over Drive turn off while driving on the highway?
While driving on the highway going about 70mph. the over drive turned off giving a thud feeling like the car down shifted. then the over drive would not turn back on. there is a light saying "O/D off"
Most likely a worn torque converter lockup clutch or TCC solenoid. That "thud" is very indicative of that. When it starts to go it can cause a loss of top gear/overdrive first. Read more here on symptoms of a bad lockup clutch: http://www.freeengineinfo.com/tcc-torque-converter-clutch-lockup-conve.htm
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