Many types of multi-speed automatic transmissions are available for motor vehicles. In automatic transmissions, shifts from one operating mode, range, or ratio to another operating mode, range or ratio may be controlled electronically by computer circuitry, programming logic, and fluid hydraulics installed in the vehicle. In general, an assembly of electro-hydraulic actuators (such as solenoids), hydraulic valves and fluid passages makes up an electro-hydraulic portion of the transmission control system. Electrical controls selectively actuate hydraulic valves, which control the pressure and direction of hydraulic fluid flow applied to torque transmitting mechanisms (such as clutches, brakes and the like) and other components of the transmission. The clutches or other torque transmitting mechanisms may be engaged and disengaged upon command to attain different operating modes, ranges or ratios for the vehicle.
Some of the design considerations of control systems for automatic transmissions include shift quality, fuel economy, reliability and durability over a wide range of potential operating conditions. The range of operating conditions may include changes in the weight of the vehicle load, operating temperature, weather conditions, geographic conditions (such as altitude or humidity), and driver preferences. The configurations of component parts of the transmission and/or the control system may vary. Variations may also occur in the reliability, durability and/or manufacturing tolerances of component parts of the transmission and/or control system.
In some instances, if a failure of a component occurs during vehicle operation, the transmission control system may include features that are designed to cause the vehicle to transition to a safer or more desirable state until the failure can be addressed. Certain of such features may be referred to as “limp home” capabilities. In other instances, it may be desirable or necessary to prevent the transmission from causing the vehicle to enter an operating mode that is known or believed to be undesirable or unsafe during operation. Certain of such features may be referred to as “blocking” features. In the past, these and other design features have often required additional space in the transmission assembly.