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You'll still see the Overdrive light going on and off as the ECM will still think it has control.
Safety factors you will lose and should consider when you go to manual control....
1. O/D is normally never engaged until 122* (F) coolant temp.
2. O/D shouldn't really be used in 1st in 99.999% of cases.
3. The ECM prevents downshifts over 4000 RPM.