Computer-Controlled Transmission / Transaxle Shifting

Well, its a bit difficult to explain a Vague topic as computer controlled transmissions. I say this, as there are many ways this is accomplished. As there are also quite a few different computer controlled transmissions, such as those conventional 3,4,5 speed automatics, Paddle shifting automatics and manuals, and CVT's (continously variable transmission), and paddle shifting CVT's.

Most use some form of Shift Solenoid, either electric or hydraulic in nature. Kinda like a Linear actuator.

I know this doesnt answer your question much, my apologies, there is just no short answer i can give you. I hope someone else can explain it better.

An electronically controlled transmission system is commonly referred to as a "shift-by-wire" transmission system. In some shift-by-wire automatic transmission systems, an input shaft outwardly projects from a transmission gear housing and isrotated to shift gears in the transmission. The transmission input shaft is rotated by an electric actuator coupled to the input shaft. The electric actuator selectively rotates the transmission input shaft in response to an input signal from a manually operated gear shifter or gear selector. The gear selector is typically a lever but can alternatively be in other forms such as push buttons.

The first "shift by wire" transmissions evolved from the 1970s and 80s conventional automatic transmissions.

In those transmissions a mechanical governor would spin at the same rate as the road speed. The load on the engine was sensed by a vacuum modulator. When governor pressure became higher than the amount of load sensed by the modulator, hydraulic fluid would push against a spring loaded piston in a shift valve. As the piston would move, hydraulic fluid would now move to a piston that would overcome the release springs, and force a set of clutch plates to lock onto a rotating hub.

By nature of planetary gear sets, the now spinning hub would force the output shaft to spin at a higher speed than when the hub was not locked to the clutch discs.

OK so far?

What the " shift by wire" transmissions did was to replace the mechanical hydraulic governors and vacuum modulators with electronic sensors. Speed sensors, vacuum sensors and temperature sensors. It also replaced the hydraulic shift valves with solenoid operated valves.

Now the signals that used to be sent by hydraulic pressure are being sent by electric wires. This allows more variables to be programed into the transmission's computer.

That is what has allowed paddle shifting to come into play. Because the transmission in now shifting by means of electric signals, it became a simple matter to add 2 more switches, and tell the transmission computer that when it receives a signal from switch A, it should up-shift. Or when it sees a signal from switch B, it should downshift. All of this is providing that the shift would not cause an effect that the computer knows would be wrong. Such as up-shifting while it was in reverse, or downshifting when the lower gear ratio would cause the engine to be at a speed that was over the maximum engine speed.

Sorry for the long answer. I hope I have at least answered your question. Good luck.

Thanks. Bob...and you other guys, too. The length of your explanation is just about right, in that it is the kind of info I was looking for; and it puts the previous responses in a practical context.

Recapping, so am I to understand:

(1) That a solenoid, when energized (or de-energized), only controls a valve, which, in turn, enables hydraulically actuated shifting (hydraulics being only internal to the transmission)?

(2) That the torque converter has been eliminated?

Coincidentally:

(1) The solenoid valve actuators...are these mounted inside or outside the transmission housing.

(2) Does each "gear" have its own solenoid? Or gears in tandem...analogously to shifting manual transmission?

(3) Is it now the case that, with computer control, the risk of damage caused by manually-imparted "gear"/range selection has been eliminated? (This is a question that seems to come up a lot...without definitive resolution.)

(4) Observing automatic "engine brake" shifting (where need of pedal braking is minimized or eliminated on down grades), I am reminded of "anti-runaway"/brake saving features long in use on passenger busses (think Greyhound). How do the two auto-braking systems (on cars and busses) differ, if at all? One might think that some of the car technology was borrowed from existing commercial transport technology!?

Thanks again for a very concise and informative answer. Your allusion to '80's automatics in uncanny, in that those years correspond with my next most recent new vehicle purchase...so I'm a bit retarded as to knowledge of car innovations since then.

