The evolution of modern automatic transmission and the integration of electronic controls have allowed great progress in recent years.
Modern TCUs are so complex in their design and make calculations based on so many parameters that there are an indefinite amount of possible shift behaviours This sensor sends a varying frequency signal to the TCU to determine the current speed of the vehicle.
If either the TSS or WSS fails or malfunctions/becomes faulty, the ratio will be wrong which in return can cause problems like false speedometer readings and transmission slipping.
Older transmissions use this to determine engine load, with the introduction of drive-by-wire technology, this is often a shared input between the ECU and TCU.
This sensor sends a varying frequency signal to the TCU to determine the current rotational speed of the input shaft or torque converter.
One of the most common inputs into a TCU is the kick down switch which is used to determine if the accelerator pedal has been depressed past full throttle.
When activated the transmission downshifts into the lowest permissible gear based on current road speed to use the full power reserves of the engine.
This is still present in most transmissions though is no longer necessary to use in most circumstances because the TCU uses the throttle position sensor, the rate of change, and driver characteristics to determine whether a downshift may be necessary, thus eliminating the traditional need for this switch.
This input is used to determine whether to activate the shift lock solenoid to prevent the driver from selecting a driving range with no foot on the brake.
This can modify shift behaviour to take into account the throttle is not being operated by the driver to eliminate unexpected gearchanges when the cruise control is engaged.
Many automatic transmissions lock the selector lever via a shiftlock solenoid to stop a driving range being selected if the brake pedal is not depressed.
Simple electronic-control designs (such as Ford's AOD-E, AXOD-E and E4OD) use the solenoids to modify the shift points in an existing valve body, while more advanced designs (such as the Chrysler Ultradrive and its follow-ons) use the solenoids to control the clutches indirectly, by way of a greatly simplified valve body.
Modern designs provide partial lockup in lower gears to improve fuel economy further, but this can increase wear on the clutch components.
The transmission control unit (TCU) in older automobiles with a clutchless manual transmission (without a clutch pedal) typically consists of an electrical switch connected to the gearshift, that is activated whenever the internal transmission control unit senses driver touching the gearshift to switch gears, which then primes a sensor or solenoid to impel a clutch servo, and in turn, disengages the clutch actuator so the driver can change gears.