Sport Mode and SelectShift Automatic are two different modes really. Your vehicle must be in Sport mode first to activate the SelectShift buttons for manual override but in regular Sport Mode it should work as an automatic. Per the manual:
S (Sport mode) (if equipped)Moving the gearshift lever to S (Sport mode):
Provides additional grade (engine) braking and extends lower gear operation to enhance performance for uphill climbs, hilly terrain or mountainous areas. This will increase engine RPM during engine braking.
Provides additional lower gear operation through the automatic transmission shift strategy which reacts to vehicle inputs (vehicle acceleration, accelerator pedal, brake pedal and vehicle speed).Gears are selected more quickly and at higher engine speeds.
Note: The system will stay in S (Sport) until the gear shift lever is
moved into another gear for example P (park) or D (Drive).
SelectShift Automaticâ„¢ Transmission (if equipped Your transmission must be in S (Sport) for SelectShift to operate
properly.
Hope that helps.