Control system for selectively switchable two/four wheel drive automatic transmission system shifting transfer transmission according to main transmission speed stage

For an automatic transmission system for a vehicle, comprising a main transmission unit which can be set to any one of a plurality of speed stages, an auxiliary transmission unit which is connected in series with said main transmission unit and can be selectively set to either a first speed reduction ratio or a second speed reduction ratio substantially larger than said first speed reduction ratio, and a means for manually controlling said auxiliary transmission unit to select either said first speed reduction ratio or said second speed reduction ratio,

• a transmission control system comprising:
• (a) a means for controlling said main transmission unit automatically to provide each selected one of said plurality of speed stages according to operational conditions of the vehicle;
• and
• (b) a means for controlling said auxiliary transmission unit to override said manual control means in a manner only to prevent shifting of said auxiliary transmission unit to said second speed reduction ratio according to vehicle speed, throttle opening and according to which of said plurality of speed stages said main transmission unit is set,
• wherein said override means includes means responsive to a specific combined range of vehicle speeds and throttle openings for each of said speed stages.