Sub-transmission control system for providing engine braking

In a vehicle which comprises:

• an engine for supplying rotary power,
• a transmission connected to the engine and having a plurality of speed stages,
• a sub-transmission connected in series with the transmission to receive rotary power from the engine to drive the vehicle, the subtransmission including a higher speed stage for providing a first gearing ratio from its input to its output, and a lower speed stage for providing a second gearing ratio lower than the first gearing ratio,
• and a braking system for generating a braking signal when operated;
• a sub-transmission control system which includes:
• a main control system for automatically controlling the shifting of the sub-transmission between said higher and said lower speed stages depending on ongoing sensed operating parameters of the vehicle;
• and
• a sub-control system for receiving said braking signal, said sub-control system including means for measuring the duration of said braking signal, means for determining if said measured duration exceeds a first predetermined time, and means for generating an engine braking shiftdown signal only after said determination that said braking signal has continued for more than said first predetermined time;
• said engine braking shiftdown signal being supplied to said main control system to control said main control system to shift the sub-transmission positively into the lower speed stage.