Lex

Well, not exactly. The engine turns the outer shell of the torque converter. The outer shell also turns an oil pump. All moving of transmission components is done by the hydraulic fluid. That is moving, not rotating. The rotating is transfered from the outer to the inner part of the torque converter. It is the torque converter that allows you to come to a stop while the transmission is still in gear. The torque converter is still in current transmissions.

(1) The solenoid allows hydraulic fluid to flow to a device, or blocks flow to it. It could also vent pressurized fluid to the transmission oil pan, to release a device. Most of the older spring loaded hydraulic valves are being replaced with solenoids valves. Very low electric power controls the flow of the hydraulic fluid.

(2) As stated above, the converter remains.

(1) The solenoids are mounted inside the transmission oil pan. They are mounted to an aluminum section of the transmission case with dozens of small oil passages.

(2) In the workings of a planetary gear set, there are multiple actions that can be accomplished. It is beyond my ability to try to demonstrate to you without pictures how it works, but a single rotating input shaft can run an output shaft at the same speed, slower in the same direction, or slower in reverse. When you combine multiple planetary gear sets in one transmission, along with single direction sprags, (think of a ratchet that can spin in only one direction, only toothless) there can be as many as seven different forward gears. So, the transmission may activate one, two, or more solenoids to make the transmission engage the next "gear. At the same time, it may release a device (almost always a multiple disc wet clutch.) to engage the next gear.

(3) Well sort of. In the older Allison heavy duty automatic transmission, they were just as you said, immune to engine or transmission damage. Well sort of. If you were descending a steep hill and you downshifted the transmission to the next lower gear, the transmission would not downshift if the next lower gear would end up with the engine at speed that was above the governed engine speed. To force the transmission to down shift, a driver might hit the brakes hard enough to lock and skid the rear wheels. At that time the transmission will downshift to the lower gear. when the brakes are released, the wheels will roll again with the engine now at an engine speed too high. The fact is that if one tries hard, you can still screw it up. The newer computer controlled transmissions are all about the program in the transmission. In a Buick with no high performance options, the transmission will almost surely prevent wrong gear selections. But If you have a BMW M series, or a performance Audi, you will probaply be able to shift the transmission up, or down at will. Those transmissions probably have programs that will allow you to get the most performance out of the engine, by allowing the driver to take more control of shifting choices.

(4) Today's computer controlled engine/transmission combinations just keep getting smarter. Current cruise control designs will downshift the transmission if needed to maintain the desired speed up a hill. But they will now also downshift the transmission when descending a hill, and vehicle speed increases too much. Down shifting to allow engine braking is nothing new. It has been used from the very beginning of automobiles. Heavy duty trucks rely on a few auxiliary braking systems. The most well known is the Jake Brake. The design of this is to allow the engine to build additional compression to slow the truck down. Another device used on heavy duty automatic transmission trucks is a transmission retarder. These come in 2 varieties, input and output. Input retarders require the driver to down shift the transmission to keep the retarder at the higher engine speeds needed to work efficiently. The output retarders do not require the down shifting, because the retarding is done on the road speed part of the transmission. One auxilary braking device used on medium duty trucks si an exhaust brake. By keeping the exhaust from escaping, engine speeds are slowed. One last auxilary braking device is an electric driveline retarder. By applying an electric current to a electric magnetic device on the driveshaft, the vehicle is slowed down.

These auxilary braking devices have not shown up to any great extent on passenger vehicles, with the exception of the electric retarderd that are regenitive braking devices used on the electric cars.

Wow another long winded bs session from me. Sorry.

Automobiles that are more efficient are the stated goal of several brand new vehicle makers. Ford is the latest car maker to confirm they are looking into a new way to get more proficiency on the car. Ford Motors are researching seven- and eight-speed transmissions. Transmissions that use a high number of gears are already being used in several different autos. Lexus, Audi, Bavarian Motor Works, and many other highly expensive automobiles are already using these transmissions.These high-efficiency transmissions are already getting used in BMW, Audi and Lexus cars. Here is the proof: Quest for fuel efficiency leading to more high-gear transmissions.