Manually controlled automatic transmissions / Gearbox

Published by Carl Wilson on

Most automatic transmissions offer the driver a certain amount of manual control over the transmission’s shifts (beyond the obvious selection of forward, reverse, or neutral). Those controls take several forms:

Throttle kickdown

Most automatic transmissions include a switch on the throttle linkage that will force the transmission to downshift into the next lower ratio if the throttle is fully engaged. The switch generally only functions up to a certain road speed, so as to prevent a downshift that would overrev the engine. Some transmissions also have a part-throttle kickdown, eliminating the need to “floorboard” the throttle to downshift.

Mode Selection

Allows the driver to choose between preset shifting programs. For example, Economy mode saves fuel by upshifting at lower speeds, while Sport mode (aka Power or Performance) delays shifting for maximum acceleration. The modes also change how the computer responds to throttle input.

Low gear ranges

Many transmissions have switches or selector positions that allow the driver to limit the maximum ratio that the transmission may engage. On older transmissions, this was accomplished by a mechanical lockout in the transmission valve body preventing an upshift until the lockout was disengaged; on computer- controlled transmissions, the same effect is accomplished electronically. The transmission can still upshift and downshift automatically between the remaining ratios: for example, in the 3 range, a transmission could shift from first to second to third, but not into fourth or higher ratios. Some transmissions will still upshift automatically into the higher ratio if the engine reaches its maximum permissible speed in the selected range.

Manual controls: Some transmissions have a mode in which the driver has full control of ratio changes (either by moving the selector or through the use of buttons or paddles), completely overriding the hydraulic controller. Such control is particularly useful in cornering, to avoid unwanted upshifts or downshifts that could compromise the vehicle’s balance or traction. “Manumatic” shifters, first popularized by Porsche in the 1990s under the trade name Tiptronic, have become a popular option on sports cars and other performance vehicles. With the near-universal prevalence of electronically controlled transmissions, they are comparatively simple and inexpensive, requiring only software changes and the provision of the actual manual controls for the driver. The amount of true manual control provided is highly variable: some systems will override the driver’s selections under certain conditions, generally in the interest of preventing engine damage.

Some automatic transmissions modified or designed specifically for drag racing may also incorporate a transmission brake, or “trans-brake,” as part of a manual valve body. Activated by electrical solenoid control, a trans-brake simultaneously engages the first and reverse gears, locking the transmission and preventing the input shaft from turning. This allows the driver of the car to raise the engine rpm against the resistance of the torque converter, then launch the car by simply releasing the trans-brake switch.

See also more info on Manual and Automatic Gearbox Transmissions.

Click to rate this post!
[Total: 0 Average: 0]

Carl Wilson

You won't believe it, I'm native Scotsman. Enthusiast. Car lovers. Almost finished rebuilding my Reliant Saber 🔥

0 Comments

Leave a Reply

Your email address will not be published. Required fields are marked *